It’s a struggle to stay afloat in the non-stop flow of results. As a case in point, the Mayor et al. discovery preprint for HD 40307 b, c, and d has already been up on astro-ph for several weeks, and I only just a chance to read it carefully. The paper spells out the details of the announcement made at the Nantes conference last month, and ends with some bromides that seem to telegraph that the photometric transit search for planets b, c, and d is not yet definitive:

One of the most exciting possibilities offered by this large emerging population of low-mass planets with short orbital periods is the related high probability to have transiting super Earths among the candidates. If detected and targeted for complementary observations, these transiting super-Earths would bring a tremendous contribution to the study of the expected diversity of the structure of low-mass planets.

No controversy in that paragraph. It’ll be undeniably dope when the super-Earths start materializing in transit. Given that population of hot sub Neptunes in our Galaxy is apparently more than five times larger than the human population, it’s also likely that a significant number of these planets transit bright stars, and that’s good news for JWST.

In the interim, it’s not hard to see why the jury is still out on transits for HD 40307 b,c, and d. With its period of 4.61 days, the ~4 Earth-mass HD 40307b has a healthy a-priori transit probability of ~7%. Its expected transit depth, however, is a meager 0.05%. So far, the shallowest known transit for an extrasolar planet is that of HD 149026 b, which, at 0.3%, is fully six times deeper.

A ground-based detection of transits by HD 40307b would be quite a coup indeed. Is it feasible?The parent star HD 40307 is a K dwarf that’s quite similar in both spectral type and apparent magnitude to HD 189733. We can thus draw on the HD 189733 transits to get a ball park idea of the quality of the photometric data that one might expect from HD 40307. The best published ground-based light curve for HD 189733 that I could find comes from Bakos et al. (2008), who used the FLWO 1.2m telescope in Arizona to get the time series that I’ve reproduced just below. The skimpy expected depth of a central transit by HD 40307b is shown for comparison. The situation looks daunting.

The out-of-transit data in this light curve has a reported RMS scatter of 2.6 mmag for photometric points taken every 17 seconds (binned data is shown in the figure). Naive statistics thus imply that a 0.5 mmag central transit by HD 40307b could be detected by the FLWO 1.2m with at least several sigma confidence. Life, however, is more than root N. Systematic errors are probably large enough to scotch a discovery on a single night of observation, but nevertheless, by repeatedly observing, either with multiple nights or with multiple telescopes, a detection seems within reach. And it’s worth in excess of USD 5M. (At the moment, it seems there’s little need for European or Asian observers to hedge their currency risk.)

In the event that photometric campaigns aren’t up to the task, it’s in the realm of possibility that a transit by HD 40307b could be extracted via a spectroscopic detection of the Rossiter-McLaughlin effect. Assuming a 1 km/sec rotational velocity for the star, the expected half-amplitude of the Rossiter distortion is similar to the error bars on the published radial velocities. In the following figure, I’ve dished up a simulated Rossitered data set from HARPS, superimposed (with an offset for clarity) on a blown-up version of the radial velocity plot in the paper. During a single occultation, the radial velocities can produce a ~0.85 sigma detection.

In this case, the economics are a bit steeper, but still viable. At the current dollar-euro exchange rate, I’d estimate that USD 15K is a fair price for a HARPS night. (Forgive all this yak yak about currency — as an American traveling in Europe at the moment, I’m rather shocked to be seeing $6.24 0.7l bottles of water at the airport newstand!). One would need 4 hours, or half a night to observe the transit and get adequate baseline. To be at least four-sigma sure, you’d want to rack up ~20 full transits (which would take quite a while). Factoring in the expectation value of 0.07 arising from the transit probability, this works out to a USD ~2M detection.

This morning, I awoke to an inbox full of indications that there was indeed plenty of drama in the club.

From one of our correspondents:

He threw out dozens of new systems, very graphically, on slips of paper, like playing cards, floating down on a pile on the screen. Very dramatic. But no HD numbers on those slips!

He predicts 1 to 1.5 Earth sensitivity by around 2010 (extrapolating a trend).

He has been monitoring about 400 FGK slow rotators since 2004, with HARPS.

Can do 0.5 m/s today, 0.1 m/s in near future.

Noise sources are astroseismology, which settles to about 0.1 m/s after 15 minutes of integration, and a worse one, star spots, which settle to about 0.5 m/s after 15 minutes but do not drop lower, even though theory says level should drop to about 0.1 m/s.

He says he has 40 new candidates in the 30-50 day period range, and mass less than 30 Earths.

Nevertheless, after the drama, he did report 3 Neptune-type systems, all focused on the Super Earth theme of the meeting.

The centerpiece was definitely HD 40307, a deep southern K2.5 V star only 40 light years distant, with a metallicity roughly half that of the Sun. It has three detected planets, with Msin(i)’s of 4.2, 6.9, and 9.2 Earth masses, and corresponding periods of 4.31, 9.62, and 20.45 days. It’s fascinating that these planets are close to, but aren’t actually in a 4:2:1 resonance. This is really a remarkable detection.

With 40 candidates in the pocket, the Geneva team does, however, seem to be keeping some of their powder dry, perhaps in anticipation of a low-mass transit. Here’s a link to the ESO press release, which has triggered 93 news articles and counting.

In the press release image, HD 40307d is definitely all that and a bag of chips. Puffy white clouds, azure seas, continents, soft off-stage lighting…

On December 29th 1959 at the annual meeting of the American Physical Society at Caltech, Richard Feynman gave a remarkable talk entitled “There’s plenty of room at the bottom” in which he foresaw the impact that nanotechnology could have on materials science. At the beginning of the lecture he remarked (in a vernacular that dates him to the Eisenhower era):

I imagine experimental physicists must often look with envy at men like Kamerlingh Onnes, who discovered a field like low temperature, which seems to be bottomless and in which one can go down and down. Such a man is then a leader and has some temporary monopoly in a scientific adventure.

Over the past several years, the oklo.org party line has been that the radial velocity method for exoplanet detection is similarly equipped with the potential to go down and down in planet mass, and to continue with at least a respectable share of the lead in the ongoing scientific adventure.

That said, the Doppler returns so far this year have been underwhelming. If we look at the latest planet-mass vs year of discovery diagram on exoplanet.eu (no pulsar planets, no microlenses), the detection rate seems to be holding up, but the crop of announced low-mass planets is nonexistent. Of the 22 new planets so far in ’08 that have been detected via radial velocity, 16 were initially detected by the transit surveys.

What’s up with that?

We’re seeing core accretion in action. The baseline prediction of the core accretion theory for giant planet formation is that once a planet reaches a crossover threshold, where the mass of gas and solids is equal, then rapid gas accretion ensues, and the planet grows very rapidly to Jovian size or even larger. When the galactic planetary census is complete, one thus expects a relative dearth of planets with masses in the range between ~20 and ~100 Earth masses. In the freewheelingly unrefereed forum of a blog post, I can go ahead and dispense with an analysis that takes all the thorny completion issues and selection biases into account and state unequivocally that:

(Courtesy as usual of the exoplanet.eu statistics plot generators)

Planets that do make the grade and blow up to truly Jovian size are the beneficiaries of protostellar disks that had solid surface densities that were well above the average. At a given disk mass, a disk with a higher metallicity has a higher surface density of solids, which is the reason for the planet-metallicity correlation. Disks with higher oxygen and silicon fractions relative to iron will also have high solid surface densities, which is the reason for the planet-silicon correlation. And M stars have trouble putting their Jovian cores together fast enough to get the gas while it’s still there, which is the source of the planet-stellar mass correlation.

As one pushes below Neptune-mass, these correlations should all get much weaker, and the fraction of producing stars should go way up. It’s hard, at the ~10% success rate level for a protostellar disk, to make a Jupiter, and it should be straightforward, at (I’ll guess) the 50% success level for a protostellar disk to make a Neptune.

The gap between Neptune and Saturn is the source of the current RV planet drought. At given velocity precision (in the absence of stellar jitter), it takes ~25x more velocities to detect a Neptune than to detect a Saturn. To make progress, it’s necessary to stop down the number of stars in the survey and focus on as many old, quiet K-type stars as possible. We’re talking HD 69830.

The indications at Harvard were that the Geneva group has been doing just that. In a few hours, Michel Mayor is scheduled to give the lead-off talk at the Nantes meeting on extrasolar super Earths. I’ll post a rundown of what he has to say just as soon as the Oklo foreign correspondents file their reports…

On Friday, I flew back from the Boston IAU meeting, still buzzing with excitement. On Saturday, I woke up with what might best be described as a transit-induced hangover (an entirely distinct condition from transit fever). I’d basically allowed all my professorial responsibilities to slide for a week. On my desk is a mountain of work, a preliminary exam to assemble, and a horrifying backlog of e-mail.

Ahh, but like an exotic sports car bought on credit, it was worth it. The meeting was amazing, certainly the most exciting conference that I’ve ever attended. Big ups to the organizers! Planetary transits are no longer the big deal of the future. They’re the big deal of the right here right now. Spitzer, Epoxi, MOST, HST and CoRoT are firing on all cylinders. The ground-based surveys are delivering bizarre worlds by the dozen. And we’re clearly in the midst of very rapid improvement of our understanding of the atmospheres and interiors of the planets that are being discovered.

From a long-term perspective, the conference’s biggest news was probably provided by the Geneva group, in the form of Christophe Lovis’ presentation on Tuesday afternoon. In his 15-minute talk to a packed auditorium, Lovis covered a lot of ground. I scrambled to take notes. My reconstructed summary (hopefully without major errors) runs like this:

The HARPS planet survey of solar-type stars contains ~400 non-active, slowly rotating FGK dwarfs. Observations with the 3.6-meter telescope have been ongoing since 2004, and over time, their emphasis has been progressively narrowed to focus on stars that harbor low-amplitude radial velocity variations with RMS residuals in the 0.5-2.0 m/s range. The current observing strategy is to obtain a nightly multiple-shot composite velocity of an in-play candidate during block campaigns that run for 7-10 nights.

During the first few minutes, Lovis reviewed the current status of the published results. The Mu Arae planets (including the hot Neptune on the 9.6-day orbit, see here and here) are all present and accounted for. The HD 69830 triple-Neptune data set (see here, here and here) now contains twice as many velocities, with virtually no changes to the masses and orbits of the three known planets. Long-term scatter in the HD 69830 data set is at the ~90 cm/sec level, indicating either the effect of residual stellar jitter, or perhaps the presence of additional as-yet uncharacterized bodies.

He then announced that there are currently forty-five additional candidate planets with Msin(i)<30 Earth masses, P<50 days and acceptable orbital solutions. And that’s not counting candidates orbiting red dwarfs.

He then began to highlight specific systems. To say that planets were flying thick and fast is an understatement. Here’s the verbatim text that I managed to type out while simultaneously attempting to focus on the talk:

Rumor has it that some of these systems will be officially unveiled at the upcoming Nantes meeting on Super Earths. Odds-on, with 45 candidates in play, we’ll soon be hearing about a transiting planet with a mass of order ten times Earth’s. I won’t be at the Nantes meeting, but the stands will be harboring agents of the Oklo Corporation.

The talk finished with an overview of the statistics of the warm Neptune population. Most strikingly, a full 80% of the candidates appear to belong to multiple planet systems, but cases of low-order mean motion resonance seem to be rare [as predicted –Ed.] . There is a concentration of these planets near the 10-day orbital period, and the mass function is growing toward lower masses. Significant eccentricities seem to be the rule. And finally, I think it was mentioned that the planet-metallicity correlation is weaker for the warm Neptunes than for the population of higher-mass planets.

Seems like core accretion is standing the test of time.

Note on the images: Gaspar Bakos (of HAT fame) had the cool idea of machining metal models for the planets of known radius which are correct in terms of relative size, and which have the actual density of their namesakes. HAT-P-6, for example, is constructed from a hollow aluminum shell, and with a density of ~0.6 gm/cc it would float like a boat. HAT-P-2b, on the other hand, which packs 8.6 Jupiter masses into less than a Jovian radius, has the density of lead and (not coincidently) is made out of lead. It’s startling to pick it up. CoRoT-Exo-3b, which was announced at the meeting, has a mass of twenty Jovian masses, and a radius just less than Jupiter. I guess that one will have to be made from Osmium.

Earth, at ~5.5 gm/cc, on the other hand, can be readily manufactured from a variety of different alloys.

Jonathan Fortney has the office next to mine at UCSC, and so we’re always talking about the Spitzer observations of extrasolar planets. The Spitzer Space Telescope has proved to be an extraordinary platform for observing planets in the near infrared, and during the past year, the number of published and planned observations has really been growing rapidly.

Increasingly, with the flood of data, I’ve been finding that I have trouble keeping mental track of all the photometric observations of all the planets that Spitzer has produced. Let’s see, was Tres-1 observed in primary eclipse? Did someone get a 24-micron time series for HD 149026? And so on.

So Jonathan and I decided to put together a poster that aggregates the observations (that we know of) that have either been completed, or which have been scheduled. The relevant information for each campaign includes the star-planet system, the bandpass, and the duration and phase of the observation. We wanted the information for each system to be presented in a consistent manner, in which the orbits, the stars, and the planets are all shown to scale (and at a uniform scale from system to system). As an example, here’s the diagram for HD 189733:

In putting the poster together, we were struck by the variety of different observational programs that have been carried out. Some of the diagrams, furthermore, with text removed, have a delicate insect-like quality.

(The figure just above shows Bryce Croll’s planned 8-micron observations of Transitsearch.org fave HD 17156b. Croll’s campaign will attempt to measure the pseudo-synchronous rotation period of the planet.)

I’m going to Boston next week to attend the IAU transit meeting, and so I printed out a copy of the poster to put up at the meeting:

Here’s a link to the Illustrator file and the .pdf version. Full size, it’s two feet wide and three feet tall. Going forward, I’ll update the files as new observations come in.

Uranus and Neptune have returned to nearly the configuration that they were in at the time of Neptune’s discovery in 1846. Using Solar System Live, it’s easy to see where the planets were located when Galle and d’ Arrest turned the Berlin Observatory’s 9-inch Fraunhofer refractor to the star fields of the ecliptic near right ascension 22 hours:

In 2011, Neptune, with its 165-year period period, will have made one full orbit since its discovery. Uranus, with an 84-year period, will have gone around the Sun almost two times.

Because the planets are fairly close to conjunction, Neptune has recently gone through the phase of its orbit where it exerts its largest perturbation on the motion of Uranus. This was similarly true in the years running up to 1846, and was responsible for LeVerrier’s sky predictions bearing such a stunning proximity to the spot where Neptune was actually discovered by Galle.

LeVerrier (and Adams) were quite fortunate. Without a computer, multi-parameter minimization is hard, and both astronomers cut down on their computational burden by assuming an incorrect distance for Neptune (based on Bode’s “law”). Their solutions were able to compensate for this incorrect assumption by invoking masses for Neptune that were much too large. They carried out remarkable calculations, but nevertheless, luck (in form of the fact that Uranus and Neptune had recently been near conjunction) played a considerable role.

Predictably, as soon as the real orbit of Neptune was determined, the playa haters tried to rush the stage. Benjamin Peirce of Harvard, in the Proceedings of the American Academy of Arts and Sciences 1, 65 (1847) described LeVerrier’s accomplishment as a mere “happy accident”:

I personally think that’s going a bit far. In any case, it’s interesting to compare the two independent predictions with the actual orbit of Neptune. I pulled the LeVerrier and Adams data in the following table from Baum and Sheehan’s book “In Search of Planet Vulcan” :

There’s been no shortage of hard work, and there’s been no shortage of predictions and false alarms, but nevertheless, nobody has managed to discover another solar system planet via analysis of gravitational perturbations. With the extrasolar planets, however, the prospects look a lot better. In particular, the Systemic Backend collaboration can team up with amateur observers to do the trick.

On the Systemic Backend, there are many candidate planets that have had their orbits characterized. As is usually the case with planet predictions, most of the candidates will wind up being spurious, but it’s definitely true that real planets orbiting real stars have been detected by the Backend user base. For example, Gliese 581 c was accurately characterized by the Systemic users several months before it’s announcement by the Swiss (see this post) and the same holds true for 55 Cancri f (see this post).

In the happy circumstance that a candidate planet is part of a system with a known transiting planet, then there’s an increased probability that if the candidate planet exists then it can also be observed in transit. This provides a channel for detection that completely circumvents the need for professional astronomers to carry out confirming radial velocity observations. Amateur observers are currently pushing the envelope down to milli-mag precision. Here’s an out-of-transit observation of the parent star of XO-1b by Bruce Gary:

This photometry is potentially good enough to confirm a Neptune-sized planet in transit across a Solar-type star, which is absolutely amazing.

An initial proof-of-concept observation has recently been carried out. On the systemic backend, the users have been investigating the HD 17156 system, which contains a known transiting planet. User “japf ” (José Fernandes) found that a lower chi-square fit to the published radial velocity data can be obtained if there’s a 6.2 Earth-mass companion on a 1.23 day orbit.

The best-fit eccentricity of the planet would bring it to a hair-raising 2 stellar radii of HD 17156, and if the planet is made of rock or water, it’ll be too small to detect, but nevertheless, it’s at least worth having a look. Jose sent the ephemeris to Bruce Gary, who observed on the opportunity falling on April 20, 04.5 UT.

No transit detected. This in itself was not at all surprising, given the long-shot nature of this particular candidate planet. What’s exciting, though, is that the full pipeline is now in place. There will definitely be strong candidates emerging over the coming months, and I think it’s quite probable that we’ll see a prediction-confirmation that is at least as good a match as was obtained for Neptune in 1846…

Back in 2002, Keith Horne gave a talk at the Frontiers in Research on Extrasolar Planets meeting at the Carnegie Institute in Washington and showed an interesting table:

At that time, there were more than two dozen active searches for transiting extrasolar planets, but only a single transiting planet — HD 209458 b — had been detected. Transits were generating a lot of excitement, but paradoxically, the community was well into its third straight year with no transit detections. The photometric surveys seemed to be just on the verge of really opening the floodgates, with a total theoretical capacity to discover ~200 planets per month.

It’s been six years, and the total transiting planet count is nowhere near 14,000. Most of the surveys on the table have had a tougher-than-expected time with detections because of the large number of false positives, and because of the need to obtain high-precision radial velocities on large telescopes to confirm candidate transiting planets. Indeed, the surveys that were sensitive to dimmer stars have largely faded out. It’s just too expensive to get high-precision velocities for V>15 stars. With the exception of the OGLE survey (which had been set up to look for microlensing during the 1990s, and which had established a robust pipeline early on) none of the surveys that employed telescopes with apertures larger than 12 cm have been successful. The currently productive photometric projects: TrES, XO, HATnet, and SuperWASP all rely on telescopes of 10 to 11 cm aperture to monitor tens of thousands to hundreds of thousands of stars, and all are sensitive to planets transiting stars in the V~10 to V~12 magnitude range. This magnitude range is the sweet spot: there are plenty of stars (and hence plenty of transits) and the stars are bright enough for reasonably efficient radial velocity confirmation.

Yesterday, SuperWASP rolled out 10 new transits at once, dramatic evidence of the trend toward planetary commoditization and of the fact that it’s getting tougher to make a living out on the discovery side. The detection of new planets is growing routine enough that in order to generate a news splash, you need multiple planets, and the more the better. This inflationary situation for new transit news is highly reminiscent of where the Doppler surveys were at seven years ago. For example, on April 4, 2001, the Geneva team put out a press release announcing the discovery of eleven new planets (including current oklo fave HD 80606b).

I’d like to register some annoyance with this latest SuperWASP announcement. There are no coordinates for the new planets, making it impossible to confirm the transits. There is no refereed paper. The data on the website are inconsistent, making it hard to know what’s actually getting announced. I was astonished, for example, that WASP-6 is reported on the website to have a radius 50% that of Jupiter, and a mass of 1.3 Jovian masses:

That’s nuts! If the planet is so small, why is the transit so deep? And a 2200 K surface temperature for a 3.36d planet orbiting a G8 dwarf? Strange. Perhaps the radius and mass have been reversed? In addition, there are weird inconsistencies between the numbers quoted in the media diagram and in the tables. For example, the diagram pegs WASP-7 at 0.67 Jovian masses, whereas the table lists it at 0.86 Jovian masses. WASP-10 has a period of 5.44 days in the table and 3.093 days in the summary diagram. Putting out a press release without the support a refereed paper is never a very good idea, even when there’s a danger that another team will steal your thunder with an even larger batch of planets.

Despite the difficulty in getting accurate quotes from the exchange, it’s interesting to see how the ten new planets stack up in the transit pricing formula. Using the data from the new WASP diagram (except for the 0.66 day period listed for WASP-9) and retaining the assumption that USD 25M has been spent in aggregate on ground-based transit searches, the 46 reported transits come out with the following valuations:

The ten new WASP planets (assuming that the correct parameters have been used) contribute about 1/10th of the total catalog value. There will likely be interesting follow-up opportunities on these worlds from ground and from space, but its unlikely that they’ll rewrite the book on our overall understanding of the field.

It’s interesting to plot the detection rate via transits in comparison to the overall detection rate of extrasolar planets. (The data for the next plot was obtained using the histogram generators at the Extrasolar Planets Encyclopaedia, which are very useful and are always up-to-date.)

It’s a reasonable guess that 2008 will be the first year in which the majority of discoveries arrive via the transit channel, especially if CoRoT comes through with a big crop. Radial velocity, however holds an edge in that it’s surveying the brightest stars, and (so far) has been responsible for progress toward the terrestrial-mass regime. I think that we might be seeing planets of only a few Earth masses coming out of the RV surveys during the coming year. Certainly, everything else being equal, a planet orbiting an 8th magnitude star is far more useful for follow-up characterization than a planet orbiting a 13th magnitude star.

Image Source.

The range of planetary orbits that are observed in the wild is quite a bit more varied than the staid e < 0.20 near-ellipses in our own solar system. For regular oklo readers, the mere mention of Gl 876, 55 Cancri, or HD 80606, is enough to bring to mind exotic worlds on exotic orbits.

Non-conventional configurations involving trojan planets have been getting some attention recently from the cognescenti. Even hipper, however, is a configuration that I’ll call the 1:1 eccentric resonance. Two planets initially have orbits with the same semi-major axis, but with very different eccentricities. Conjunctions initially occur close to the moment of apoastron and periastron for the eccentric member of the pair.

Here’s a movie (624 kB Mpeg) of two Jupiter-mass planets participating in this dynamical configuration.

At first glance, the system doesn’t look like it’ll last very long. Remarkably, however, it’s completely stable. Over the course of a 400-year cycle, the two planets trade their angular momentum deficit back and forth like a hot potato and manage to orbit endlessly without anyone getting hurt.

Here’s an animation (1 MB Mpeg) which shows a full secular cycle. The red and the blue dots show the planet positions during the two orbit crossings per orbit made by one of the planets. It’s utterly bizarre.

These animations were made several years ago by UCSC grad student Greg Novak (who’ll be getting his PhD this coming summer with a thesis on numerical simulations of galaxy formation and evolution). As soon as we can get the time, Greg and I are planning to finish up a long-dormant paper that explores the 1:1 eccentric resonance in detail. In short, these configurations might be more than just a curiosity. When planetary systems having three or more planets go unstable, two of the survivors can sometimes find themselves caught in the 1:1 eccentric resonance. The radial velocity signature of the resulting configuration is eminently detectable if the planets can be observed over a significant number of orbital periods.

Image Source.

Stefano, and Eugenio and I have been completely immersed in several time-critical projects during the past few months, and as a result, the frequency of posts here on oklo.org has not been as high as I would like. We’re starting to see our way clear, however, and very shortly, there’ll be a number of significant developments to report. Also in the cards is a major new release of the console, and a refocus on the research being carried out on the systemic backend. In any case, sincere thanks to all the backend participants for their patience.

Oklo regulars will recall all the excitement last fall surrounding the discovery of transits by HD 17156b. The transit was first observed on September 10th by a cadre of small telescope observers, and was then confirmed 21.21 days later on October 1.

Jonathan Irwin at Harvard CfA has led the effort to analyze and publish the October 1 observations of the transit. The work recently cleared the peer-review process, and was posted on the web a few days ago. (Here’s a link to the paper on astro-ph.)

The night of October 1 was plagued by atrociously aphotometric conditions across the North American continent, and most of the observers who tried to catch the transit were clouded out. Southern California, however, had reasonably clear skies, and three confirming time series came from the Golden State. The Mount Laguna observations were taken from SDSU’s Observatory in the mountains east of San Diego, the Las Cumbres observations were made from the parking lot of the LCOGT headquarters in Santa Barbara, and Transitsearch.org participant Don Davis got his photometry from his backyard in suburban Los Angeles.

The aggregate of data from the October 1 transit allowed us to refine the orbital properties of the planet, and additional confirming observations in a paper by Gillon (of ‘436 fame) et al have given a much better characterization of the orbit.

Because of the high orbital eccentricity, the planet should have very interesting weather dynamics on its surface. Jonathan Langton’s model predicts that the planet’s 8-micron flux should peak strongly during the day or so following periastron passage as the heated hemisphere of the planet turns toward Earth.

By measuring the rise and subsequent decay of the planet’s infrared emission, it’ll be possible to get both a measure of the effective radiative time constant in the atmosphere as well as direct information regarding the planet’s rotation rate. Bryce Croll is leading a team that successfully obtained time on the Spitzer telescope to make the observations.

In another interesting development, a paper by Short et al. appeared on astro-ph last week which proposes the existence of a second planet in the HD 17156 system. The Short et al. planet has an Msin(i) of 0.06 Jupiter masses and an orbital period of 111.3 days. It’s quite similar to the slightly more eccentric (and hence dynamically unstable) version of the HD 17156 system proposed by Andy on the Systemic Backend last December, which was based on the radial velocities and transit timing then available:

The existence of a second planet in the HD 17156 system would be extremely interesting! The immediate question, however, is, how likely is it that the second planet is actually there?

To make an independent investigation, it’s straightforward to use the downloadable systemic console to fit to the available published data on HD 17156. I encourage you to fire up a console and follow along. Now that the Irwin et al. paper is on the web, we have the following transit ephemerides:

These can be added to the HD17156.tds transit timing file in the datafiles directory. The file should be edited to look like this:

When the HD17156v2TD system is opened on the console, it shows both the radial velocity and the transit timing data.

It’s quick work to dial in a one planet fit to the RV and transit timing data. I get a system with the following fit statistics:

The required jitter of 2.12 m/s indicates that a one planet fit to the data should still be perfectly adequate, since the star (which is fairly hot and massive) has an expected stellar jitter of order 3 m/s. Nevertheless, the residuals periodogram does show a distinct peak at ~110 days:

Using the 110 day frequency as a starting point, one finds that ~0.1 Mjup planets do indeed lower the chi-square. I’ve uploaded an example two planet fit to the systemic backend that harbors a second planet in a 113 day orbit and a mass of 0.13 Jupiter masses. Its periastron is aligned with that of planet b, and the RMS has dropped down to 3.08 m/s (for a self-consistent, integrated fit). The implied stellar jitter is a bargain-basement 0.59 m/s, which is almost certainly too good to be true.

When I do an F-test between my one and two planet fits, the false alarm probability for planet ccomes in at 38%. It’s thus fairly likely that the second planet is spurious, but nevertheless, it certainly could be there, and it’ll be very interesting to keep tabs on both the transit timing data and the future radial velocity observations of this very interesting system…

Image Source.

There’s a provocative paper up on the astro-ph today. Ignasi Ribas and two collaborators are reporting the “possible discovery” of a 4.8 Earth mass planet in an exterior 2:1 mean motion resonance with the transiting hot Neptune Gliese 436b. Planet four three six b is the well-known subject of great consternation, great scientific value, and many an oklo.org post. (For the chronological storyline, see: 1 (for background), 2, 3, 4, 5, 6, 7, 8, 9, and 10.)

Here’s the basic idea. Ribas et al. note that a single-planet fit to the Maness et al. (2007) radial velocity data set (which is listed as gj_436_M07K on the systemic console) has a peak in the residuals periodogram at P~5.1866 days:

Using this periodogram peak as a starting point, they get a keplerian 2-planet fit that lowers the reduced chi-square from ~4.7 to ~3.7. They then point out that this detection can potentially be confirmed by measuring variations in transit timing. In their picture, the presently-grazing transit has come into visibility only within the last 2.5 years or so, as a result of orbital precession. The transit light curve should thus be showing significant variations in duration as well as deviations from a strictly periodic sequence of central transit times.

This will be a huge big deal if the claim holds up. For starters, it’ll provide a natural explanation for Gl 436b’s outsize eccentricity. And everyone’s been on the lookout for a strongly resonant transiting system with a short orbital period. For the time being, though, I’m withholding judgment. As a first point of concern, Ribas et al. are presenting a keplerian fit to the radial velocities. Yet for the orbital configuration they are proposing, it’s absolutely vital to take planet-planet interactions into account. One can see this by entering their fit into the console. (Use a mean anomaly at the first RV epoch 2451552.077 for planet b=40.441 deg, corresponding to their reported time of periastron of Tp_b=HJD 2451551.78, and a mean anomaly for planet c=268.14 deg, corresponding to their reported value of Tp_c=HJD 2451553.4.) One can also dial in a long-term trend if one wants, but this isn’t necessary. Once the fit is entered, the reduced chi-square is 3.7. Activate integration. (Hermite 4th-order is the faster method.) When the planets are integrated, their mutual interactions utterly devastate the fit, driving the reduced chi-square up to 85.018. Using the zoomer and the scroller, you’ll see that the integrated radial velocity curve and the keplerian curve start off as a good match, but then rapidly get completely out of phase.

In order to examine the plausibility of a two-planet fit in 2:1 mean motion resonance, one needs to fit the radial velocity data with integration turned on. It is also important to include the existing transit timing data in the fit (and to do this, it’s best to use the most recent, so-called unstable version of the console). Over at Bruce Gary’s amateur exoplanet archive (AXA), there are now three transit timing measurements listed, with the latest obtained by Bruce himself this past New Years Eve. The HJD measurements of central transit should be added to the gj436.tds file, along with the HJD 2454280.78149 +/- 0.00016 central transit time measured by Spitzer.

Ideally, the Spitzer secondary transit timing data should also be included, but at the moment, the distribution version of the console does not have the capability to incorporate secondary transit measurements. One approach would be to get a self-consistent fit, and then see whether the epoch of secondary transit matches that observed by Spitzer.

Have fun…

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Discoveries relating to transiting extrasolar planets often make the news. This is in keeping both with the wide public interest in extrasolar planets, as well as the effectiveness of the media-relations arms of the agencies, organizations, and universities that facilitate research on planets. I therefore think that funding support for research into extrasolar planets in general, and transiting planets in particular, is likely to be maintained, even in the face of budget cuts in other areas of astronomy and physics. There’s an article in Saturday’s New York Times which talks about impending layoffs at Fermilab, where the yearly budget has just been cut from $342 million to $320 million. It’s often not easy to evaluate how much a particular scientific result is “worth” in terms of a dollar price tag paid by the public, and Sean Carroll over at Cosmic Variance has a good post on this topic.

For the past two years, the comments sections for my oklo.org posts have presented a rather staid, low-traffic forum of discussion. That suddenly changed with Thursday’s post. The discussion suddenly heated up, with some of the readers suggesting that the CoRoT press releases are hyped up in relation to the importance of their underlying scientific announcements.

How much, actually, do transit discoveries cost? Overall, of order a billion dollars has been committed to transit detection, with most of this money going to CoRoT and Kepler. If we ignore the two spacecraft and look at the planets found to date, then this sum drops to something like 25 million dollars. (Feel free to weigh in with your own estimate and your pricing logic if you think this is off base.)

The relative value of a transit depends on a number of factors. After some revisions and typos (see comment section for this post) I’m suggesting the following valuation formula for the cost, C, of a transit:

The terms here are slightly subjective, but I think that the overall multiplicative effect comes pretty close to the truth.

The normalization factor of 580 million out front allows the total value of transits discovered to date to sum to 25 million dollars. The exponential term gives weight to early discoveries. It’s a simple fact that were HD 209458 b discovered today, nobody would party like its 1999 — I’ve accounted for this with an e-folding time of 5 years in the valuation.

Bright transits are better. Each magnitude in V means a factor of 2.5x more photons. My initial inclination was to make transit value proportional to stellar flux (and I still think this is a reasonable metric). The effect on the dimmer stars, though was simply overwhelming. Of order 6 million dollars worth of HST time was spent to find the SWEEPS transits, and with transit value proportional to stellar flux, this assigned a value of two dollars to SWEEPS-11. That seems a little harsh. Also, noise goes as root N.

Longer period transits are much harder to detect, and hence more valuable. Pushing into the habitable zone also seems like the direction that people are interested in going, and so I’ve assigned value in proportion to the square root of the orbital period. (One could alternately drop the square root.)

Eccentricity is a good thing. Planets on eccentric orbits can’t be stuck in synchronous rotation, and so their atmospheric dynamics, and the opportunities they present for interesting follow-up studies make them worth more when they transit.

Less massive planets are certainly better. I’ve assigned value in inverse proportion to mass.

Finally, small stars are better. A small star means a larger transit depth for a planet of given size, which is undeniably valuable. I’ve assigned value in proportion to transit depth, and I’ve also added a term, Np^2, that accounts for the fact that a transiting planet in a multiple-planet system is much sought-after. Np is the number of known planets in the system. Here are the results:

HD 209458 b is the big winner, as well it should be. The discovery papers for this planet are scoring hundreds of citations per year. It essentially launched the whole field. The STIS lightcurve is an absolute classic. Also highly valued are Gliese 436b, and HD 189733b. No arguing with those calls.

Only two planets seem obviously mispriced. Surely, it can’t be true that HAT-P-1 b is 10 times more valuable than HAT-P-2b? I’d gladly pay $85,507 for HAT-P-2b, and I’d happily sell HAT-P-1b for $969,483 and invest the proceeds in the John Deere and Apple Computer corporations.

Jocularity aside, a possible conclusion is that you should detect your transits from the ground and do your follow up from space — at least until you get down to R<2 Earth radii. At that point, I think a different formula applies.

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As the academic quarter draws to a close, it gets harder to keep up a regular posting schedule. This year, certainly, the difficulty has nothing to do with a lack of exciting developments associated with extrasolar planets.

A few unrelated items:

It appears that the HD 80606b Spitzer observations went smoothly, and that the data has been safely transmitted to Earth via NASA’s Deep Space Network. It is currently in the processing pipeline at the Spitzer Science Center. When it clears the pipeline, the analysis can start.

Back in September, I wrote a post about Bruce Gary’s Amateur Exoplanet Archive. This is a web-based repository for photometric transit observations by amateurs. With the number of known transits growing by the month, there’s a planet in transit nearly all of the time. Over 90 light curves have been submitted to the archive thus far. For transiting planets such as HD 189733b or HD 209458b, which have significant numbers of published radial velocity data, it’s very interesting to take the transit center measurements from Bruce’s archive and use them as additional orbital constraints within the console. The September post gives a tutorial on how to do this.

It really is turning out to be a banner year for extrasolar planets. As we head into December, this year is averaging more than one planet per week. The detection rate is more than double that of the previous four years.

The plot above gives a hint that Saturn-mass planets might wind up being fairly rare, as one might expect from the zeroth-order version of the core accretion theory. (For more information, this series: 1, 2, 3, 4, 5, 6, and 7 of oklo posts compares and contrasts the gravitational instability and core accretion theories for giant planet formation.)
Also, if you give talks, here’s a larger version of the above figure.

Another interesting diagram is obtained by plotting orbital period vs. year of discovery:

It’s possible that this diagram might be hinting that true Jupiter analogs are relatively rare. Could be that the disks around metal-rich stars are able to form Jovian mass planets and then migrate them in, while stars with subsolar metallicity form ice giants beyond the ice line. In this scenario, our solar system lies right on the boundary between the two outcomes.

It could also be the case that there are a whole slew of true-Jupiter analogs just on the verge of being announced. Time will tell.

And as always, it’s interesting to spend time with the correlation diagram tool over at exoplanet.eu.

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It’s sometimes a little weird to realize that my daily schedule is dictated by the orbits of alien planets. HD 80606b went through periastron passage at 07:00 UT last Tuesday, with the Spitzer Space Telescope’s rattlesnake’s eye vision trained intently upon it. Over the past few days, it’s been hurtling away from the star, gradually reducing its velocity as it climbs up the gravitational potential well of the star.

At 07:45 UT on Monday morning, HD 80606b is scheduled to go through inferior conjunction. In the 1.6% a-priori geometric chance that the orbital plane of the planet is in near-perfect alignment with the line of sight to the solar system, then it will be possible to observe the planet in transit. The 1.6% transit probability is fairly high for a planet with a period of 111 days, but much lower than the 15% probability that a secondary eclipse can be observed. If the planet is undergoing secondary eclipse, then we’ll know as soon as the Spitzer data comes in.

Back in early 2005, Transitsearch.org coordinated a campaign to check for transits of HD 80606b. At that time, there were fewer radial velocities available, and so the transit window was less well constrained. A number of observers got data, and there was no sign of transit, but the coverage was not good enough to rule out a transit. I’m thus encouraging observers to monitor HD 80606 during the next 48 hours on the off chance that it can be observed in transit. Given the small chance involved, it seems appropriate to refer to the transit probability in terms of basis points. As in, “In ’05, we got about 40 basis points. That means there’s still 120 basis points out there to collect.”

HD 80606 is a visual binary. The companion, HD 80607, provides a good comparison star in telescopes with a large enough aperture under good seeing conditions. For most observers, however, the light from the two stars is combined. A transit by HD 80606b is expected to have a depth of order 1.4%, and (if it’s a central transit) will last about 16 hours. It’s a long-shot for sure, but worthwhile and fun nonetheless.

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As I write this, it’s JD 2454425.219 (17:16 UT, Nov. 20 2007). HD 80606 b whipped through periastron a little more than 10 hours ago, and the Spitzer Space telescope is literally just finishing its 31-hour observation of the event. Next comes the downlink of the data to Earth on the Deep Space Network, and then the analysis. Definitely exciting!

The Spitzer Space Telescope is scheduled to run out of cryogen in early 2009. When the telescope heats up, we’ll lose our best platform for mid-infrared observations of hot extrasolar planets, and so there was a palpable urgency last week as everyone prepared their proposals to meet the submission deadline for Spitzer’s last general observing cycle. During the next few years, there is going to be intense development of detailed 3D radiation-hydrodynamical models for simulating the time-dependent surface flows on extrasolar planets. These models will need contact points with hard data. It’s thus vital to bank as wide a variety of observations of as wide a variety of actual planets under as wide variety of different conditions as possible. A number of fascinating exoplanet observing proposals were submitted last week by a variety of highly competent teams. I’m urging that they all be accepted!

Most of the exoplanet observations that have been done with Spitzer have focused on tidally locked transiting planets on circular orbits. HD 189733b, HD 209458b, TrES-1 and HD 149026b are the flagship examples of this class. In the past year, however, eccentric transiting planets have started turning up. Gliese 436b (e=0.15) was the first, followed by HAT-P-2b (e=0.5), and HD 17156b (e=0.67).

Drake Deming, Jonathan Langton and I decided that the most interesting proposal that we could make would be for Gliese 436 b. This is the Neptune-mass, Neptune-sized planet transiting a nearby red dwarf star. Here’s the to-scale diagram of the 2.644-day orbit:

After Gliese 436b was discovered to transit last spring, it triggered a Joe Harrington’s standing Target of Opportunity program. Both a primary and a secondary transit were observed (see this post) which confirmed the startlingly high eccentricity, and which allowed an estimate of the planet’s temperature (or, more precisely, the 8-micron brightness temperature). This turned out to be 712±36 K, which is significantly higher than the ~650 K baseline prediction.

The hotter-than-expected temperature measurement could arise from a number of different effects (or combinations of effects). By measuring the secondary eclipse, you strobe one hemisphere of the planet. If there are significant temperature variations across the surface of the planet, then a high reading might arise from chancing on the hotter side of the planet. Alternately, the effective temperature implied by measuring the energy coming out at 8-microns could be seriously skewed if the spectrum of the planet has deep absorption or emission bands at the 8-micron wavelength. Another possibility is that we’re observing tidal heating in action. Gliese 436b is being worked pretty hard in its eccentric orbit, and it should be generating quite a bit of interior luminosity as a result. If its structure is similar to Neptune, then a 712K temperature is completely understandable.

Io, of course, is subject to a similar situation. Here’s a K-band infrared photo of Io in transit in front of Jupiter:

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Gliese 436b is in pseudo-synchronous rotation, and spins on its axis every ~2.3 days. The eccentricity of the orbit leads to an 83% variation in the amount of light received from the star over a 1.3 day timescale. This leads to a complicated flow pattern on the surface.

Here’s what Jonathan Langton’s model predicts for the appearance of the hemisphere facing Earth at five successive secondary eclipses:

Globally, the hydrodynamical model produces a statistically steady-state flow pattern that is dominated by a persistent eastward equatorial jet with a zonally averaged speed of ~150 meters per second. This eastward flow in the planet’s frame produces a light curve in the lab frame that has a ~3 day periodicity. This period is significantly longer than both the planet’s orbital period and the planet’s spin period. Our Spitzer proposal is to observe a sequence of 8 secondary transits in hopes of confirming both the amplitude and the periodicity of this light curve.

It’s certainly the case that our current hydrodynamical model is not the definitive explanation of what these planets are doing. I won’t be at all surprised if the flux variation from eclipse to eclipse is more complicated than what we predict. I’m highly convinced, however, that the model is good enough to indicate that the situation on Gliese 436b will be interesting, dynamic, and complex. The actual variation in the real observations will provide an interesting and non-trivial constraint that a definitive model of the planet will need to satisfy. The observations, if approved, will thus be of great use to everyone in the business of constructing GCMs for short period planets.

Stay tuned…

The flurry of activity surrounding the detection of the HD 17156b transits, combined with the start of the academic quarter here at UCSC, caused me to fall way behind on my stack. All of a sudden, over two weeks have passed with no oklo.org posts. I think that’s a record, unfortunately.

And it’s not as if there’s been no new exoplanet news. The past two weeks have seen the announcement of two more new transiting planets from Gaspar Bakos’ HATnet project. “Yo, what up TrES?” HAT-P-5b has a period of 2.79 days, and looks good in a 400-pixel wide everything’s to scale diagram:

There are enough transiting extrasolar planets now, so that it’s interesting to look for trends in the planetary properties. At Jean Schneider’s exoplanet.eu site, there’s a nifty set of php routines that make it very easy to dial up all the different correlation diagrams. With the inclusion of HD 17156b, HAT-P-5b, and HAT-P-6b, a plot of planetary radii vs. stellar metallicity is pointing to an interesting trend. It’s quite apparent that the metal-rich stars tend to harbor smaller planets. This seems to be indicating that metal-rich disks are yielding planets that have highly enriched heavy element fractions, which in turn is giving us an important clue into the planet formation process.

If we ignore Gliese 436b, which is far smaller in both mass and radius than all the other known transiting exoplanets, then there’s a fairly obvious hint of two separate sequences in the diagram — a large-radius sequence, and a small-radius sequence. Feel free to voice your opinion in the comments section…

It’d certainly be nice to get more transits by planets orbiting bright parent stars. To that end, it’s important to stress that literally every single planet transiting a V<13 parent star is located north of the the celestial equator. It’s pretty clear that the southern hemisphere Doppler-wobble planets have not been fully followed up with photometric campaigns. I’m thus keen to get the Southern-Hemisphere Transitsearch.org corps out on the sky during the coming austral summer. First on the list is HD 76700b. This 0.20 Jupiter-mass planet orbits with a period of 3.970985 days, and has an extensive and fairly recent set of published radial velocities. I just updated the orbital fit, and found that the transit windows are still quite narrow. A simple bootstrap analysis shows that the uncertainty in the time of the transit midpoint is not much wider than the expected duration of the transit itself. The star is just coming visible in the early morning, and so it should be straightforward to either confirm or rule out a transit for this particular planet.

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I’m happy to report that HD 17156b is observable in transit, and that Transitsearch.org observers played the key role in the discovery.

Regular oklo.org readers are familiar with HD 17156 b. This planet has an orbital period of 21.2 days, which is nearly four times longer than any other known transiting planet, and an eccentricity of e=0.67 (even higher than HAT-P-2b’s eccentricity of e=0.5). The geometry of the eccentric orbit has a periastron angle of 121 degrees, which means that the planet is quite close to the star as it perforates the plane containing the line of sight to Earth. Here’s a scale model showing the planet (at equally spaced intervals), the star, and the orbit:

Early last week, I got e-mail from Mauro Barbieri, an Italian post-doctoral researcher who is working on the CoRoT satellite, and who’s based at LAM in Marseille, France. In his spare time, Mauro has been writing articles for popular astronomy magazines in Italy, and has worked to coordinate Italian amateur astronomers for participation in Transitsearch campaigns. On the night of Sept. 9/10, he recruited four Italian amateur astronomers to monitor the transit window.

Two observers in Northern Italy were clouded out prior to the start of the transit, but two others, D. Gasparri and C. Lopresti, were able to observe through much of the night in Central Italy. Their data looked promising, showing clear ingresses at the same time as the ingress was observed by Jose Almenara in the Canary Islands. Ron Bissinger, observing from Pleasanton, California, and seven hours farther west, was able to start observing just after the transit ended.

As it happened, Roi Alonso, another CoRoT postdoc, is good friends with Jose Almenara, who made the Canary Island observations on Sep 9/10. Barbieri and Alonso did a careful analysis of the three transit-bearing data sets, and concluded that the transit is present at 3-sigma, 5.3-sigma, and 7.9-sigma, respectively. Almenara’s data, in particular, is excellent, despite of the fact that high winds occluded part of the mid-transit time series. By last Friday, on the strength of the detections, we had begun drafting a paper that discusses the discovery.

The Almenara egress is particularly convincing (the bottom time series shows the result of subtracting out the best-fit transit signal):

Barbieri’s and Alonso’s fit to the data from Sep. 9/10 implies the following properties for the planet and the transit. The model consistently takes into account the eccentric character of the planetary orbit:

The fit to the data suggests that the radius of HD 17156b is just a bit larger than the radius of Jupiter. This is fully in line with our theoretical models of the planet. HD 17156b experiences strong tidal forces during its periastron passages. This tidal heating might be observable in the form of excess infrared radiation, but it is not serving to inflate the planet beyond its expected radius.

Needless to say, we were quite excited by the quality of the fit. Everything seemed to hang together quite well, but confirmation was essential. The full transit would be visible across the United States and Canada. I wrote to the transitsearch.org mailing list, urging observers to monitor the star through the night of Sep. 30/Oct. 1. Dave Charbonneau of Harvard had been following oklo.org, and was impressed by the Sep 9/10 data. He worked very hard to organize and coordinate observers, and he’ll be leading a follow-up paper that uses data from all the transits to improve the planetary and the orbital characterization. Dave was very generous in offering to notify us if a transit was confirmed by the cadre of observers that he’d recruited.

Sunday morning was perfectly clear in Santa Cruz. Ron Bissinger lives in Pleasanton, just forty miles to the Northeast, and was ready to observe. His pipeline is quite automated, and so if he could observe, I knew that we would rapidly rule out or confirm a transit.

Late Sunday afternoon, I went running, and noticed that a gloomy bank of clouds was visible over the Pacific to the west:

At dusk, it was still clear, but the weather forecast did not look good. The bands of clouds that had stayed offshore to the Northwest were beginning to slide across the skies. By midnight, it was evident that the Bay Area would not be producing useful photometry. Furthermore, all of Arizona and New Mexico were rained out. Observer after observer reported in to say that they had not gotten data. The only positive report of clear skies came from Dave, graduate student Philip Nutzman, and postdoc Jonathan Irwin, who had set up a small telescope on the roof of a Harvard/CfA building in Cambridge MA.

By Monday morning, however, it was clear that several observers had indeed managed to obtain data. In addition to the Harvard roof observations, Bill Welsh and Abhijith Rajan had obtained usable photometry from the Mt. Laguna Observatory run by San Diego State University. Dave reported to me that on Sunday, while Dave was visiting the Zoo with his daughter, Bill had called with the news that they had managed to secure a night on the telescope and were at that moment driving up the mountain. In addition, a report came from Tim Brown of Las Cumbres observatory that while their Hawaii site had been weathered out, observations had been successfully made from parking lot of the observatory headquarters in Santa Barbara. And in addition, Don Davies, a Transitsearch.org observer in Torrance California had obtained a 10,000 CCD frames under good sky conditions.

On Tuesday evening, I got a phone call from Dave. The transit was clearly visible in both the Cambridge and the Mt. Laguna data. Thirty minutes later, I got an e-mail from Don Davies, who, in the early stages of analysis was seeing a clear transit-like signal at the expected time. We signed on Davies as a co-author, added Dave’s personal communication to the paper draft, and submitted the discovery. The paper will be showing up on astro-ph today. Here’s a link to a .pdf version:

Barbieri et al. 2007, AA, submitted (157 kb)

As for HD 17156b itself, the transit should present a number of exciting opportunities for follow-up observations. The large orbit leads to a 26-fold orbital variation in the amount of flux received from the parent star. This should drive complex weather on the surface, and indeed, even the night side of the planet should be glowing from its own radiation. Here’s a frame from Jonathan Langton’s most recent simulation of the planet which shows the night side hemisphere:

And here’s a one-orbit 1 MB animation of the surface flow patterns, glowing and roiling with their own emitted heat.

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Tonight, Sept. 30/Oct 1, is the night to follow-up to confirm whether HD 17156b can be observed in transit. Earlier this morning, I sent the following e-mail to the Transitsearch Observers list:

Hello Everyone,

I’d like to alert you to an important follow-up opportunity TONIGHT to observe HD 17156 for a possible transit by its companion planet. North American Observers are best situated for the event.

HD 17156 b has been the topic of several blog posts on oklo.org, see: [1], [2], [3], [4], [5].

Photometry taken by Jose Manuel Alemenara Villa on the Sept. 9/10 opportunity was suggestive of a possible transit with duration 169 minutes, a photometric depth of 0.007 magnitudes, and a mid-transit time of HJD ~ 2454353.614. These values are all quite close to what one would expect if HD 17156b is really transiting.

If the event observed by Alemenara Villa is due to a transit, then the next transit will be centered at HJD~2454374.83 (CE 2007 October 01 07:55 UT Monday) with the transit beginning at about 06:30 UT.

Observing should start as soon as possible this evening, and observers are encouraged to take photometry for as long as possible.

My fit to the published radial velocities predicts a transit midpoint centered at HJD 2454374.87 (CE 2007 Oct. 01 08:52 UT Monday), with a +/- 0.3d uncertainty in the time of central transit. The Alemenara Villa event sits nicely inside this window.

Thanks very much!
best regards,
Greg

It looks like much of the Southwest is clouded out, and although the skies outside here in Santa Cruz are currently cobalt blue, it’s predicted that clouds and even rain will materialize after midnight. SoCal, however, and many locations in the midwest and east look good to go. Here’s a selection of California predictions from the clear sky clock. This is a cool graphical tool for use in scheduling observations. Dark blue is good, white is bad.

In the last post, I pretty much wrote off HD 17156 b, which was the subject of last week’s transitsearch.org photometric follow-up campaign. Ron Bissinger observed the star during the latter part of the transit window, and saw no evidence of a transit. Tonny Vanmunster wrote with the news that Belgium was clouded out.

Soon after the post went up, however, Jose Manuel Almenara Villa of the Instituto de Astrofisica de Canarias posted a comment:

Hi Greg,

I observed HD17156 in the transit window. Unfortunately the night was windy, affecting the small telescope so the photometry is not so clear as we would wish. Anybody else observe?

It’s possible that I have a central transit. I can show you some plots if you want. I will try to observe again on December 3 (I think that is my next opportunity).

Regards,
Jose

On Saturday, Jose sent me his photometric plots, I should point out that he emphasized once again that the night was windy. In his plots (I’ve rewritten the labels in illustrator so that they show up better on the narrow blog-page format) the black dots are individual observations (R filter, 7 s exposures), the red dots bin 6 observations, and the blue dots bin 12 observations.

On the night before the night of the transit window, he got baseline photometry which shows considerably less scatter, and which does a nice job of showing his excellent photometric technique:

He fit a simple trapezoidal transit template to his data. The resulting fit has a duration of 169 minutes, a depth of 0.007 magnitudes, and a mid-transit time (HJD) ~ 2454353.614. These values are all quite close to what one would expect if HD 17156 b really is transiting. The possible event ends just prior to the start of Ron Bissinger’s time series.

So what to think? It’s most important to reiterate Jose’s point that the weather was not particularly good, and that a block of critical data is missing during the event itself. I myself have contracted transit fever several times in the past, and have built up sufficient immunity to refrain from getting too excited. I think a conservatively realistic assessment would say that there’s still an 11% chance that HD 17156 b transits are occurring, and that the uncertainty in the window has been narrowed down significantly. Over the long run, if transitsearch.org is going to succeed, then its important to stay cautiously optimistic. The good thing about a transit is that it repeats with clockwork regularity (barring the unlikely, but tough-luck situation where dynamically induced precession of the node induces transit seasons.) The next chance to observe HD 17156 during the transit window falls to North America on Oct. 1, where hopefully there’ll be multiple observers on the sky. We’re bad – We’re Nationwide…

To end on a heartfelt note, I think that the global collaborative efforts that go into these transitsearch campaigns have been both fun and inspiring, even when the result is the high-probability flat-line light curve. It would be exciting, though, if Jose ends up leading a discovery paper with the other participating observers as co-authors.

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It’s not looking good for transits by HD 17156 b. Ron Bissinger of Pleasanton, California obtained a block of photometric data that covered a significant chunk of the transit window. His time series lasts from JD 2454353.68 through 2452353.88, and shows no hint of an event:

His observations were taken just after the peak of the transit midpoint histogram:

No word yet on whether anyone in Europe or the eastern US were able to observe during the first half of the window. If you got data, let me know.

Also, the Gliese 176 window has opened up. If you’ve got a telescope, a CCD, and a free evening, you know what to do!

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It’s 01:58 UT Sep. 10, and HD 17156 has moved into its transit window. Hopefully photometric transit observers across Europe have clear skies. If you’re collecting data, drop us a note on the comments page!

Sep 09, 2007 Europe Satellite Map Source.

Most of California looks pretty good for catching the latter part of the transit window once it gets dark tonight. I was up on Mt. Hamilton last night, and even though it was clear, there was a strong smell of smoke in the air. Bits of gray ash from the nearby forest fires were floating down like snow, and so they couldn’t open the dome of the 36-inch.

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The odds of a HD 17156 transit are 10.9%, so its best not to get hopes up too high. Its always good to have the next candidate ready to go, and as luck has it, there’s another good one in the hopper.

Endl et al. have published a preprint describing the discovery of a Neptune-mass planet orbiting the nearby red dwarf star Gliese 176 (aka HD 285968). This discovery is further evidence in favor of the core-accretion prediction that Neptune planets should be common around low-mass stars whereas Jovian-mass planets should be relatively rare. Endl et al.’s new planet has an orbital period of 10.24 days, an a-priori transit probability of 3%, and an expected transit depth of 0.4%. This is a low-amplitude signal, but it is nevertheless accessible to many experienced amateur astronomers. The discovery paper makes no mention of a photometric transit search, making this planet a very attractive Transitsearch.org candidate. The star is located at RA 04:43, Dec +18:57, and the next transit window is centered on Sep. 15, 2007.

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The HATNet survey’s latest single, “3b” landed on the charts last week at #12. This hot (Teff~1053K) new disk shows a definite metal influence, which makes sense, given that [Fe/H] for the parent star is an Ozzy-esque +0.27. You can get a free download of the paper from the Extrasolar Planets Encyclopaedia.

The past twelve months has seen the inventory of known transiting planets more than double, as wide-field surveys such as TrES, Exo, and HATnet start to reach the full production end of their observational pipelines. As the number of planets reaches the threshold for statistical comparisons, interesting trends (or possible trends) have started to emerge.

By far the most remarkable correlation, however, has been with respect to sky location. Among the fourteen fully announced transiting planets orbiting stars with V<14, every single one is located north of the celestial equator.

Looks like there’s some opportunity down under…

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On Thursday and Friday of last week, the Dow Jones Industrial Average jumped nearly 2%. Given the soaring price of oil and the subprime mortgage crisis, many students of the financial markets were puzzled by this seeming burst of irrational exuberance.

A visit to exoplanet.eu, however, suggests that investors and speculators were placing buy orders in response to the rapid recent increase in the number of known planets. During the first two quarters of ’07, the extrasolar planet detection rate has been running more that 100% above the rate reported for the most recent full fiscal year.

When asked about the impact of the new discoveries, one metals trader was quoted, “Well, Mate, the Marketplace has been pricing in the core-accretion theory for several years now. That means we’re looking at a Z of ~0.1 for each one of these planets coming in, so that’s roughly 30 Earth masses of ore per extrasolar planet. If we use the solar gold assay, that works out to one quintillion ounces of new proven reserves for each discovery. With gold at $660, we’re starting to talk real money.”

Jocularity aside, the raft of new planet discoveries is having a noticeable impact on the correlation diagrams that can be explored at the exoplanets.eu site. One (likely statistically insignificant) curiosity is the lack of Saturn-mass planets in this year’s crop to date. At the low-mass end, Neptunes such as Gl 674b are turning up with increasing frequency, and the detection-rate for Jupiter-mass planets and above also remains strong. This dichotomy is very much in line with a key prediction of core-accretion in its simplest form. The rapid gas accretion that occurs once the planet mass reaches ~30 Earth masses should tend to make Saturn-mass planets relatively rare.

Another interesting diagram results when one plots the masses and eccentricities of the known RV-detected planets. A glance at the resulting diagram indicates that low-mass planets tend to be on more circular orbits. Could this be hinting at two populations of planets and (perhaps) two different formation mechanisms? It’s hard to tell. Much of the effect comes from the fact that low-mass planets need to have short periods in order to be detectable. Short-period planets, in turn, are far more affected by tidal circularization of the orbits. The plot is also reflecting the still-mysterious (but well known) shortage of high-mass hot Jupiters.

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The Transitsearch collaboration has been active since 2001, and has fallen somewhat short of success. When reporters from the likes of space dot com call, they always want to know, “How many planets have you guys discovered?”

“Zero.”

The project has, however, been of some value. It’s helped publicize the fact that small telescopes can be of remarkable utility in carrying out photometric follow-up observerations. The basic strategy of checking Doppler-detected planets at the predicted transit times has proved its worth for the Swiss with the transits of Gl 436 b. But the fact is unavoidable. Transitsearch needs to step up several levels if it’s going to compete.

I’m thus in the midst of implementing a major overhaul of the site resources. To get away from the tonight-we’re-gonna-html like it’s 1999 feel, I’ve given the website a new look. Check it out.

Not everything is in place yet, but the server that hosts the systemic backend is now also keeping the candidates tables up to date. The ephemerides are incrementally updated every ten minutes, and so the transit window column now has a much finer resolution. It gives a quick overview of which planets are transiting (or potentially transiting) right now.

A Transitsearch observer seeking to get a first detection of a transiting extrasolar planet still starts at a major disadvantage. The radial velocity survey teams all have in-house photometric observers who monitor their candidate stars prior to announcement, and they thus have first dibs on the stars that are most likely to pan out with transits. This vertically integrated strategy will continue to monopolize the detection of hot Jupiters like HD 209458b, HD 149026b, and HD 189733b that transit bright stars.

Ideally, we need to get an open-source dedicated radial velocity observatory up and running to really feed transitsearch and the systemic backend, and we are looking at avenues to make this happen. In the interim, however, we can tap the growing fit database on the systemic backend for suitable candidate planets that have not yet been published in the literature. There are a number of planetary candidates that have low false-alarm probabilities and are dynamically stable (see also here).

To get things started, I’ve taken two candidate planets — HD 19994 c and HD 216770 c — from the probable planet discoveries page on the backend wiki, and reproduced the fits on the downloadable console. With a fit in hand, it’s straightforward to use the bootstrap utility to compute errors on the orbital parameters, and to produce transit ephemerides and observing windows. These first two candidates are listed in a table on the Transitsearch website, and we’ll be adding many more potential planets in the near future:

HD 216770 “c”, for example, has a period of 12.456 +/- 0.019 days, and Msin(i)~60 Earth Masses. If it exists, it has a 3.1% chance of transiting, and would likely produce a transit depth of a bit more than 1%. The radial velocity data set for HD 216770 is several years old, and so the transit window has, frustratingly, widened to about 8 days.

Let’s try to identify additional candidates that are (1) dynamically stable, (2) have Msin(i)>0.05 Jupiter Masses, (3) F-test statistics below 0.2, and (4) periods less than 100 days. If you find them, add them to the backend wiki, or as comments to this post.

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Gl 436 b orbits its parent star in a short 2.64 days, and the discovery of transits indicates that its physical properties are quite similar to Neptune. The theoretical expectation is thus completely clear cut. “That orbit is circular, Son. Tidal dissipation has long since damped out that eccentricity.”

The data, however, stubbornly insist otherwise. When I do a one-planet fit to the radial velocities (incorporating the constraint on the mean anomaly imposed by Gillon et al.’s observation of the transit midpoint) then the distribution of bootstrap fits indicates e~0.13 +/- 0.03:

[Note: Stefano and Eugenio have been cranking away on the downloadable console code base, and the current beta-test version on the backend now contains a slew of new features, including a revved-up Hermite integrator and the ability to incorporate transit timing observations into the orbital fits. The user interface has been completely overhauled in order to maintain usability with the rapidly expanding feature set. We’ll be putting up some posts very soon that demo all this bling. In the interim, though, I definitely recommend downloading a copy and taking it for a test-drive.]

The latest console version.

In this post from last week, I looked at the possibility that gl 436 b’s eccentricity is being maintained by as-yet unpublished planets. There’s a hint of a long-term trend in the data that indicates a large and distant companion.

The lowest chi-square fit to th gj437_M07K data set (by user Schneidi) reduces the magnitude of the long-term trend by using a pair of planets on 53 and 399 day orbits.

In Schneidi’s fit, the bulk of the perturbation on planet b is provided by the 53-day plant “c” which also has close to a Neptune mass. In last week’s post, I looked at this model in gory detail. If the 53-day planet exists, and if its orbital plane is aligned for transits, then the transit will occur around June 7th.

For two planets like Gl 436 b and c, which aren’t in mean-motion resonance, and which aren’t on crossing orbits, the long-term evolution of the orbits is well-described by an approximation worked out by Laplace and Lagrange in the 1770s. In the Laplace-Lagrange theory, the gravitational interactions between a set of planets are assumed to be effective over a “secular” timescale that is much longer than the orbital periods of the planets themselves. The planets can thus be treated as flexible elliptical wires of varying mass density (highest near apoastron where the planets spend more time, and lowest near periastron where the least time is spent). The planets are able to trade eccentricity back and forth while keeping their semi-major axes fixed (orbital angular momentum is exchanged, but not orbital energy).

Last week, I was wondering whether the secular interchange of eccentricity could provide a mechanism for b to offload angular momentum as it tidally dissipates its orbital energy. If such a mechanism were effective, then it might explain why b’s orbit is still eccentric.

To look at this, I used a “double averaging” approximation to do a long-term numerical evolution of the 2-planet system in the presence of tidal damping. With this approach, one uses the Laplace-Lagrange theory to advance the system forward over a secular timestep of hundreds to thousands of years. After each secular timestep, one then applies tidal dissipation (modify semi-major axis and eccentricity so as to decrease the energy of planet b while conserving its angular momentum). Then one takes another secular timestep, etc. This approach should provide a reasonable picture of the orbital evolution so long as the secular time scale (thousands of years) is much shorter than the tidal evolution time scale (millions of years or more).

The answer is immediately clear. The presence of a 53-day planet “c” doesn’t stave off tidal circularization. In the graph above, I’ve assumed a Neptune-like tidal Q of 10,000 for b. The high-frequency secular exchange of angular momentum is of no use for maintaining b’s eccentricity. The orbit is circularized on an e-folding timescale of ~10 million years — much shorter than the current age of the star.

Guess I’m just not hip to where b’s scoring its e.

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First, I squandered literally years of opportunity to coordinate a photometric follow-up transit search on Gl 436. Then I managed to incorrectly report the circumstances of the detection on the initial version of this (now corrected) oklo post! Naturally, I’m feeling sheepish, and my situation bears a distant echo to that of John Herschel (son of William), who was partly to blame for the inadequate coordination of an observational follow-up to John Couch Adams’ predictions of Neptune’s location.

Following the stunning news from the Continent of LeVerrier’s prediction and Galle’s successful detection of Neptune, Herschel likely realized at once that Neptune’s discovery would have gone to England had only he pressed Adams’ case more assiduously. In an October 1, 1846 letter to the London Athenaeum, Herschel hems and haws in a somewhat disingenuous effort to wriggle out of the uncomfortable situation that he had put himself in.

“The remarkable calculations of M. Le Verrier – which have pointed out, as now appears, nearly the true situation of the new planet, by resolving the inverse problem of the perturbations – if uncorroborated by repetition of the numerical calculations by another hand, or by independent investigation from another quarter, would hardly justify so strong an assurance as that conveyed by my expression above alluded to. But it was known to me, at that time, (I will take the liberty to cite the Astronomer Royal as my authority) that a similar investigation had been independently entered into, and a conclusion as to the situation of the new planet very nearly coincident with M. Le Verrier’s arrived at (in entire ignorance of his conclusions), by a young Cambridge mathematician, Mr. Adams; – who will, I hope, pardon this mention of his name.”

Herschel also wrote urgently to his friend William Lassell, a wealthy beer brewer from Liverpool, and a skilled observer who owned a fine 24-inch reflector. Herschel exhorted him to partially salvage the situation for himself and for Britain through a search for “satellites with all possible expedition!!”

Lassell began observing Neptune immediately, and within a week had spotted what was later confirmed to be Neptune’s satellite Triton. This, however, did little to assuage the court of British public opinion, and Challis, Airy, and Herschel were savaged for their inaction. “Oh, curse their narcotic Souls!” wrote Adam Sedgwick, professor of Geology at Trinity College.

[I culled these anecdotes from my favorite book on the topic of Neptune, Vulcan, LeVerrier and 19th-century dynamical astronomy; “In Search of Planet Vulcan — The Ghost in Newton’s Clockwork Universe” by Richard Baum and William Sheehan.]

Unfortunately, even with the exertion of all possible expedition, the detection of satellites orbiting Gl 436 b is a long shot. Large moons orbiting a planet only 0.02 AU from the parent star are almost certainly dynamically unstable (as shown here), and would, in any case, require exquisite photometry to detect. But one can, however, investigate the possibility that Gl 436 b might point the way toward other detectable planets in the system.

The first clue that Gl 436 might harbor more than one planet comes from planet b’s considerable, e~0.16, eccentricity. It’s surprising to find a P=2.644 day planet on a non-circular orbit. Given that its tidal quality factor, Q, is likely similar to Neptune’s, it should have circularized a long time ago — unless there’s a source of ongoing gravitational perturbation.

Gl 436 b’s high eccentricity means that, like Jupiter’s moon Io, it’s experiencing a lot of tidal heating. It’s internal luminosity is likely of order 10^20 Watts, which is in the rough ballpark of the amount of energy that the planet intercepts from the red dwarf parent star. Another interesting consequence of the non-zero eccentricity is that b will have a pseudo-synchronous spin period. That is, tidal forces will have forced the planet into a rotational period of 2.29 days, which allows it to optimally show one face to the star during periastron passage when the tidal forces are strongest. Jonathan Langton has done a simulation of the surface flow pattern (assuming a water-vapor atmosphere). The following 1.1MB animations (“eastern” view, and “western” view) trace two full orbits in the planet’s frame, and show the slow synodic drift of the baking daylit hemisphere.

If there’s a perturbing companion to Gl 436 b, then it’s a reasonable guess that it lies in roughly the same orbital plane, meaning that there’s a non-negligible chance of transit. It would certainly be nice if such a transit could be predicted in advance…

The first task is to look at whether the published radial velocity data set for Gl 436 gives any hint of additional planets. Going to the “Real Star” catalog on the systemic backend, and calling up the “gj436_M07K” dataset shows a wide variety of fits that have been submitted by systemic users over the past nine months:

Unlike the case of Gl 581c, there’s no particularly compelling evidence for a second planet. In sifting through the various fits that have been submitted, one finds that a second planet with a mass similar to Uranus and a period of 53 days is probably the most likely candidate perturber, and using the console, I find an unpublishably high false-alarm probability of 49% for a planet “c” with these properties. (The discussion boards on the systemic backend indicate that the systemic users have also arrived at this conclusion.)

On the other hand, however, a coin-flip isn’t half-bad odds, and what better low-stakes venue than a blog for an analysis? Let’s go ahead and assume that the 53-day candidate is really there.

At the current time, the console software isn’t configured to incorporate transit information into radial velocity fits. In particular, when one has a transit, one gets (1) an excellent determination of the period, and (2) an accurate ephemeris of the moment when the transiting planet and the parent star both lie on the line of sight to the Earth. Condition (2) provides a constraint on the fit that replaces the transiting planet’s Mean Anomaly as a free parameter. I have a Fortran code (that I wrote for an analysis of the orbit of HD 209458b) that handles this situation, and so I can carry out a self-consistent two-planet fit that takes advantage of the transit ephemeris for b reported in the Gillon et al. paper. This 2-planet fit (based on the 53-day Uranus suggested by the fits submitted to the systemic backend) has a chi-square statistic of 3.09, and an RMS scatter of 3.91 m/s. The orbital parameters of the planets are: P_b=2.64385d, P_c=53.57724d, e_b=0.1375, e_c=0.2281, omega_b= 347.999 deg, omega_c=185.146 deg, M_b=0.0697 M_jup, and M_c=0.0417 M_jup. The Mean Anomaly of the putative planet “c” at JD 2451552.077 is 100.69 degrees.

One would certainly prefer to see a beefier perturber for Gl 436 b. When I compute the Laplace-Lagrange 2nd-order secular theory for the above system (including the effects of general relativistic precession) I find that b’s eccentricity cycles between e_min=0.135 and e_max=0.160 with a period of 13,000 years. This is much shorter than the time scale for orbital circularization, but it’s not immediately clear to me whether the secular perturbations from c would be able to maintain such a large eccentricity for b over billions of years. Does anyone know the answer offhand? That is, if b and c both formed with sizable eccentricities, would the secular interaction prevent circularization by providing c with a mechanism to offload angular momentum?

In any case, if c is for real, and if its orbital plane is properly aligned for central transits, then they will occur on (all times UT):

ingress JD: 2454152.02 2007, Feb 20, 12:34
egress JD: 2454152.19 2007, Feb 20, 16:29

ingress JD: 2454205.60 2007, April 15, 2:24
egress JD: 2454205.77 2007, April 15, 6:24

ingress JD: 2454259.18 2007, June 7, 16:14
egress JD: 2454259.34 2007, June 7, 20:14

ingress JD: 2454312.75 2007, July 31 06:04
egress JD: 2454312.92 2007, July 31 10:04

I’m now doing a more detailed analysis to see if c can maintain the observed eccentricity of b over the long term. If it’s a go, then I’ll run a bootstrap calculation to determine the probable error on the above predictions. It might be useful, however, to mark down June 6th-8th on the calendar.

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I’m still astounded by the dramatic detection of the transit of Gl 436b, and I’m working on some posts that sort through the scientific results and implications that this discovery is generating.

GJ 436b was found using the same basic strategy that led to the detection of the transits of HD 209458b, HD 149026b, and HD 189733b. First, the planet is located with the radial velocity technique. Doppler velocities, of course, do not give the inclination of the planetary orbit, but they do give a prediction of when transits would occur if the line of sight to the system lies within a small enough angle of the planet’s orbital plane.

Short-period planets have higher a-priori chances of being observed in transit (a 12% probability is typical for a hot Jupiter on a short-period orbit) and so in general, most of the RV-detected planets with orbits of less than a week are checked photometrically for transits by members of the discovery team before the planet is publicly announced. The discovery teams found the transits of HD 209458b, HD 1409026b, and HD 189733b. Dramatically not so, however, with Gl 436b.

Note: In the initial version of this post, I jumped to some incorrect conclusions about how the Gl 436 discovery was made. This article on swissinfo caused me to infer that the initial April 2nd detection of the transit was a postcard-perfect story of an independent small-observatory follow-up of the variety encouraged by transitsearch.org. It turns out, however, that the OFXB telescope is tightly linked to the Geneva program. The Gl 436 detection was made in the course of an ongoing systematic survey of the known planet-bearing M-stars and K-stars, and of as-yet unannounced new candidates discovered by HARPS and SOPHIE. Michael Gillon, lead author on the Gl 436 paper, and the lead scientist for the photometric follow-up effort was kind enough to correct my facts.

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Literally seconds after I pressed the submit button on this afternoon’s TrES-3 post, the telephone rang. Eugenio.

“Did you see astro-ph? the Swiss have a transiting Neptune around Gl 436!”

I stayed up almost all night last night finishing my NASA PGG proposal, and so the cogs in my brain were turning rather slowly.

“Gl 436?” I asked, confused, “That doesn’t sound right. You’re sure it’s not Gl 674?”

But he was right. It is Gl 436 b that’s transiting, and this is easily the biggest planet-related discovery so far this year. New results from the Swiss team have been coming so thick and fast that it’s hard to even keep them all straight. Let me be the first to offer my heartfelt — and let’s admit it, envious — congratulations.

First, the hard facts that we’ve all been waiting for. The planet has a mass of 23 Earth-masses and an orbital period of 2.64385 days. It orbits a red dwarf star 33 light years away. The temperature on the planet is somewhere in the oven-cleaning neighborhood of 600K (327K, 620F). No habitability news stories on CNN for this fine fellow. The transit depth is a healthy 0.6%, which implies that the the planet’s radius is ~25,000 km. That’s four times that of the Earth, and essentially identical to the 24,764 km radius of Neptune.

The Neptune-like radius indicates that the planet is largely composed of water. This means that it formed beyond the snow-line in Gl 436’s protoplanetary disk and then migrated inward to its present location.

Remarkably, Gl 436b has been known for over two years. In the original discovery paper (on which yours truly was a co-author) there are a number of photometric observations of the star taken over a long period. When the data are folded at the orbital period of the planet, no transit was visible. It looks like systematic effects associated with the analysis of long-baseline photometry may have resulted in the baby being thrown out with the bathwater. In retrospect, that clump of points just to the right of the predicted transit interval may actually be the transit.

Lesson learned. Even if folded photometry shows no sign of a transit, it’s important to follow up with a time-series that covers an entire predicted transit window. This object is within reach of dozens of amateur observers, and it has been sitting in the transitsearch.org candidates table since 2004. Had I pushed for observations of this planet in the same way that we pushed for Gl 581 b and Gl 876 b and c, then we would have gotten it. But to the Victor belongs the prize, and I’m thrilled that this long-awaited Neptune-mass transiting planet has turned up.

The opportunities for follow-up on this discovery are enormous. First, the eccentricity of Gl 436 b appears to be alarmingly high. Single planet fits to the radial velocity data indicate e=0.16. The orbit, however, should have been tidally circularized quite a while ago. It’s likely that there’s additional perturbing bodies in the system. To get a discussion going, look at this fit by user flanker on the systemic backend. There’s an urgent need to start fitting this system to get multiple-planet fits that have (1) low chi-squares and (2) low F-test statistics for planets beyond the known transiting planet “b”. If you find a good fit, upload it to the backend.

Gl 436 should be placed under constant photometric surveillance. If you’re capable of doing sub-1% photometry, please get out there on the sky whenever the night is clear and Gl 436 is at low air mass. If there are additional planets in the system, then it’s completely possible that they are transiting as well.

In addition, it’s very important to collect the best possible time-series data for future Gl 436 transits. By timing when the transits occur, it will be possible to derive the orbital elements of significant additional perturbing bodies. This endeavor is within the reach of careful amateur and small-telescope observers.

And then there’s the Rossiter effect:

Folding the downloadable systemic console‘s gj436_M07K dataset at 2.64385 days shows two points that have (possibly) had their velocities altered as a result of being taken during transit:

And finally, no more major discoveries this week, please! I’ve got to finish my Kepler Participating Scientist proposal to NASA, which is due on Friday.

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Looks like this week is good for at least one new transiting planet. Francis O’Donovan (Caltech) and his collaborators have just announced the discovery of TrES-3 and it’s a hot property. The planet is nearly twice Jupiter’s mass, and has a radius about 30% larger than Jupiter. The most remarkable characteristic of the planet is its extremely short orbital period. Thirty one hours, twenty minutes and fifty five seconds. I’ve learned that shady Glenngary Glen Ross-type operators have begun promoting real estate on this planet. Don’t get suckered in! TrES-3 is undergoing orbital decay as a result of tidal evolution, and sooner or later it’s going to merge with its parent star.

TrES-3 exhibits a nice symmetry. The radius of the central star is 16.5% of the planet’s orbital radius, and the planet’s radius is 16.5% the radius of the star. The transit itself is practically a grazing transit, which leads to a bell-shaped light curve. As soon as this post goes up, I’ll add the ephemerides to the transitsearch.org candidates page. With a whopping 2.5% transit depth and a transit just about every day, this is a great starter world for Northern Hemisphere observers who want to bag their first extrasolar planet.

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The CoRoT satellite fired off its first planetary dispatch today. Here’s a link to the CNES press release. It now appears that CoRoT has the photometric sensitivity to eventually reach down to planets of approximately Earth-size, and in the immediate near future, the mission stands a good chance at bagging the first discovery of a transiting sub-Neptune mass planet.

The prospect of seeing a 10-Earth mass planet in transit has everybody all worked up, and for good reason. The moment a transiting example of a planet like Gl 581 b (or c) turns up, then we’ll know whether it formed in-situ (in which case it’ll be small and thus made of rock and iron) or whether it migrated in from colder regions of the protoplanetary disk (in which case it’ll be relatively large and thus made mostly of water).

Here’s a slightly reworked version of the light curve accompanying the press release.

So far, there doesn’t seem to be such a thing as an “average” extrasolar planet. Nearly every new world that turns up has at least one unusual, completely unexpected characteristic. This week so far has been no exception. Hat-P-2b sports an extraordinarily high orbital eccentricity. X0-2b appears to have a very large complement of heavy elements, which gives it a comparatively high density and a comparatively small radius. CoRoT-Exo-1b is distinguished by its enormous size.

The CoRoT press release quotes a radius of 1.68 Jupiter radii for their 1.3 Jupiter-mass planet. The planet’s orbital period is short (only 1.5 days) and its surface temperature is high — probably ~1500-1800K — but its still quite a bit larger than the 1.45 Jupiter-radius that our models predict. A powerful internal heat source seems to be necessary to get the planet up to the large observed radius.

Or alternatively, the star may be somewhat smaller in size than the best-fit value. It’s notoriously difficult to get accurate radii for stars that don’t have parallax measurements.

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Man, when it rains it pours! This week’s big planet news is the announcement of a second transiting planet from the HATNet project.

HAT-P-2b orbits the bright nearby star HD 147506, which means that there will be all sorts of opportunities for detailed follow-up. For those who want to get in on the action, the midpoint of the next transit will occur at 3 PM on May 3rd (UT). The planet’s orbital eccentricity is a whopping e=0.5, the planetary mass is high (8 Jupiter masses) and the orbital period is a relatively long — for a transiting planet — 5.63 days. In fact, just about the only aspect of this world that isn’t remarkable is its radius. Preliminary indications are that the planet is 10-20% larger than Jupiter, exactly as theoretical models predict.

Had HAT-P-2b turned up on the scene with a large radius a la HD 209458b, or with a small radius (such as that observed for HD 149026b), then it would have signaled that something is seriously awry with our understanding of planetary structure. The interior of an 8-Jupiter mass planet is dominated by electron degeneracy pressure, which leaves little room for large variations in the planet’s overall size. It doesn’t matter if there’s tidal heating. It doesn’t matter if there’s a 50-Earth mass core. The radius of an 8-Jupiter mass planet should maintain a zen-like lack of perturbation in the face of all that optional bling that causes lesser planets to run off track. It’s thus very reassuring to see that HAT-P-2b is meeting its radial obligation.

The weather on this planet is going to provide an amazing opportunity for Spitzer. Even as I write this, our processors are roaring to the tune of a full-scale hydrodynamical simulation of the flow patterns on the surface.

UPDATE: I put this post up, went to bed, and woke up to news of yet another transiting planet, XO-2b. See the Extrasolar Planets Encyclopaedia, and the astro-ph preprint. In this case, the planet, which has a mass of 0.6 Jupiter masses and an orbital period of 2.6 days, seems to have a sub-Jovian radius, suggesting a 20-40 Earth mass core of heavy elements. A heavy burden of heavy elements in this case is not too surprising, given that the V=11 K0V parent star has a metallicity nearly three times that of the Sun.

I see that transitsearch.org veterans Ron Bissinger, Mike Fleenor, Bruce Gary, and Tonny Vanmunster are all on the author list of discoverers, Congratulations, guys!

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No word yet on whether anyone flew down to Tahiti last Monday to observe GJ 674.

For Northern Hemisphere observers who want some action closer to home, there’s a cool opportunity to check HD 80606b for transits starting essentially right now.

HD 80606b is a favorite here at oklo.org (see e.g. here). The planet went through periastron passage last week, and is now just on the verge of inferior conjunction with the Earth. The a-priori geometric odds of observing a transit are 1.6%. In 2005, transitsearch.org ran a campaign on the star, and while some useful photometry was obtained, the entire transit window was not covered. If HD 80606b happens to show central transits, then the duration of the event will be ~18 hours and the photometric depth will be ~1.4%. At any one location on Earth, one would be able to observe only the ingress or the egress.

The best fit to the published radial velocity data indicates a mid-transit time of 11:07 April 17, 2007 UT. This midpoint is uncertain by roughly half a day, which means that observations starting now and ending on April 18th will be useful.

No word yet on whether that newly discovered 11 Earth-mass (and possibly rocky) planet orbiting GJ 674 is transiting or not.

The next opportunity is coming up on April 11th 13:17 UT. Given GJ 674’s location in the sky at RA 17:29, Dec -46 54, the South Pacific has by far the best view of the next event. Anyone willing to jump on the next plane to Tahiti with a Meade LX200 and an SBIG ST-7 in their checked baggage?

The current Tahitian weather forecast for the transit window calls for scattered clouds with a 20% chance of rain:

Not exactly the best conditions for obtaining 0.5% photometry, but not completely hopeless, either. I’m interpreting the current forecast as indicating there’s a 1/3rd chance that the weather will be cooperative. This means that if you fly to French Polynesia and set up your telescope in the hotel parking lot, you’ve got a 1 in 60 shot at walking away with the biggest exoplanet discovery of the year.

Even at 60:1 odds, there’s a case to be made that the trip is a good investment. According to the CoRoT website, the CoRoT satellite will detect “a few tens” of large rocky planets for a price tag of roughly 100 Million USD. That’s ~3 million per large rocky transiting planet.

A trip to Tahiti tomorrow, on the other hand, costs out at under 4K, and involves a more clement destination than Baikonur. In fact, when I dialed up a spur-of-the-moment expedition on expedia, I was informed that the price had just gone down:

The expectation value for the Tahiti mission, therefore, is a comparative bargain at $240,000 per transiting planet.

Assuming that you can show up at LAX by ~10pm this evening (Monday) a direct flight on Air Tahiti Nui gets you in to Papeete at 5:10 Tuesday morning. There’s plenty of time to grab a taxi to the luxe Le Meridien Tahiti, where you can take a refreshing nap in your “over water bungalow” set on one of Tahiti’s few sand beaches. Follow your late afternoon dip in the pool with dinner at Restaurant Le Carre, with its trendy atmosphere and refined A la carte dishes. After dinner, there’s still plenty of time for drinks at the L’Astrolabe Bar, where they’ll likely pick up your tab while you regale the hip-yet-distinguished clientele with astronomical bon mots. Indeed, you’ll likely have an admiring circle of new-found friends as you set up your scope in the parking lot and expertly obtain darks, flats, and baseline photometry, prior to observing well into astronomical twilight.

It’ll then be time to retire to your bungalow for some well-deserved rest. You’ll have the rest of the week to analyze your data and hopefully send that discovery e-mail to the IAU. It’ll be impossible for anyone on Earth to scoop your discovery until the next transit window on April 16th, at which point you’ll be flying home (having upgraded to first class for the long-haul flight back to LA).

What’s that you say? No money for your trip? No Problem. As soon as the market opens this morning, just write a few at-the-money April calls on a precariously high-flying tech stock to raise the necessary cash.

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Word up! Chalk a jet-fresh Neptune on the boards — the Swiss’ve done it again.

The red dwarf GJ 674 lies 14.67 light years away. Minus 49 Dec. Only 53 known stars are closer to the Sun, and at V=9.382, GJ 674 is slightly more than twice as bright in the optical as its far more famous cousin GJ 876. With ~35% of the Sun’s mass, it’s packing more heat as well.

According to the Bonfils et al. discovery preprint posted to astro-ph yesterday, GJ 674 is accompanied by a sub-Neptune mass planet on a 4.6938 day orbit. Bucking the recent trends, the paper doesn’t contain a tabulation of the radial velocities. Eugenio, however, made dextrous use of the Dexter to scrape them off the figures, and they’re now safely packaged into the downloadable Systemic Console. The star has also been added to the “Real Stars” catalog on the Systemic Backend. The internal errors on the velocities are mostly below 1 m/s, which is impressive, given that each data point is based on a 15-minute integration of a rather dim star.

This discovery is a exciting for several reasons. Most immediate, is the fact that the planet does not yet seem to have been fully followed up photometrically to check for transits. At first glance, such an effort might appear to be hampered by the fact that the star is young enough to show significant photometric variability in synch with its 35-day rotation period. A central transit, however, would have a duration of only ~80 minutes — much shorter than starspot-induced variations — and would generate a clearly detectable dip of at least ~0.5% photometric depth.

Transitsearch.org has observers in Australia, South Africa, and South America, and so I’m hoping that they can quickly take advantage of this opportunity. The next transit window is centered about 15 hours from now, on April 06, 2007 at 20:38 UT. Here’s looking at you, Perth. The ephemeris table showing all the upcoming opportunities is at transitsearch.org. Based on a radius estimate for the star of 0.35 solar radii, the geometric transit probability is ~5.0%. Roll that twenty-sided die.

It’s fair to say that the next major discovery in the exoplanet game will likely be the detection of transits of a short-period Neptune-mass planet. Quite a few players are scrambling to be the first in the door. If it isn’t done from the ground during the next 6-months, then it’s likely that CoRoT will take the prize.

There’s a large difference in radius between sub-Neptune-mass planets made from rock and iron and sub-Neptunes composed mostly of water:

A Neptune transiting one of the brightest M-dwarfs in the sky would be a huge big deal. Hundreds of citations, Dude. Even if there’s no transit, this planet will likely be an excellent candidate for observation in the long-wavelength Spitzer bands, and fortunately there’s one more GO cycle before that cryogen runs out.

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Most hot Jupiters have orbital eccentricities near zero because the tidal forces exerted on them by their parent stars are strong enough to rapidly circularize their orbits. Any planet whose orbit has been circularized should also be spin-synchronous, and so like our Moon with respect to the Earth, it should turn on its axis once every trip around the star. Synchronicity lends each hot Jupiter a permanent day and night side. This likely imparts a profound effect on both the planetary weather, and on the brightness of the planet when viewed in the infrared at different orbital phases.

All of the planets observed so far with the Spitzer Space telescope have nearly circular orbits, and hence are in (or are very near) the spin-synchronous state. We’re waiting to hear the results of our Spitzer GO-4 application to observe the highly eccentric planet HD 80606b, during an upcoming ‘606 day. If our observing proposal gets a thumbs-up, it’ll dramatically broaden the range of conditions under which planets have been observed. Very shortly, I’ll be posting the results of calculations that Jonathan Langton and I have been doing which predict what the light curve of HD 80606 should look like during the periastron passage in the various Spitzer bands. Here’s a sneak preview of how the temperature distribution on the planet might evolve over a 36-hour period as seen from a direction consistent with our line of sight from the Earth:

In looking over the latest officially published additions to the catalog of extrasolar planets, I noticed that there’s a very interesting object — HD 118203b — that straddles the extremes of the circular hot Jupiters and the ultra-eccentric HD 80606b. This planet was discovered in 2005 by the Swiss Team, has an orbital period of 6.13 days, a mass at least twice that of Jupiter, and a well-determined eccentricity, e=0.3. HD 118203b therefore won’t be spin-synchronous. Rather, as is also the case with HD 80606b (see the diagram here), it’ll have been forced into a state of pseudo-synchronous rotation, in which it does its best to keep one face toward the star during the periastron passage. Its day should be 64.8% as long as its year:

Higher resolution .eps version here.

Which raises a rather interesting question: Why is HD 118203’s eccentricity so high?

Assuming that the planet has a similar structure to Jupiter, the equations of tidal dissipation (see here for a discussion) indicate that the planet’s orbit should circularize in a mere 10-20 million years. This time scale is surprisingly short because the parent star is a subgiant with a radius ~1.5 times larger than the Sun. Something must be exerting a very strong perturbation to keep this planet’s e up.

In their discovery paper, Da Silva et al remark that the residuals around the best 1-planet Keplerian fit to the data are very large. It’s quite straightforward to verify this with the downloadable Systemic console (try it!) Da Silva and company were able to improve their fit by including a linear drift of 49.7 meters per second with their one-planet model. This corresponds to adding the effect of an outer planet that has been observed for only a small part of a single orbit. (The 43 published velocities span a period of 1.1 years.) They speculate that an outer as-yet-uncharacterized planet provides the gravitational perturbation that maintains the high eccentricity for the inner planet.

Last year, Fred Adams and I wrote a computer code (see these papers 1, 2) that includes the effect of general relativistic corrections on long-term planet-planet gravitational interactions. It’s easy to use this program to calculate what the long-term influences of various companion planets would have on HD 118203 b’s eccentricity. I ran a few trial cases, and quickly found that the interactions produced by companions that also provide the observed linear drift in the radial velocities don’t seem to be strong enough to explain HD 118203b’s high eccentricity. Could there be another explanation?

This is the sort of situation where the collaborative systemic back-end is extremely useful. I had a look at the stable fits that have been submitted so far for HD 118203. The best stable, self-consistent fit was uploaded back in October by the user Flanker, and has a reduced chi-square statistic of 1.96:

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This fit might point in an interesting direction for some further inquiry. Instead of using a linear trend to soak up the residuals to the one-planet fit, Flanker added two additional planets. One of the planets has a mass of 0.3 Jupiter masses and is orbiting with a period of 15 days. Its periapse is nearly aligned with the periapse of the inner planet. The resulting short-period secular interaction may well be strong enough to keep the eccentricity of the innermost planet high in the face of tidal dissipation. Flanker’s model also contains an outer planet with an orbital period of 244 days and a minimum mass 0.6 times that of Jupiter.

I think it’s worthwhile to explore additional models for this system that contain planets with short enough periods to intereact strongly with the 6.13 day planet. If the perturbing body has a relatively short-period orbit, then its presence will not be hard to verify with additional radial velocity observations of the star. And also, if Spitzer’s cryogen holds out, HD 118203b might be a very interesting target for a full-phase campaign.

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If you’re a new visitor to the site, welcome aboard! Yesterday’s post talks about the systemic collaboration, and gives an overview of how you can participate.

The interpretation of radial velocity data sets is confounded by the existence of many different model planetary systems that all do a good job of fitting the data from a given star. If you really want to know whether a particular fit is the correct interpretation of the system, then you need to wait for (or make) more observations to see if your fit’s predicted radial velocity curve is confirmed.

For a real planetary system orbiting a real star, it can take years for enough confirming observations to be made, and so it’s useful to have as many criteria as possible for evaluating whether a particular fit is a contender. Orbital stability provides one such criterion.

On the backend, there are many orbital models that have been submitted that give excellent fits to the given data sets. For example, the four configurations shown in the picture just below are all acceptable models for the 14 Her system.

One immediately notices that these orbital configurations look “crowded”. The orbits make close approaches and sometimes even cross. If we let these model systems run forward in time, then we find that the mutual gravitational pulls between the planets lead to catastrophe within a few decades or less. Instead of behaving in an orderly fashion, the orbits execute motions like this:

which lead inevitably to collisions and ejections. While it’s theoretically possible that we happen to be observing a particular system just before it experiences disaster, Occams razor strongly suggests that wildly unstable fits are likely spurious. We can safely exclude any configuration that lasts for only a tiny fraction of the stellar ages (which are generally in the 2-10 billion year range).

Participants in the systemic collaboration can evaluate the stability of their models by using the “check long-term stability” function on the console. Stefano has also recently implemented a robot that crawls through the systems residing in the backend database and integrates all of the submitted fits. So far, it has sorted out which systems are unstable on timescales of less than a century, and as time goes on, it’s pushing the integration times to longer horizons. It turns out that a 100-year integration can catch a majority of the systems that eventually go unstable. After that, we expect roughly equal numbers of systems to be lost in each factor-of-ten increase of integration time.

Although we don’t expect to see orbital instabilities play out on our watch, it’s nevertheless likely that planet-planet interactions and their associated instabilities have played an important past role in sculpting the systems that we now observe. For example, Eric Ford and his collaborators have published a highly plausible theory for the formation of the Upsilon Andromedae planetary system that involves a dramatic instability. In their scenario, the system starts out with four planets, and eventually ejects one of them. The outer two survivors are left stunned and reeling, and the dynamical imprint of the disaster survives to the present day. They’ve made an engaging animation (available here) that shows the action blow-by-blow.

This brings up a relevant question. If orbital instability exists among the extrasolar planets, might our own solar system eventually go unstable? Is it possible that Earth will find itself getting dramatically tossed around the solar system in the manner that was experienced by the unfortunate Upsilon Andromedae E?

The question isn’t new, and the stability of the solar system has been at the forefront of interest for the last 350 years. It was first tackled by Newton, who wanted to understand how the orbits of the Jupiter and Saturn would behave over long periods if their mutual interactions were taken into account. Newton put a lot of effort into the problem, and eventually decided that:

To consider simultaneously all these causes of motion, and to define these motions by exact laws admitting of easy calculation exceeds, if I am not mistaken, the force of any human mind.

Newton’s fame, and the fact that he’d written off the problem as too difficult, was a big motivation for succeeding generations of mathematicians. Pierre Simon de Laplace eventually solved the problem of the motions of Jupiter and Saturn, and fully explained their orbits to the accuracy that could be observed in the late 1700s. In Laplace’s model, the solar system is completely stable, and the inherent predictability of his planetary motions contributed to the concept of a rational determinism, and the idea of a clockwork universe.

During its first three hundred years, the problem of the stability of the solar system was attacked using pen and paper. In the past few decades, however, the advent of computers has provided a powerful new tool. We can now make accurate simulations of the trajectories of the planets through space, and look in detail at the solar system’s possible futures. By the 1980s, when hardware and algorithms had progressed to the point were it was possible to integrate the planets millions of years forward into the future, it was found that the solar system is chaotic in a sense originally envisioned by Poincaré. If the position of a planet, the Earth say, is given a tiny change in the computer, then as millions of years elapse, this slight perturbation grows erratically larger. If Earth is displaced in its orbit by a centimeter, then, after several million years, Earth will likely be located somewhere within 2 centimeters of where it would have been had it been given no push at all. After several million years, the degree of uncertainty doubles again, this time to 4 centimeters.

Worrying about such tiny buildups of uncertainty in the position of Earth on its orbit sounds utterly absurd. Nevertheless, like interest compounding in a forgotten account, the accumulation of uncertainty is guaranteed to eventually become significant. After a hundred million years, which is much less than the 4.5 billion year age of the solar system, the position of Earth in its orbit becomes completely impossible to predict. For times 100 million years in the future, we have no firm knowledge of Earth’s trajectory. We have no idea whether January 1, 100,000,000 AD will occur in the winter or in the summer, or even whether Earth will be orbiting the Sun at all.

Poincaré’s great insight was that the realistic physical description of non-trivial systems can involve what we now call chaotic behavior. The weather is an excellent example. Overnight weather forecasts are generally quite accurate. Three-day forecasts are certainly of some utility. Two-week forecasts, on the other hand, are essentially worthless. Although we have a very clear understanding of the laws of physics that govern the behavior of Earth’s atmosphere, we can’t sample global weather conditions with enough precision to make forecasts accurate beyond a few days. If you let out a deep sigh at the complexity of it all, then the air current that you exhale will spur subtle deviations in the flow of air and moisture of the Earth’s surface that become increasingly magnified over time. The aggravated swirl of air from a slap at a mosquito can career into divergences that visit a hurricane on Miami rather than spinning it out into oblivion over the North Atlantic. Although we can’t accurately predict how the pattern of weather fronts and daily high temperatures will look on the 10:00 p.m. News two weeks from today, we do have some idea of what the weather will be. If it is in the middle of the summer, Texas will be hot. Duluth, in January, will be cold. The pattern of erratic day-to-day weather is superimposed over solidly predictable seasonal and regional climates.

We can thus ask the question: Are the movements of the planets predictably chaotic in the same sense as the weather? That is, over billions of years, will the planets wander only within circumscribed bounds, or is a more wild chaos, with orbit crossing, ejections, collisions and the like – a real possibility?

The answer will be a statistical statement. To high probability, the planets will remain more or less on their present courses until the Sun becomes a red giant. Exactly how high a probability is not fully clear. Stay tuned…

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The discovery of new planets is rarely clear cut. No sooner does a new world (Vesta, Neptune, Pluto) emerge, than the wrangling for the credit or the naming rights starts. And it’s usually possible to find a reason why the prediction (or even the planet itself) wasn’t really valid in the first place.

The trans-Uranian planet predicted by Urbain J. J. Le Verrier and John Couch Adams happened to coincide quite closely with Neptune’s actual sky position in September 1846, but the orbital periods of their models were too long by more than 50 years. Le Verrier’s predicted planetary mass, furthermore, was too large by nearly a factor of three, and Adams’ mass prediction was off by close to a factor of two.

In England, following the announcement of Neptune’s discovery, and with the glory flowing to Le Verrier in particular and France in general, the Rev. James Challis and the Astronomer Royal George Airy were denounced for not doing enough to follow up Adams’ predictions, “Oh! curse their narcotic Souls!” wrote Adam Sedgwick, professor of geology at Trinity College.

Nowadays, with the planet count up over 200, the prediction and discovery of a new world doesn’t quite carry the same freight as it did in 1846. No editorial cartoons, no Orders of Empire, and no extravagant public praise to the discoverer, such as that heaped by Camille Flammarion on Le Verrrier, who wrote, “This scientist, this genius, has discovered a star with the tip of his pen, without other instrument than the strength of his calculations alone!”

Nevertheless, I don’t want to be shoehorned into the ranks of the “narcotic souls” as a result of not properly encouraging the bringing to light of any potential planetary discoveries in the systemic catalog of real stellar radial velocity data sets. As of Dec. 30th, 2006, over 3,680 orbital fits have been uploaded to the systemic backend. It’s definitely time to start sifting carefully through the results that the 518 registered systemic users have produced. Over the next few weeks we’ll be introducing a variety of analysis and cataloging tools that will make this job easier, but there are some interesting questions that can be answered right away. Foremost among these is: what are the most credible (previously unannounced) planets in the database?

The backend uses the so-called reduced chi-square statistic as a convenient metric for rank-ordering fits:

In the above expression, N is the number of radial velocity data points, and M is the number of activated fitting parameters. As a rule of thumb, a reduced chi-square value near unity is indicative of a “good” fit to the data, but this rule is not exact, and should hence be applied with caution. The observational errors likely depart from a normal distribution, and more importantly, the tabulated errors don’t incorporate the astrophysical radial velocity noise produced by activity on the parent star. Furthermore, it’s almost always possible to lower the reduced chi-square statistic by introducing an extra low-mass planet.

Eugenio recently implemented the downloadable console‘s F-test, which can provide help in evaluating whether an additional planet is warranted. The F-test is applied to two saved fits and returns a probability that the two fits are statistically identical. As an example, pull up the HD 69830 data set and obtain the best two planet fit that includes the 8.666-planets and 31-day planets. Save this fit to disk. Next, add the 200-day outer planet and save the resulting 3-planet fit to disk (using a separate name). Clicking on the console’s F-test button allows the F-test to be computed using the two saved fits:

In the case of HD 69830, there’s a 1.7% probability that the 2-planet fit and the 3-planet fit are statistically identical. This low probability indicates that the third planet is providing a significant improvement to the characterization of the data. It’s likely really out there orbiting the star.

So here’s the plan: Let’s comb through the systemic “Real Star” catalog, and find the systems that (1) contain an unannounced planet(s) in addition to the previously announced members of the system (see the exoplanet.eu catalog for the up-to-date list). (2) have a F-test probability of less than 2% of being statistically identical, and (3) are dynamically stable for at least 10,000 years. If you find a system that meets these requirements, post your findings to the comments section of this post.

Disclaimer: this exercise is for the satisfaction of obtaining a better understanding of the planetary census, and also for fun. When the planets do turn up, I’m going to sit back with a bottle full of bub and enjoy any scrambles for priority from a safe distance.

Happy New Year, y’all!

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When J. Edgar Hoover was getting on in years, his aides would often tell scheduled visitors to his office that he was unable to meet with them because he was “in conference”. In reality, this meant that Hoover was napping at his desk.

It might seem that the refrain of, “we’re busy working on the systemic back-end” is an equally convenient euphemism for long lapses between posts on the front end. Nevertheless, we have been busy getting the new oklo xserve quad xeon up and running. The whole site has now been replicated and tested, the server is live and on air, and very shortly, we’ll be flipping the switch. Can’t wait, man!

With the vast increase in processing power afforded by the xserve, we’ll be able to provide a much more extensive suite of research tools to oklo visitors. In particular, it’ll be possible to dynamically generate the kinds of correlation diagrams that are currently only available from our estimable continental competition: exoplanet.eu.

It’s always interesting to look through the latest versions of the correlation diagrams to see whether the various trends and hints of trends are holding up. The a-e plot is worth examining, as is the plot that charts the number of planetary discoveries per year over the past decade. As of today, exoplanet.eu lists 192 planets that have been detected with the radial velocity method. Plotting the masses of these planets against their periods on a log-log plot (and running the resulting screenshot through Illustrator) yields the following:

For Keplerian orbits, the relationship between the radial velocity half-amplitude of the parent star and the orbital period of the planet is given by:

If we assume that the mass of the planet is negligible in comparison to the mass of the star and if we further assume edge-on, circular orbits around solar mass stars, then we get the dashed lines in the figure that show detection thresholds for K=3 m/s and K=1 m/s. The three planets orbiting HD 69830 stand out in this diagram as the most striking discoveries of 2006.

To the eye, there are two curious clusters of planets in the diagram. At short periods (P~3d) we have the hot Jupiters. Most of these have masses (times the sine of the unknown inclination) somewhat less than Jupiter. At longer periods (P>100d) we have a second prominent clump of planets. These are the Eccentric Giants, and their masses average out at a significantly higher value (between 2 and 3 times the mass of Jupiter). Part of the difference in mass is due to selection bias, but nevertheless there is a real effect. Like the planet-metallicity connection, this effect is telling us something about either planet formation or planet migration (probably the latter).

Anyone got an idea regarding what’s going on? Let’s get a discussion going in the comment section. Over the past week, I’ve been flooded by depressingly clumsy attempts at comment spam from single-minded robots with mechanical enthusiasms for satellite TV service and online poker, e.g. “Great blog, keep it comming.” It’d be nice to see some signal in the noise…

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The California Carnegie planet search team posted a data-rich paper on astro-ph this week. The new article is scheduled to appear in the February 2007 issue of the Astrophysical Journal. Eugenio, exercising his usual diligence, has added the new velocity tables to both the downloadable systemic console and the systemic back-end stellar catalog.

The highlight of the paper is a new two-planet system orbiting HIP 14810, a metal-rich solar-mass star lying 53 parsecs away. The inner planet in the system has a period of 6.66 days, and tips the scales with least 3.84 Jupiter Masses. The outer planet is less massive (Msin(i)=0.76 Mjup), and goes around the star every 95.3 days.

The secular interaction between the two planets compels them to trade angular momentum back and forth. As a result, the inner planet cycles between an eccentricity of 0.02 and 0.15 on a relatively short 5000-year timescale. It’s currently in the high-eccentricity phase of its orbit. The large radial velocity signal-to-noise for the planet means that its eccentricity can be measured quite precisely (have a look at it with the console). The fact that the orbit is clearly non-circular would be strong evidence for the presence of planet c, even if there weren’t enough data to detect c directly. If planet b was the only significant planet in the system, its orbit would have circularized via tidal dissipation on a timescale that is less than the age of the star.

Short-period planets with masses greater than three Jupiter masses are intrinsically rare. Tau Boo b (with a mass of at least 3.9 Jupiter masses and an orbital period of 3.3 days) is the only other object with roughly similar properties. By contrast, 32 planets with periods of less than a week and minimum masses less than Jupiter’s mass are currently known.

In my opinion, the two most robust statistical correlations that have emerged from the first decade of extrasolar planet detection are (1) the planet-metallicity connection and the (2) dearth of high-mass short-period planets. The planet-metallicity correlation makes perfect sense. It’s the natural, expected outcome of the core-accretion process and the fact that Jovian-mass (as opposed to Neptunian-mass) planet formation is a threshold phenomenon. The paucity of high-mass short-period planets, on the other hand, is both weird and completely unexplained. It’s telling us something about the process of planetary formation and migration. We just don’t know what it is.

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Giant planets are interesting. Terrestrial planets are more interesting. Habitable terrestrial planets are the most interesting of all, and it’s nearly guaranteed that we’re living in the age when the first genuinely Earthlike worlds beyond our solar system will be discovered. The only question is which technique will wind up doing it. The big money is on space-based transit photometry, but I think that ground-based RV might take the prize.

The Systemic Challenge 004 system was designed to be a futuristic idealization of what the Sun’s reflex velocity would like if it were observed with high precision from a neighboring star for more than two decades.

The individual radial velocity uncertainties for the 1172 velocities the Challenge 004 datset are each of order 10 centimeters per second. Errors this small are still safely smaller than the sub-meter per second precision that is currently being obtained by the Swiss team (with HARPS) and the California Carnegie team (at Keck). Given the rapid improvement in the radial velocity technique over the past decade, however, it’s not at all unreasonable to expect instrumental precisions of 10 cm/s fairly soon. Many console users were able to extract the four largest-amplitude solar-system planets — Jupiter, Saturn, Earth, and Venus — out of the challenge004 dataset, suggesting that it’s only a matter of time before instrumental precisions and observational baselines arrive at the threshold where truly habitable, Earth-mass planets can be detected from the ground using the radial velocity technique.

A potential show-stopper for this rosy predictive picture is the astrophysical radial velocity noise produced by the stars themselves. If you want to detect a planet with the mass and period of Earth (which induces a radial velocity half-amplitude of only 9 cm/sec) then you need to be assured that the star is quiet enough for the low-amplitude terrestrial planet signal to be detectable. It’s therefore natural to ask the question: what does the Sun’s reflex velocity look like?

The GOLF experiment on the SOHO satellite provides one set of measurements. A massive time-series of radial velocity observations (from 1996 through 2004) has been published, and is now publicly available. The data set contains over seven million radial velocities taken at a 20-second cadence. The main goal in obtaining this data was to study the Sun’s spectrum of p and g-type modes, which show strongest oscillations at periods of a few minutes.

Three alternate calibrations of the GOLF dataset are posted on the project website. Two of these have clearly been processed to filter out low-frequency, long-period radial velocity variations. It’s interesting, however, to look at what the one unfiltered dataset suggests is happening over timescales of a year or more. I sampled the unfiltered data at a cadence of one velocity measurement per several days, and then loaded the resulting time-series into freshly downloaded version of the systemic console:

According to the above time series, the Sun is a pretty noisy star. I scoured the papers on the GOLF site, and could not find any discussion regarding how much of the variation shown above is believed to come from instrumental effects and how much is belieived to be actually intrinsic to the Sun. The fact that both the scatter and the amplitude of the variations seem to be increasing during the run of the data tend to indicate that intrumental effects relating to the aging of the detector play an important role. If anyone has more specific information on this issue (or if anyone is aware of a preferred calibration) please post to the comments section of the post.

What happens to the detectability of planets that are placed in the GOLF time series? To date, the most precise RV detection of an extrasolar planetary system is the Swiss Team’s discovery of the three Neptune-mass planets orbiting HD 69830. As a control experiment, we relabeled the published HD 69830 dataset at systemic003, and placed it on the backend for Systemic users to evaluate. As expected, nearly all of the twelve submitted fits recovered the published configuration, with chi-square reaching down to about 1.20.

For the systemic004 system, we took the published HD 69830 3-planet orbital model and integrated it forward in time to make a synthetic radial velocity curve. We then perturbed this curve with noise values drawn from the unfiltered GOLF dataset (We averaged the velocities into 15-minute blocks to simulate rapid-fire multiple observations that average over high-frequency p-modes). As of Sunday night, there have been 21 fits uploaded for systemic004 [thanks, y’all, -ed.]. None of them manage a chi-square below 2.5, and aside from the innermost planet, none of them make a convincing case for the presence of the planets that were placed in the dataset. Log in to the backend, call up systemic004 from the “real stars” catalog, and you’ll see what I mean.

The conclusion, then, is that if the GOLF data-set gives a realistic determination of the intrinsic radial velocity variation of the Sun, then the Sun is a far noisier star than HD 69830 (and other similarly old, early K-dwarfs). Indeed, you would even be hard-pressed to believe the presence of Jupiter in the GOLF time-series, unless you’ve got the luxury of waiting for at least several Jovian orbital periods.

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Update: The original version of this post tagged systemic003 as the target system and systemic004 as the control system. It’s actually the reverse. In any case, we’re interested in getting multiple fits to both systems. -GL

No post today, just a request:

We’ve been doing an analysis of the detectability of low-mass planets around certain types of stars. In the course of this work, we’ve generated a radial velocity data set, systemic004, which may (or may not) harbor a planetary system. I’d like to ask everyone to (1) download the latest version of the console, and (2) submit your fits to the systemic004 system to the backend. We’ve also included a control system, systemic003, which may look familiar. It would be very useful to have your fits to that system as well.

Thanks in advance! Once we get a batch of fits, I’ll write a post that explains the motivation underlying this request…

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UCSC just put out a press release on the Systemic Project, so if you’re a first-time visitor to oklo.org, welcome aboard. You’ll find information about the project in the list of page links just to the right.

Last week, during my visit to Harvard CfA, I talked to Eric Ford, who has been exploring the idea of searching for trojan companions to extrasolar planets. He pointed out that the discovery of a body in a trojan configuration with a known extrasolar planet would provide an important test of theories of hot Jupiter formation. Here’s a link to his paper.

One way to make a hot Jupiter is to form the planet through the standard core-accretion method at a large radius in the protostellar disk. In this scenario, a newborn gas giant planet starts as a core of rock and ice, which grows to a size of 5-10 Earth masses and begins to rapidly accrete gas from the surrounding nebula. As the planet increases in size, it begins to clear a annular gap in the parent disk. Hydrodynamical simulations (such as the ones reported here, and reproduced in the illustration below) show that L4 and L5, the so-called trojan points located 60 degrees ahead and 60 degrees behind the forming planet, are the last regions of the gap to be cleared out.

It’s possible that co-orbital planets can form from the slow-to-clear material at L4 and L5. When the gas is gone, these objects will remain in stable trojan orbits.

If a pair of planets is caught in a trojan configuration, then they will migrate inward together through the disk, and the migration process will not cause them to become dynamically unstable. Eric points out that the observed presence of a trojan companion to a hot Jupiter would thus be evidence that the hot Jupiter arrived at its short-period orbit via migration. Other possibilities for forming hot Jupiters, such as dynamical instability followed by orbital circularization, do not allow for trojan companions.

A trojan pair of planets presents an interesting conundrum for planet hunters. Normally, a single planet on a circular orbit goes through its radial velocity zero point at the moment when the planet lies on the plane containing the line of sight from the Earth to the parent star. If we have a trojan, however, a planetary transit will be offset from the radial velocity zero point, which is associated with the orbit of a “ghost body” that combines the gravitational effect of the primary planet and its trojan companion. Using the console, try obtaining a two-planet perfect trojan (60 degree separation) fit to a well-known hot Jupiter data set such as that for HD 187123. You’ll find that it’s perfectly possible. The resulting configurations have utterly indistinguishable radial velocity signatures.

Trojans can be detected, however, if the primary planet happens to transit. The presence of the trojan companion can be inferred by measuring the lag between the center of the transit and the zero crossing of the radial velocity curve. For planets with an equal mass ratio, this would amount to a full 1/12 of an orbital period (6 hours for a 3-day orbit).

Which brings up an interesting project for transitsearch.org observers. Most of the known hot Jupiters have been checked photometrically for transits. These transit searches, however, are performed in the time window surrounding the radial velocity zero point. In the (admittedly unlikely) case that some of these objects are trojan pairs with near-equal mass ratios, the transits would have been missed using this approach. To fully rule out transits, one should cover the full 1/6th of an orbital period surrounding the nominal predicted transit time…

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Hey Everybody, if you’ve been working on your fits to the second Systemic challenge system, send them to the e-mail address listed on the web-page given in the Sky and Telescope article.

I’ve been hearing many rumors floating around that there’s going to be a big planet announcement next week. Unfortunately, oklo.org has not yet joined the privileged ranks of the mainstream science press, so I haven’t been privy to any advance looks at the result that’s gonna come off the wire. I did hear, though, that it’s going to be an STScI announcement, so I did a little anticipatory detective work.

STScI runs HST, which means that the result will be the product of Hubble observations. In order to make Hubble observations, one needs to manage to get a block of fiercely competitive Hubble time, which means you need to write a successful Hubble proposal. The abstracts of winning Hubble proposals are posted publicly on NASA ADS. A quick search yields the following accepted proposal abstract (Proposal ID #10466), submitted by Dr. K. C. Sahu:

We propose to observe a Galactic bulge field continuously with ACS/WFC over a 7-day period. We will monitor ~167, 000 F, G, and K dwarfs down to V=23, in order to detect transits by orbiting Jovian planets. If the frequency of “hot Jupiters” is similar to that in the solar neighborhood, we will detect over 100 planets, more than doubling the number of extrasolar planets known. For the brighter stars with transits, we will confirm the planetary nature of the companions through radial- velocity measurements using the 8-m VLT. We will determine the metallicities of most of the planet-bearing stars as well as a control sample, through follow-up VLT spectroscopy. The metallicities of the target stars range over more than 1.5 dex, allowing for a determination of the dependence of planet frequency upon metallicity–a crucial element in understanding planet formation. We will be able to discriminate between the equally numerous disk and bulge stars via proper motions. Hence we will determine, for the first time, the frequencies of planets in two entirely different stellar populations. We will also determine for the first time the distribution of planetary radii for extrasolar planets for both these populations. Parallel observations with NICMOS will provide ultra-deep near-infrared images of a nearby bulge field, which will be used to determine the stellar luminosity and mass functions down to the brown-dwarf regime. The data will also be useful for a variety of spinoff projects, including a census of variable stars and of hot white dwarfs in the bulge, and the metallicity distribution of bulge dwarfs.

I looked at Sahu’s web page at STScI, where he writes:

At present, my research is mainly focused on a large HST program which involves monitoring of about 300,000 stars towards the Galactic bulge using the Advanced Camera System (ACS) on board HST, to search for extra-solar planets. The results are due to appear in the October 5, 2006 issue of Nature.

So clearly, the ACS data have been reduced, and it’s an excellent bet that they’re planning to announce the transit candidates that have emerged from their 7 days worth of ACS photometry. The number of transits to be announced is almost certainly more than two. This week’s announcement of WASP-1b and WASP-2b certainly didn’t produce a noticeable media splash, so there must be a lot of planets in the announcement. And given the past history of HST microlensing planet detections, I bet it’ll be the case that some of the parent stars of the soon-to-be-announced new transiting planets have indeed undergone a fairly rigorous spectroscopic follow-up with the VLT.

I think that spending a whole week of ACS time to stare at stars in the galactic bulge is a fairly worthwhile use of the HST (although I bet a lot of extragalactic astronomers might disagree). Here’s my take: In the mid-1990’s, it was believed that the stars of the galactic bulge are very metal-rich. In 1994, for example, McWillian and Rich 1994 reported an average bulge star metallicity of 0.2 “dex”, that is, ~60% greater than the solar value. More recently, however, Fulbright et al. 2006 have revised the average metallicity of the bulge downward to a value of -0.1 dex (~80% of the solar value). It thus appears that the metallicity distribution of the stars in the bulge is roughly similar to the metallicity distribution of stars in the solar neighborhood.

We know from Debra Fischer and Jeff Valenti’s work that the rate of short-period giant planet occurence is a strong function of stellar metallicity:

All other things being equal, we can use the above diagram to inform an estimate of the number of planets that Sahu and company will announce. The hot Jupiter occurence rate in a solar-neighborhood type metallicity population is of order 0.7%. About 10% of hot Jupiters will be observable in transit. About half of those transits will be clobbered by the effect of a binary companion sharing the pixel and driving the detection below threshold. For a 7-day survey, about 60% of the hot Jupiters will actually get picked up, given constant coverage and good control of detector systematics (which HST certainly has). This means that Sahu should see (167,000)x(0.007)x(0.1)x(0.5)x(0.6)=35 transiting planets.

The problem, however, will be that there are many events which will look like planet transits, including grazing eclipsing binary stars, transiting M-dwarf stars, and the surprisingly common situation where a background eclipsing binary star shares the pixel with a foreground target star, a so-called blend situation. Dave Charbonneau and his collaborators have written extensively about all of the different pitfalls that can cause a wide-field transit survey to turn up false positives.

So my guess is that there will be of order 200 transit candidates in the ACS data for 167,000 stars. The brightest and most promising of these will have been sent to the VLT for spectroscopic follow-up. If the sensitivity limit of the survey (as stated in the proposal abstract) is V~23, then the candidate stars will likely have V~20-21. Even with the VLT, it’s tough to get accurate radial velocity measurements for stars this dim. So a lack of an observed binary stellar companion will probably be taken as a confirmation of the presence of a planet. (This is all complete speculation on my part.) Going even further out on a limb, my guess is that they have ~100 stars that show transit signatures, and which do not have a spectroscopically detectable binary stellar companion. Although it’ll be hard to further sort the wheat from the chaff, I’ll harbor a guess that 1/3 of the planets that will likely be announced are bona-fide.

Assuming that this is what actually occurs at the press conference, then we’ll have a very interesting result — not so much about the planets (which will be hard to characterize owing to the dim parent stars) — but because of what it tells us about the formation of the galactic bulge. Right now, there are several competing theories for how the bulge formed. One possibility is that scattering of stars by the Milky Way’s galactic bar has populated the Milky Way’s bulge with stars. Another possibility is that the bulge stars are the result of many disrupted globlular cluster or dwarf-galaxy like objects.

A measurement of the planet population of the bulge stars can allow us to distinguish between these two possiblities. If the planet occurance rate is similar to the galactic neighborhood (which I’m guessing will be the gist of the press conference) then the bulge stars are likely to have formed under low-density conditions. This would favor a bar-scattering type of scenario. If the planet occurence rate is zero or very low (which is unlikely, given that they are having a press conference) that would imply that the stars formed in a high-density environment. A crowded star formation leads to a ultraviolet ionizing radiation field that makes it difficult for planets to form and then migrate inward to become hot Jupiters.

There was a remarkable study done with HST in the late 1990s, and published by Gilliland et al. 2000. HST obtained a 8.3-day photometric time series for 34,000 stars in the globular cluster 47 Tucanae. The data, when reduced, show a total absence of transiting planets. This result shows the power of both the environment variable (the 47 Tucanae stars formed in an intensely irradiated region of star formation) and the metallicity variable (the metallicity of the 47 Tucanae stars is about 20% the solar value).

Finally, it’s always good to look at costs. According to the Wikipedia, the total cost of building, launching, servicing, and running HST has been of order 6 billion dollars. It started working as planned in 1994, and will thus have ~15 years of fully functional use. The seven days of ACS time were therefore worth 7.6 million dollars. This is comfortably more than the cost of building a special-purpose telescope to probe the terrestrial planets that are almost certainly orbiting Alpha Centauri B. (For more information, see these oklo.org posts: 1, 2, 3, 4, 5.)

In June 2002, I saw Keith Horne give a review talk at the Scientific Frontiers in Research on Extrasolar Planets Meeting at the Carnegie Institute of Washington. He showed a slide (an updated version of which is here) that listed 23 planetary transit surveys that were in operation at that time. He had asked the investigators running each survey to send him the number of new transiting planets per month that could be expected to turn up. The grand total rang up to a whopping 191 fresh planets per month, or 2,292 planets per year.

For a number of reasons, those numbers haven’t quite panned out, but it nevertheless finally looks like we’re entering a phase where the planetary yield from wide-field transit surveys is starting to ramp up dramatically. Today’s astro-ph mailing has a paper by Collier Cameron et al. entitled “WASP-1b and WASP-2b: Two new transiting exoplanets detected with SuperWASP and SOPHIE”, describing the discovery of P=2.15 d and P=2.52 d planets transiting ~12th magnitude stars. As of this month, astronomers have been hauling in transits at a rate of one per week.

Dave Charbonneau at CfA dropped me an e-mail this morning:

Great chance to catch WASP-1 tonight from the western US. We will pursue it from Mt. Hopkins and Palomar, but thought you might want to give a heads up to transitsearch.org. This one is very much in need of a great light curve, as the current estimates of the planetary radius range from “smallish” to “huge”, with an error bar that is depressingly large.

He’s definitely right about all that. Multiple photometric data sets will be of considerable use in constraining the system parameters. It’s also going to be very important to get a better handle on the properties (Mass, Radius, and metallicity) of the WASP-1b and 2b parent stars. We’d really like to know how these two new planets fit into the overall trends that are starting to emerge among the aggregate of transiting planets.

Ephemerides for both of the WASPs have been added to the transitsearch.org candidates table, and the (rather meagre) published tables of radial velocity measurements have been added to the Systemic back-end. Given the short orbital periods, I don’t think it’ll be very long at all before small telescope observers start producing confirmation light curves.

Image Source.

As we work to get the systemic collaboration off the ground, I’ve been letting transitsearch.org coast along on basically its own devices. That will change fairly soon as we incorporate the transitsearch.org site into the oklo.org backend. This will allow for (among other things) creation of transit ephemeris based on submitted fits, and a tighter collaboration among observers.

In the interim, there’s an important opportunity coming up on October 20th, 2006 for transitsearch.org observers to see whether the highly eccentric planet HD 20782 b can be observed in transit. The a-priori probability is 3.6% that we’ll make a huge (and I mean big deal) discovery. Here’s a link to the oklo post from last April describing the system. I’ll be sending out a campaign notice to the transitsearch.org e-mail list shortly, and I’m hoping that AAVSO will want to collaborate on this campaign as well.

A large number of amateur and small-telescope observers have been catching transits recently. Here’s an overview of some of the results that have showed up in my in-box over the past two months:

Last year, transitsearch.org got an important opportunity to follow up on a high-probability transit candidate. The target was the P=3.34 d planet in the close triple star system HD 188753 that was announced by Konacki, 2005 and which received quite a bit of notice in the press.

Konacki’s planet, which has a minimum mass of 1.14 Jupiter masses, and a predicted radius of ~1.1 Jupiter radii, orbits a 1.06 solar-mass star (HD 188753A) at a distance of ~0.05 AU. The star A is in turn separated by 12 AU (e=0.50) from a binary pair (HD 188753B “a” and HD 188753B “b”). The binary pair B consists of stellar components with 0.7 and 0.9 solar masses in a 155-day e=0.1 orbit (separation of 0.67 AU). The presence of a hot Jupiter orbiting HD 188753A was very surprising, because the presence of the binary B leads to severe difficulties for conventional planet formation theories.

Observations in 2005 by Ron Bissinger and other transitsearch.org observers indicated that it is very unlikely that HD 188753A “b” is transiting (the a-priori probability was 11.8%). Further confirmation of a lack of transits came this July from Joe Garlitz of Elgin Oregon, who writes:

Attached is a chart made from data on HD 188753 taken from 04:45 to 09:30 July 21 [UT]. There is no suggestion of a transit during this time within the capacity of this data. It is difficult to get a good data set since there are no stars within the field at a magnitude similar to HD 188753.

Garlitz’s full figure is here, and his website is here.

One disadvantage of having 200-odd planets rattling around in the planet catalog, is that it’s getting hard to keep all the HD numbers straight. HD 188753 doesn’t show transits, but HD189733 most dramatically does. On July 30th, Donn Starkey, of Auburn, Indiana sent a nice light curve of HD 189733 during the JD 2453946.7 transit:

More detail regarding Starkey’s results can be found on his website.

On August 27th, I received a dispatch from Veli-Pekka Hentunen of Varkaus Finland. Summer has finally wrapped up in Finland, and Taurus Hill observatory (featured in this post from last May) is again open for business:

Last weekend, we began to continue exoplanet transit observing after the long Finnish light summer. On the night of August 26-27 w observed our first XO-1b transit at the Taurus Hill Observatory, Varkaus (obs. code A95). We were able to catch only about half of the entire transit because the object was quite low in the north-western sky, and the altitude decreased from 30 to 15 degrees during the course of observation. Our light curves and observing information are given on our English website.

In a follow-up e-mail on September 17th, Hentunen reported that they had observed a full transit for HD 189733 on the night of September 16th-17th:

Starting on September 9th, Tonny Vanmunster, Kent Richardson, and Ron Bissinger all reported observations of the newly discovered TrES-2 transit. Tonny and Kent’s observatons are detailed in this oklo post, whereas Ron’s TrES-2 light curve looks like this:

Don Carona at Texas A&M also sent a light curve of a TrES-2 transit, obtained (under less-than-ideal weather conditions) from the Physics Dept. at College Station. Here’s an unbinned excerpt from his reduced high-cadence time series:

Notice the the lack of a flat bottom for the TrES-2 transit (which is more obvious to the eye when the data is binned). TrES-2 crosses the star with a high “impact parameter”, which means that the planet does not occult the central portion of the star as seen from Earth. Stellar Limb Darkening is responsible for the remarkable smoothness of the dip.

On September 19th, Bob Buchheim of Altimira Observatory sent a very nice HD 189733 lightcurve:

Along with the photometry, he sent a report of an entirely new strain of transit fever:

Last night I monitored HD189733 for a photometric transit signature. A bit of an embarassing story: I noticed that it was nicely placed and “in window”, so I set up the telescope and went to bed … but I forgot to check if transits had already been detected for this star. Imagine my surprise at the resulting deep transit signature! (See attached graph). Oh, well, now that I’ve checked, I see that I’m a year late with this “discovery”.

Bob’s experience reminds me that I’ve got to update the various results pages for the transiting candidates. The transitsearch results page for HD 189733 b reports (erroneously, and by default) that “No photometric transitsearch has yet been reported for this system”. Yikes!

Finally, just a few minutes ago, I got an e-mail from Arto Oksanen, who was the first amateur to observe an HD 209458b transit (on Sept. 16th, 2000). Six years later, it looks like he’s also the first amateur to observe HAT-P-1b

I observed the end of transit of HAT-1b last night at Hankasalmi Observatory, Finland. The weather was not very good, but the egress was well visible. The observing instrument was a 40 cm RC telescope. I used V-filter with SBIG STL-1001E CCD.

Oksanen’s light curve is plotted is at this link, and is reproduced just below:

Oksanen notes that the egress seems to occur 30 minutes early relative to the published ephemeris. This could well be the case. The longer the time baseline for transit observations, the more accurate the ephemeris become. Small-telescope observations have an important role to play in this regard.

Image Source.

Chalk another transiting exo-planet up on the board. In a preprint released today, Gaspar Bakos and his colleagues in the HATnet project are announcing HAT-P-1b, a large-radius, low-density planet transiting one member of a relatively nearby, relatively bright solar-type binary star.

HAT-P-1b (which orbits the star BD+37 4734) is quite interesting for several reasons. Its 4.46529 day orbit is the longest period yet detected for a transiting extrasolar planet, and its measured radius of 1.36 Jupiter radii is alarmingly larger than the baseline theoretical prediction. The planet contains 0.53 Jupiter Masses, and has a surface temperature near 1100 K, so our models predict that it should have a radius of 0.94 Jupiter radii if it contains a 20 Earth-mass heavy-element core, and a radius of 1.09 Jupiter radii if it’s made of pure solar-composition gas. It’s thus roughly 20-30% larger than it “should” be, which means that something is providing it with a very significant source of extra interior heat.

The large radius of the planet means that the transits exhibit a ~1.5% photometric depth. Deep transits make it easier to obtain data with high signal-to-noise, which means that we can look forward to very accurate follow-up measurements for this system. The presence of a nearly identical companion star at a separation of 11 arc seconds should also help observers obtain good differential photometry. The star is up now, and its well-placed for Northern Hemisphere observers. I don’t think it’ll be long before we see confirmations rolling in from small-telescope observers worldwide. If you’re interested in observing it, the ephemeris table is here.

Where could the extra source of heat be coming from? One possibility is tidal dissipation related to the circularization of an eccentric orbit.

The theory of tides can rapidly slip into thousands of pages of detailed mathematical analysis. Many of the interesting ideas, however, are close to the surface. In the introduction to his still-useful 1898 popular book, “The Tides”, Sir George H. Darwin, the son of the naturalist, and Plumian Professor at Cambridge, wrote:

A mathematical argument is, after all, only organized common sense, and it is well that men of science should from time to time explain to a larger public the reasoning behind their mathematical notation. To a man unversed in popular exposition it needs a great effort to shell away the apparatus of investigation and the technical mode of speech from the thing behind it.

I would actually argue that the situation is quite the opposite. I think it’s easier and better to get a colloquial, heuristic understanding first, and then make an attempt to put the ideas on a sound mathematical basis.

For a planet that (like Hat-P-1b) has an orbital period of less than a month or so, it’s expected that tidal forces will rapidly bring the planet into synchronous rotation, in which the planet spins once on its axis every orbital period. For a circular orbit, this means that the planet always keeps one face to the star, just as the Moon keeps one side toward the Earth. If the orbit is eccentric, however, the planet will not manage to keep one side directly facing the star. Because the planet spends more time at the far point of its orbit, it does more “turning” there, and remarkably, it turns out that the planet always keeps one face pointed toward the empty focus of its elliptical orbit. From the point of view of the star, a fixed point on the planet is seen to librate. The Moon’s orbit is slightly eccentric, and so we can see this effect in time-lapse animations of lunation. See here and also (if you’re not inclined to motion sickness, here).

The tidal bulge of the planet, on the other hand, is forced to always point toward the star, as it’s the star’s gravity that is producing the differential gravitational attraction. This means that once per orbit, the tidal bulge is rocked back and forth across the planet, which produces serious internal heating. The size of the bulge also increases and decreases once per orbit due to the varying distance between the planet and the star, and the resulting oscillation also contributes to the amount of internal tidal heating.

The amount of tidal energy deposited in the planet increases with the square of the orbital eccentricity. A reasonable model for the physical properties of HAT-P-1b indicates that an eccentricity e=0.09 is adequate to generate enough tidal heating to expand the planet to its observed size of 1.36 Jupiter radii. The problem, is that the orbit should circularize on a timescale of less than 1 billion years, whereas the system seems to be about 4 billion years old. Therefore, if a significant orbital eccentricity exists, then there must be a mechanism to maintain the eccentricity. The best candidate is another planet further out in the system.

We can take a quick look to see whether such a situation holds water. A second planet in the system will exert gravitational perturbations of HAT-P-1b, which will cause its eccentricity to vary with time. The picture to have in one’s head is to imagine that the planets are viewed on a timescale that is much longer than an orbital period. If one takes the long view, the motion of the planet is effectively blurred out along the orbit, and one can model the planet as an elliptical wire of varying mass density — heavy near apastron where more time is spent, and light near periastron where the planet spends less time. The interaction between the two planets can then be modeled as the interaction between two flexible elliptical wires. When one does this, and ignores the moment-by-moment motion of the planet, one is making a “secular approximation”. Secular theory was developed to a high art in the 1770s by Laplace and Lagrange, and we can make use of their work to quickly look at how the eccentricities of two mutually interacting planets vary with time.

Over at the transitsearch.org, I have a program which computes a 2nd-order secular theory for each of the known multiple-planet systems, and plots the resulting eccentricity variations as part of the candidate information table . (To see the plots of the secular variations, click on the “planet column” for a multiple planet system such as Upsilon Andromedae).

An advantage of using the secular approximation to the dynamics is that one can quickly scan through a range of planet-planet configurations and find out what the perturbative interactions look like. If I add a second planet to the HAT-P-1 system with a period of 50 days, a mass of 0.06 Jupiter masses, and an orbital eccentricity of e=0.30, I get the following long-term variation in eccentricities, in which the eccentricity of planet b has a time-averaged value close to e=0.09:

Is it possible to hide a second planet in the system while remaining consistent with the (still sparse) set of published radial velocity measurements? The downloadable console is ideally suited to quickly answering such a question. It turns out that it’s very easy to get a perfectly adequate 2-planet fit to the data in which a hypothesized perturbing outer planet is capable of maintaining a time-averaged eccentricity e=0.09 for the inner planet. One such fit looks like this:

corresponding to a system configuration that looks like this:

It would be exciting if a planet like the one in the above diagram (or its dynamical equivalent) could be detected. This would tell us that we’re on the right track in obtaining a better understanding of the wide range of observed radii among the known transiting planets. Probably the fastest way to detect the perturbing planet is through accurate timing of the intervals between successive transits. If an outer planet is tugging on the inner planet, then the orbit will fail to be perfectly periodic, and variations will be observed in the amount of time that passes between transits. Matt Holman and Norm Murray have written a paper that describes how this works. They give a rule-of-thumb equation for the transit-to-transit variations that one can expect. For a planet similar to HAT-P-1b, with semimaor axis a1 and period P1, and a pertubing planet with simimajor axis a2 (where a2>a1), period P2 and mass M2, they find that the typical variation of the interval between successive transits is given by

where

For HAT-P-1b and HAT-P-1c (as hypothesized above) delta T works out to about 5 seconds. Variations of this magnitude are readily detected from the ground, and a number of research groups will probably jump right on this problem.

The transit game is getting to be a competitive global business. No sooner is a new transit announced than amateur astronomers worldwide are on the sky to obtain follow-up observations. Tonny Vanmunster, of Landen Belgium and Ron Bissinger of Pleasanton California are generally among the first to check in with confirmations. Vanmunster nailed TrES-1 a mere week after its announcement in 2004. Last summer, Bissinger caught HD 149026b on literally the day it was announced. In the case of TrES-2, which was announced yesterday, it looks like Vanmunster has snagged the prize. “What’s up, Cali?”

[Actually, it was both cloudy and the middle of the day in Pleasanton while Vanmunster was on the sky. But there’s a transit tomorrow night, Sept. 10/11 PDT, that Ron’ll likely catch.]

Here’s Vanmunster’s light curve. The transit was in progress at dusk in Belgium, so he was able to observe only the latter part of the event.

Vanmunster writes:

Here are some technical details : observations were made at CBA Belgium Observatory, using two 0.35-m f/6.3 Celestron telescopes, each equipped with an SBIG ST-7XME CCD camera. I simultaneously made unfiltered and R-band observations (hence the 2 telescopes). The included light curve is unfiltered, and each dot in the curve is the average value of 5 successive observations (binned). The gray lines show the standard deviation (about 4 millimag on average). Exposure time was 15 to 20 sec.

The egress is very evident in the light curve, and happened right at the predicted time. The transit depth was approx. 0.0155 mag, which again corresponds well with the value published in the discovery paper.

Follow-up observations such as the ones made by Vanmunster and Bissinger can be very scientifically useful. For example, Vanmunster’s 2004 observations allowed us to get an improved estimate of the TrES-1 planetary radius, and he co-authored a journal article with us on that topic. Both Vanmunster and Bissinger were involved in the discovery of X0-1, and both are co-authors on the recent Shankland et al. paper which I’ll talk up in an upcoming post.

Image Source.

Last week, I started a series of posts that will examine the feasibility of detecting a habitable terrestrial planet orbiting Alpha Centauri B. We want to do this from the ground, using the proven radial velocity technique. Our strategy will be to build a ~1 meter telescope in (probably) Chile with a high-precision spectrometer. The telescope will be used exclusively to obtain velocities of Alpha Centauri B, night after night, whenever the star is above the horizon, and whenever the weather is good.

At La Silla, Alpha Centauri never quite sets below the horizon. From roughly October through December, however, the star is generally too low in the sky to be adequately observed. This generates a yearly periodicity in a simulated 5-year radial velocity time series:

(Note that in the original version of last week’s post, I posted an incorrect file version of this figure. The plot has now been replaced in the post with the correct plot.)

The 96076-point time series shown above was generated by Eugenio under the assumption that we can obtain the same radial velocity precision that the HARPS spectrograph obtained on HD69830 (which is a near-twin to Alpha Centauri B). The average radial velocity error is 0.8 m/s, and the observing cadence is 200 seconds. The periodogram shows crystal clear evidence of three of the four terrestrial-mass planets in the simulated system.

The large peak in the periodogram at P=347 days corresponds to a planet with half an Earth mass. The two interior peaks are produced by planets with masses similar to Mars. If the planetary masses are doubled, that is, if the largest planet in the system has one Earth mass, then the evidence from the time series is even more overwhelming (the data set is available on the downloadable console as AlphaCenB_m2Y5):

.

Now I know that scientific progress is the important issue at stake here, and I’m very excited about the upcoming Kepler mission, and the fact that it will be detecting Earth-sized planets in the habitable zones of 12-14 magnitude stars. That’ll be cool. Nevertheless, we’d like to beat Kepler to the punch. We’d like to bag the first habitable Earth-mass planet from the ground. Can we do it?

Kepler is currently scheduled for launch in October 2008. To be confident of an Earth-mass planet in the habitable zone of one of their target stars, they’ll need to see four successive transits. This means that a reasonable date for their big press conference is Dec. 21, 2012. That gives us six years.

There’s a big difference between noisily posting synthetic observations on a blog, and actually observing Alpha Centauri with a purpose-built ~5 million dollar dedicated telescope in the Southern Hemisphere. It’s safe to say that we won’t be seeing first light a year from now, and therefore we won’t have a five-year data stream by the time the current long count is up.

On the other hand, the above periodogram for the Earth-mass planet constitutes an incredibly strong detection. We can be confident in the presence of the planet with considerably less data.

So consider the following example. Let’s say that instead of obtaining HARPS-like 0.8 m/s precision, we’re only able to get 3 m/s precision. This could be the result of Alpha Centauri B showing more short-term jitter than expected, or because we don’t have enough dough to commision an absolutely state-of-the-art spectrometer. Let’s also assume that we only observe for two years rather than five. In this case, the periodogram looks like this:

We’ll definitely need to do a proper statistical analysis, but from appearances alone, “chi-by-eye”, the planet is clearly still there at the many sigma level.

Image Source.

Everything that we know about planet formation indicates that both Alpha Centauri A and Alpha Centauri B should be accompanied by terrestrial planet systems. Long-term integrations show that the dynamical environment is stable. Simulations using the Wetherill-Chambers method show that the accretion of terrestrial-sized bodies should proceed with an equal or greater ease than was the case in our own solar system. The metallicity of Alpha Centauri is significantly supersolar, which points toward the availability of plenty of raw material for forming terrestrial planets.

The terrestrial planets orbiting Alpha Centauri A and B were likely assembled from dried-out planetesimals. Pertubations from Proxima, however, would have stirred up Alpha Centauri’s circumbinary analog of the Kuiper belt, providing a mechanism for the delivery of volatiles to terrestrial bodies orbiting A and B.

In a string of posts last month [most recent here], we laid out the case for the existence of terrestrial planets in the Alpha Centauri system. We argued that if one of these planets has an Earth-mass and a habitable orbit, then it is detectable with a flat-out effort by the HARPS spectrograph. We based our argument on the fact that HARPS was recently used to produce an amazing detection of three Neptune-mass worlds orbiting HD 69830 — an old, chromospherically quiet K0V star that is a near-exact twin of Alpha Centauri B.

This presents quite an interesting situation. HARPS, at La Silla, and the AAT in Australia are the only instruments in the world that currently could conceivably be used to make the detection. The APF telescope can’t see the Southern Sky, and the wider astronomical community would never allow one of the VLTs to be commandered for an all-out effort on one charismatic system. Furthermore, both HARPS and AAT are currently hard at work on large-scale radial velocity surveys. This means that [1] we’re unlikely to get scooped, and [2] we’ll have to build a special-purpose telescope if we want to explore the Alpha Centauri planetary system.

Eugenio and Aaron and I have begun a detailed feasibility study of a dedicated, state-of-the-art radial velocity campaign on Alpha Centauri B. So far, the results have been encouraging — nothing resembling a showstopper has turned up yet. The purpose of the next few posts is to report what we’ve learned so far. We’ll be posting data files so that interested readers can replicate (and if they want to, extend) our work using either their own routines or the downloadable systemic console.

We’re extremely lucky to have a metal-rich KOV dwarf star lying just 1.3 parsecs away. Alpha Centauri B is roughly 100 times brighter than any other equally desirable radial velocity candidate star. The presence of Alpha Centauri A on its 80 year orbit, however, poses a complication for the fitting procedure of radial velocities from B. This problem seems readily solvable, however, and we’ll examine it in much more depth in one of the upcoming posts. Here, we’ll assume that the long-term large-amplitude radial velocity signal arising from A has been correctly filtered out of the data.

Our first step, then, was to invent some terrestrial planetary systems. Our systems have been produced with a Wetherill-Chambers method accretion code, and are both dynamically stable, and organically farmed in the Alpha Centauri AB environment.

Once a terrestrial planetary system has been created, it is observed with Eugenio’s TAC code. The TAC code is provided with a location on Earth (we’re producing synthetic data from telescopes located at La Silla and at the South Pole), and the position of the target star on the sky. Based on the readout time for the HARPS spectrograph, we assume an observing cadence of 200 seconds. Radial velocities are obtained whenever (1) the simulated weather is clear, (2) the Sun is more than 102 degrees from the zenith, and (3) Alpha Centauri is at less than 2.5 air masses. With its declination of -60 degrees, Alpha Centauri is circumpolar at La Silla, which significantly improves its overall observability. Observational errors are drawn from a normal distribution implied by the RMS residuals to the HD 69830 fit. We’re proactively aware that this is only a first approximation. (Over at my other job, we’ve been spending a fair amount of time thinking about autoregressive conditional heteroskedasticity.)

After running the TAC code, we find that 96,464 radial velocities are obtained in a trial five-year observing session. That’s one helluva RV data set. The complete time series has been added to the datafiles directory of the downloadable systemic console. You can access it by selecting “other” from the bottom of the system menu and opening the file AlphaCenB_Y5.sys. Note that because of the large size of this data set, the console function will be very slow. Patience is required. If you want to use your own software, the time series is AlphaCenB_Y5.vels.

When 96,076 velocities are all displayed in the data window, the resulting plot shows the yearly modulation of observability:

If you zoom to the highest time resolution, you can see the blocks of radial velocities obtained on successive nights:

The periodogram says it all:

The peak at 351 days corresponds to a half Earth-mass planet. The three neighboring peaks correspond to smaller planets having masses on the order of Mars.

Aaron tweaked the code for the folding window so that large datasets can be usefully manipulated. When the data is folded at 351 days, the periodicity is ever so faintly visible. Thanks to Joseph Fourier, however, the planets are clearly, unmistakeably detectable in the periodogram.

In the upcoming posts, we’ll talk in detail about the results of fitting to this data, and how the fit compares to the actual planetary system under observation. We’ll also look at modeling systematic error, non-gaussian noise, uncertainty, biases and so forth. These complications will inevitably erode the size of the peak in the above periodogram. Then we’ll implement a double-blind experiment on a set of 10 individual time series. Then we’ll talk about the confounding factors introduced by the binary companion. Then we’ll talk strategy.

Back in March, I wrote about how the systemic console could be used to locate a tentative second planet orbiting 51 Peg. The power in the residuals periodogram, and an extensive Monte-Carlo analysis show strong evidence for a Saturn-mass world in a habitable-zone orbit. I was thus quite excited on Saturday when the California-Carnegie group released a heavy-hitting catalog paper that includes (among many other interesting things) 256 re-analyzed Lick Observatory velocities for 51 Peg. These velocity data points are of high quality, with generally small errors.

After using the console to fit out 51 Peg b, the residuals periodogram for all the data still shows a strong secondary peak. The period has shifted, however, to 345 days, and the relative power has declined somewhat with the addition of the new Lick data:

When the residuals are folded at a 345 day period, there’s a visible hint of periodicity:

Note, however, that there are a huge number of points in the right-hand part of the diagram. These are primarily points taken at Lick during the Fall of 1995, just after the Swiss discovery announcement. There’s a scarcity of points near the middle of the diagram resulting from the near-match between 345 days and one year. The star can’t be observed when it’s near the Sun in the sky.

Using the console to get a 2-planet fit, the preferred mass of “c” is still about a Saturn mass, but the eccentricity has increased to e=0.59. This is a worrisome development. The system is still perfectly stable, and an eccentricity of 0.59 is certainly within the range exhibited by exoplanets. The problem is that the fit has adjusted itself so that the pronounced radial velocity swings of the outer planet tend to occur where there is no data.

The Lick data therefore seem to have taken some of the air out of 51 Peg c. It would be good to see whether there’s any hint at all of planet “c” if we consider just the Lick velocities. This is easy to do. Just delete the line containing “51peg_ELODIE04.vels” from the 51peg_B06L.sys file in the datafiles directory that comes with the downloadable console.

Don’t worry if you screw something up. You can toss out a busted console and download a fresh one because they’re free! The only problem with downloading a lot of consoles is that the Internet is not something you just dump something on. It’s not a truck. It’s a series of tubes. And if you don’t understand those tubes can be filled, and if they are filled, when you try to get your new console out, it gets in line and its going to be delayed by anyone that puts into that tube enormous amounts of Internets, enormous numbers of consoles.

When one looks at the Lick data set by itself, the one-planet fit has a required jitter of only 3.6 m/s. This is pretty well in line with the level of astrophysical non-planet noise that would be expected from a star like 51 Peg. There’s really nothing in the Lick-only data to suggest that the model needs a second planet. The strong 350-day residual periodicity is thus present in the Swiss data, but not in the Lick data. In particular, the residuals periodogram to the one-planet fit shows absolutely nothing of interest near 350 days

In my post from last March, I wrote that:

It could very well be true that the 356. day periodicity is due to a systematic effect in the data that has nothing to do with a planet. I would not be surprised at all if this is the real explanation. That is, the observational results might be subject to a small seasonally dependent effect produced by the telescope — as a straw-man example, the temperature of the instrument might have a slight effect on the measured radial velocity. The variation could also have an astrophysical source that has nothing to do with a second planet.

The latest data release from the Lick group certainly seems to favor this conclusion. A telescope-dependent effect is highly unlikely to be present in two independent data sets. The fact that the periodicity is present in only one set of data points toward and Earth-based, rather than a 51 Peg c-based explanation for the periodicity. In the older post, I also wrote that:

As a working scientist, I’ve found that about 95 to 97% of the seemingly publishable “discoveries” that I stumble across end up being spurious for one reason or another. Hope springs eternal, but it always pays to temper one’s enthusiasm!

Continues to be true.

Saturday was an epic day for the radial velocity consuming public. Paul Butler and the California-Carnegie planet search team published a blockbuster paper in the Astrophysical Journal, and it looks like the first weekend’s gross is gonna be huge. The paper announces the detections of five new planets, and publishes re-analyzed and (in many cases) greatly expanded radial velocity data sets for no less than eighty three planet-bearing stars. The velocities are all available in machine-readable tabular form. No dextering, no unfolding, no typing, no postscript extractions. As an added plus, the paper also provides the latest estimates for jitter, mass, metallicity, and vsin(i) for all of the tabulated stars.

Needless to say, there’s a great deal of interesting data in this compendium. The updated 55 Cancri velocities, for example, should aid the characterization of a fully self-consistent model of that system. The slew of fresh velocities will be of great help in constraining the uncertainties in the transit predictions for planet bearing stars.

I dug right in to see how the 51 Peg system (described in a series of posts detailed here) is holding up. There are now 256 new and updated velocities from Lick Observatory to complement the 153 published Swiss velocities. The time-series shows a well-sampled mixture of long-term cadence and intensive monitoring.

Needless to say, 51 Peg b is still present with a vengeance. The power spectrum of the combined 409-point data set has a certain overwhelming 4.231 day character:

The data set phased at 4.2307 days shows a very nice sinusoid. About a thousand orbits have been folded down to make this plot:

So how does 51 Peg “c” fare in the new dataset? I’ll post an analysis tomorrow. If you’re impatient, though, you can use the downloadable console to investigate what the new data has to say.

Image Source.

Several weeks ago, the planet count at the extrasolar planets encyclopedia notched up by one with the announcement by Artie Hatzes and his collaborators that Pollux (Beta Geminorum) is accompanied by a ~2-3 Jupiter mass planet on a 590 day orbit. This world has been under construction for a long time. The first published radial velocity data point for the star dates back to Nov. 15th, 1980, and Hatzes et al. brought 55 new radial velocities to the table to seal the detection. The planet was independently confirmed by Reffert et al., who (in a preprint posted July 7th) deliver an additional 80 high-precision velocities.

All told, there are now seven published datasets, and all are available on both the on-line and downloadable versions of the console. When folded together, at a 593 (1.6 year) period, a full quarter century’s worth of radial velocity data show the planet quite nicely.

After the Sun, Pollux is the 17th brightest star in the sky. It’s prominently visible both because it’s close (34 light years) and also because it’s an intrinsically bright K0III giant star. Pollux is about 1.9 times more massive than the Sun, and is already coming to the end of its life. It has left the main sequence, and is beginning its long trek up the red giant branch of the Hertzsprung Russell diagram.

In last Saturday’s post, I wrote about predictions of the core-accretion hypothesis with respect to planet formation. The ability to quickly build a Jovian-mass planet depends on the surface density of solid material in the protostellar disk. A lot of solids leads to rapid buildup of cores, and hence the ability of planets to achieve rapid gas accretion before the protostellar disk dissipates. (The spiral wave-induced evolution of marginally gravitationally stable disks leads one to expect that disk masses will correlate with the masses of the central stars, see this paper for a lot more discussion.) All other factors being equal, one expects that Jupiter-mass planets will be rarer around stars that have significantly less mass than the Sun, and that conversely, Jupiter-mass planets will be more common around planets of somewhat higher mass than the Sun. (Note that for really massive stars, the luminosity of the star itself will rapidly photo-evaporate the disk, which will cause problems for giant planet formation via core accretion).

Unfortunately, it gets increasingly harder to apply the radial velocity detection method to Main Sequence stars that are considerably more massive than the Sun. The higher temperatures of these stars lead to weaker spectral lines. Weaker spectral lines make it hard to get really accurate radial velocities. Higher mass stars also tend to be fast rotaters, which further smears out the lines, and they are often subject to pulsations which can mimic the radial velocity signature of an orbiting planet. Above about 1.3-1.4 solar masses, it thus becomes hard to survey main sequence stars for planets.

Luckily, however, a trick can be used to assess the planet frequency for high-mass stars. As a star that has ~1.5-3 solar masses ends its main-sequence hydrogen burning life, its core begins to contract and its outer layers swell up and cool down. The atmosphere of the star then regains the wealth of spectral lines that can be used to make accurate radial velocity measurments, and hence detect planetary companions. The core accretion theory predicts that planet hunting around such giant stars should be a highly profitable enterprise.

Image source.

Four out of five astrophysicists surveyed recommend the core-accretion theory to those interested in planet formation theories.

Oklo regulars know that I lean toward core-accretion over gravitational instability as an explanation of the dominant mode of planet formation. I think that core-accretion does a superb job of explaining the planet-metallicity connection, and I don’t think that the initial conditions that underlie hydrodynamical calculations that show disk fragmentation are physically realistic.

The key aspect of core-accretion is that it is a threshold phenomenon. If a planetary core reaches a Neptune-like mass of ~10-20 Earth masses while there is still gas in the protoplanetary disk, then it will rapidly accrete that gas, and (in most cases) increase its mass by a factor of ten or more. On the other hand, if a core reaches a Neptune mass after the gas is gone, then the growth will cut off, and the core will end its days as a modest ice giant.

The amount of time that it takes for a core to reach the phase of rapid gas accretion depends sensitively on the amount of solid material that is available in the disk in the form of planetesimals. A disk with a high surface density of solids is capable of rapidly assembling a core, thereby forming a Jovian-mass gas giant quite quickly. Recent simulations suggest that an average protostellar disk surrounding a star of solar metallicity will lie right at the threshold of being able to manufacture a Jovian planet. This result gives a satisfying mesh with the observations. As stellar metallicity exceeds solar, the fraction of stars with detectable Jovian-mass planets increases very rapidly. Disks that form their Jovian planets early-on are better able to migrate them into the terrestrial region where they can easily be detected. Stars of solar mass and metallicity will tend to have giant planets that remained, like Jupiter, more or less where they formed. Stars with subsolar metallicity will rarely be accompanied by Jupiter-mass planets.

In addition to explaining the planet-metallicity connection, core-accretion provides a number of other testable predictions. Our simulations suggest that a growing planet orbiting a star with 40 percent of the Sun’s mass will require more than 10 million years to “go Jovian.” After 10 million years, however, the gas in most protostellar disks is long gone. The core-accretion theory predicts, therefore, that low-mass red dwarf stars should very often be accompanied by Neptune-mass planets but should almost never have Jupiter-mass companions.

The surface density of solids in a protostellar disk is correlated with metallicity, and some heavy elements are more important than others. Oxygen, for example, in the form of water ice, is of fundamental importance for building cores. At given mass and overall metallicity, therefore, a disk that is naturally rich in oxygen should be better able to form Jupiter-mass planets. Silicon-rich disks too, should have an enhanced capacity for building gas giants.

Everything takes longer than you think it’s going to take.

I thought it would be a relatively straightforward task to collect and assemble all of the published radial velocity data sets together in a uniform format. Turns out (as is often the case) that I was overly optimistic. It’s been a major effort to get an authoritative radial velocity catalog into shape. Eugenio, however, has been extremely persistent and methodical, and the job is now essentially done. Datasets for 155 stars accompanied by published planets are now available on (1) the downloadable console, (2) the web-based console, and (3) on the systemic back-end. Many of these data sets are now available in ASCII format for the first time; Eugenio made extensive use of the Dexter applet to extract data from papers in which the radial velocities have been hitherto published only as plotted points.

As far as systems with published planets that are not on the console go, we’re definitely scraping the bottom of the barrel. This morning, Eugenio sent me an update on where he’s at with the last dregs. Basically, there are a dozen planet-bearing stars that still need to be added to the console. In most of these cases, we either can’t find any listing of data, or the data is available only in the form of a phased plot that can’t be disentangled:

0. HD114762: the two references I was able to find have (or appear to have) no uncertainties since the “planet” is likely a brown dwarf, I’ll still skip this one for now.

1. HD41004: hierarchical quad system: A(K star)-2.5 M_J/B(M star)-BD. Swiss give table of velocities for both A and B but no uncertainties.

2. Tau Bootes: This still looks like a hopeless cause.

3. GL86: phased velocities only.

4. HD11964: missing data?

5. HD122430: missing data?

6. HD196885: missing data?

7. HD34445: missing data?

8. HD59686: Only announced at American Astronomical Society Meeting; Still listed as Mitchell et al. 2004, ApJ, submitted.

9. HD73256: phased velocities only (do not confuse with HD 73526).

10. HD89307: missing data?

11. Tres-1: Phased velocities only.

Without a doubt, there are some easy as-yet unannounced and as-yet unpublished planets ripe for the picking off of the console menu. Two months ago, I wrote a series of posts showing that 51 Peg almost certainly has a second Saturn-mass planet in a habitable orbit. This planet was uncovered after only a few minutes of work on the console. This afternoon, Eugenio and I looked at four or five data-sets (basically at random) and found a nice planet candidate that we’re planning to write up in one of this week’s posts.

There are more than 6 billion people on Earth, and only a handful of them have discovered a planet. Here’s your chance.

Remember the scene in Blade Runner in which Deckard successively zooms and enhances a digitized photograph found at a crime scene? In 1982, it seemed to epitomize the fashionably sleek high tech, and it left a strong impression on me.

In this post from last February, I wrote about the protostellar disks in Orion that were imaged by the Hubble Space Telescope in the mid-1990s. One of these disks has achieved nearly iconic status (at least among those of us who give talks on planet formation). The following image shows it viewed edge-on and in silhouette against a background of glowing nebular gas. Only a faint smudge of red hints at the central star embedded within the disk.

This disk is roughly 17 times larger than the orbit of Neptune. It’s also considerably larger than the orbits of 2003 UB-313 and Sedna, which I’ve integrated and placed on top of the image for comparison.

The Hubble press release that accompanies the disk image highlights a number of different protostellar disks, or “proplyds”, many of which are being strongly photoevaporated by radiation from the nearby high-mass stars of the Trapezium cluster. This region is faintly visible to the naked eye as the unresolved middle “star” of Orion’s sword:

A growing body of evidence suggests that our own solar system may well have formed in a similarly disruptive environment. Analysis of meteorites shows evidence that radioactive atoms with short half-lives (in particular, Aluminum 26) freshly ejected from a nearby supernova in the birth cluster may have been incorporated into our solar system’s protostellar disk. The scattered orbits of bodies such as Sedna also hint that the solar system may have had a close encounter with another star at an early time in its history. Close encounters only occur in a dense stellar environment.

On the Hubble press release page, there is a 22.97 MB Tif image of the entire Trapezium region. When the full image is displayed at laptop-screen resolution, it isn’t clear where the protostellar disks actually are. If, however, you slog through the full download and open the image in a program like Photoshop, then, like Deckard, you can zoom in with successively higher resolution to find the disks shown in the press release. The resolution in the 23 MB Tif image is not the full resolution provided by the actual mosaic of images, but it’s high enough to enable discovery of a lot of detail. A leisurely exploration of the image with pan and the zoom controls gives an amazing sense of the overall structure of the stellar nursery. The three-dimensionality of the cluster is easier to visualize. You can sense that the disk is suspended in empty space, in a slow, arcing free-fall through the cluster, making it somehow easier to grasp that this is an image of new worlds in the process of creation.

Literally every astronomical worker, amateur and professional alike, who has carried out a serious photometric search for planetary transits is familiar with the symptoms of at least one of the two common strains of transit fever.

In the egressia strain, one observes a photometric time-series in which a transit seems to be ending just as the observations are starting:

The ingressia strain has different symptoms, but is equally infectious. Near the end (or sometimes near the middle of an observing session, it appears from the time-series photometry that a planet is entering transit. In the most common form of the syndrome, the star generally either descends into the murk of high air-mass or is overtaken by dawn before the transit has finished:

A third, somewhat rarer variety of the fever, known as rossiteria, has also been described. This strain is most commonly contracted by theorists; one finds radial velocities in the literature taken during a transit window which seem to show clear evidence of the Rossiter-McLaughlin effect:

I came down with my first serious case of transit fever (later diagnosed as egressia with complications due to rossiteria) nearly four years ago. The symptoms were brought on by HD 217107, the first candidate planet-bearing star observed by the transitsearch.org network. On the night of August 6th, 2002, photometric data was sent by both an observer in Pleasanton California, and from the KAIT automatic telescope at Lick Observatory. The fits to the radial velocity data indicated that the HD 217107 b transit window was scheduled to begin at 2:40 am PDT, and amazingly, both data sets showed a photometric dip right at the predicted time. To seemingly clinch the case, Debra Fisher also used the Lick 3-meter telescope to obtain five radial velocity measurements of HD217107 during and before the predicted transit ingress. When the spectra were analyzed, the velocities came back with a pattern consistent with the expected Rossiter-McLaughlin effect! [The data is available at this webpage]

To say that I was excited was an understatement. It was my first time out. I had no natural resistance. Tim Castellano, Debra, and I all came down with a full-blown case of transit fever. The period of HD 217107b is 7.127 days, which means that successive transits are spaced one week and 3 hours apart. Tim and I were on the verge of flying with a Meade LX-200 to Hawaii to observe the Aug. 13th transit from the parking lot of the Keck Observatory (That plan, fortunately, was canceled by a combination of high ticket prices and Joe Miller, the cooler-headed then-director of UCO/Lick).

By mid-September, observers in the Canary islands obtained a data set which clearly shows that HD 217107b does not transit. Huge disappointment. At that time, HD 209458 b was still the only known transiting extrasolar planet, and so the second detection would have been a very big deal.

With hindsight, having been innoculated against the transit fever, it’s clear that the transit interpretation was ambiguous in all three of our data sets. For example, in the case of the radial velocities from the Lick 3-meter, a new CCD detector for the spectrograph had just been installed, and so the overall zero point of the velocities relative to the predicted radial velocity curve was a free parameter. If the points are all moved down by 10-15 m/s, then the transit feature turns into ordinary scatter in the data. Likewise, with the photometric data, it was clear in retrospect that systematic effects were at work.

The transit detection problem is tough in part because it’s extraordinarily easy for systematic effects to seemingly conspire to produce an apparent signal. I would not feel confident in announcing a transit until I’ve seen multiple full-transit light curves. On the other hand, though, the false alarms play an important role. They get observers out on the sky, and spur the collection of enough data to truly rule out an event. This certainly wound up being the case for GJ 876, HD 168746, and a number of other candidates.

An early case of transit fever was contracted by U. J. J. LeVerrier, the Nineteenth-century French mathematician famous for the dynamical calculations that led to the prediction and subsequent discovery of Neptune. After the Neptune discovery, LeVerrier turned his attention to explaining the precession of Mercury’s perihelion, and found that the effect could be explained by the presence of a small intra-Mercurial planet that he named Vulcan. After a thorough literature search, LeVerrier unearthed five separate observations of the solar-disk transits by this planet, all at times consistent with predictions. In the following 1877 communication to the Monthly Notices of the Royal Astronomical Society, he’s basically saying, how could all those observations (which agree with theory, no less) possibly be wrong?

As everyone now knows, LeVerrier’s Vulcan doesn’t exist. The 43 extra seconds of Mercurian perihelion precession that had bothered LeVerrier so severely are explained by modifications to classical Newtonian gravity by Einstein’s general theory of relativity.

Tristan Guillot and his colleagues have just published a paper, “A correlation between the heavy element content of transiting extrasolar planets and the metallicity of their parent stars” which explores an interesting new hypothesis for resolving the size problem for transiting hot Jupiters.

Readers of oklo.org are well aware that our theoretical understanding of the radii of hot Jupiters isn’t all it could be. For example, the transiting planets TrES-1 and HD 209458 b have very similar masses and surface temperatures, and yet HD 209458 b has a radius that is roughly 25% larger than TrES-1’s. In a previous post (see also this post), we outlined some of the hypotheses that might explain this discrepancy in radii.

Guillot et al’s idea is that the mass of a planetary core is a very steep function of stellar metallicity. That is, doubling the metallicity of the parent star leads to a 5-10 fold increase in the amount of mass contained in the cores of any short-period planets in orbit around the star. Larger core masses lead to smaller overall planetary radii at given mass, and so, in the Guillot et al. picture, planets orbiting metal-rich stars will, in general, be considerably smaller than planets of equal mass orbiting metal poor stars.

They present the following graph to support their hypothesis. It shows the difference in observed planetary size from the baseline theoretical expectation on the y-axis, and the parent star metallicity on the x-axis. They note (and the eye notes) that there is a trend in the diagram; planets orbiting metal-rich stars (as exemplified dramatically by HD 149026b) tend to be smaller than predicted, and planets orbiting stars of near-solar metallicity (e.g. HD 209458b tend to be larger than predicted.

Figure adapted from Guillot et al. 2006.

But is this correlation really present? To get a qualitative sense of whether it is or not, I took the planetary radius and parent star metallicity values for the 9 transiting planets in the Guillot et al plot and redrew them from their implied uncertainty distributions to make alternate, statistically equivalent versions of the plot in their paper. I also made control plots in which I assumed that there is no underlying radius-metallicity effect, only noise from measurement uncertainty. I made four plots of the first variety, and four plots of the second control variety. They are shown below. Can you identify which plots are the control plots?

Image source: designonline.se

Today’s astro-ph mailing contains a paper by McCollough et al. detailing the discovery of a new transiting planet orbiting a sun-like star lying in Corona Borealis. Dubbed “XO-1” — easily the coolest name yet for an exoplanet– this world has a year that lasts 3.941534 days, and a mass roughly 90% that of Jupiter. The temperature at the scalding, toxic, vortex-riddled cloudtops should be a torrid 1083 K. It ain’t no habitable world, folks, but as the fifth extrasolar planet found to transit a relatively bright (V=11) parent star, it’s big news nonetheless. Bright parent stars are great for planetary characterization, because they make detailed follow-up a lot easier to carry out.

The preliminary indications are that this planet, which has a measured radius of 1.3 plus or minus 0.1 Jovian radii, is somewhat larger than expected. Our theoretical models predict that XO-1 should have a radius of 1.05 Rjup if there’s a 20-Earth mass core, and 1.11 Rjup if the planet is core-free. If the large radius is confirmed, then we’ll be faced with the same radius problem that we’re facing with HD 209458 b (as explained in this oklo post from Dec. 2005). An interesting clue may be provided by the fact that the XO-1 parent star, like HD 209458 is not particularly metal rich.

The paper lists four amateur observers as co-authors. Three of them, Tonny Vanmunster, Ron Bissinger, and Bruce Gary, are long-time transitsearch.org participants. P.J. Howell is the fourth amateur co-author on the list. As usual, the photometry that these guys are getting is excellent:

Figure adapted from the McCollough et al. paper.

The paper describes in detail how the photometry from the amateur observers was able to effectively leverage and speed up the discovery, and how the ready-made network provided by the small-telescope observers eliminated the need for an expensive robotic follow-up observatory.

Sounds to me like it’s time to crack open a bottle of Hennessey XO. Congratulations all around, guys!

The published census of extrasolar planets grew by 1.57% today, with the Geneva Extrasolar Planet Search Teams’s announcement in the journal Nature that trois Neptunes orbit the nearby sunlike star HD 69830.

There are a number of reasons why the Swiss team’s paper is interesting. The HD 69830 system seems tailor-made for a detailed dissection with the Systemic Console. This dissection can get the oklo user-base up and running with the new beta version of the Systemic Backend. In addition, the configuration of this system has some fascinating consequences for the theory of planetary formation and evolution. It is definitely worth digging into this story for the next few posts.

First, the nuts and bolts of the announcement. The three planets, named — you guessed it — HD 69830 “b”, “c” and “d”, clock in with Msin(i) estimates of 10.2, 11.8, and 18.1 Earth masses respectively. Assuming that the system is co-planar and is being viewed close to edge-on, this places b and c squarely in the mysterious planetary mass range that falls between the ice giants (e.g. Uranus and Neptune) and the familiar terrestrial planets. Planet d is somewhat more massive, with a net bulk almost exactly equal to that of Neptune. [It’s important to remember that the inclination of any given planetary systems is more likely to be viewed edge-on (i=90 deg) than pole-on (i=0 deg) for the same reason that the Earth has more real-estate within a hundred miles of the equator than within a hundred miles of the poles.]

The published orbits of HD 69830’s planets are, however, distinctly unreminiscent of Uranus and Neptune. HD 69830 b orbits in 8.667 days, c orbits in 31.6 days, and d circles the star once every 197 days. All three have modest eccentricities.

For kicks, here’s a .wav format sound file which turns the reflex radial velocity waveform from the star into an audio signal. [Note: you can use the downloadable version of the Systemic Console to produce an audio representation of any planetary system, see this post from last week for the details.] In the sample file for HD 69830, I’ve pitched the innermost planet to a rather piercing 3 octaves above A 440. The period difference betwen the inner and outer planets leads to ~4.5 octaves of pitch difference. Planet d’s contribution can be heard as a bass drone about 2 octaves below middle C.

Indeed, the “chord” produced by these three new planets sounds terrible. Badly out of tune, to be precise. This immediately tells us that ther are no strong mean-motion resonances in the published configuration of planets. The human ear-brain system is good at on-the-fly calculation of whether the mean-motion resonance arguments are in circulation or libration. For example, this .wav file corresponds to a (synthetic) planetary system that is participating in several mean-motion resonances. When compared to HD 69830, it sounds awfully good.

As soon as I saw those periods — 8.667 days, 31 days, 197 days — I raced ahead through the paper draft to see if a photometric check for transits was carried out already by the discovery team… Excellent! There’s no mention anywhere in the paper of an attempt at a photometric search for transits of the planets. This will lend a challenging, high-profile opportunity to transitsearch.org. My guess is that the Geneva team was quite eager to get these three worlds out the door, and did not want to hold up the show with an exhaustive photometric check. There’s a real danger that you’ll wind up getting scooped if you cross all your photometric T’s before publishing your radial velocity-detected planets.

In the next post, we’ll look at what the radial velocities have to say.

Image source: NASA Visible Earth

For most people, a mention of Finland brings to mind snow, lakes, conifers, Nokia and Linus Torvalds. Here at oklo.org, however, we hear Finland and we think of top-drawer amateur astronomers. In 2000, the Finn Arto Oksanen was the first amateur to observe the HD 209458 b transit. More recently, both Oksanen and countryman Pertti Paakkonen have contributed a number of observations of TrES-1 and other stars to transitsearch.org. Finnish IP addresses are consistently among the top traffic generators on the Systemic Console.

Now, Veli-Pekka Hentunen and the Warkauden Kassiopeia ry (the Astronomical Association of Varkaus) join the ranks, with a fine observation of last week’s TrES-1 planetary transit. Their lightcurve, shown just below, was obtained with a Meade 12-inch LX200 telescope and a cooled SBIG ST8-XME CCD camera:

The photometry shows a tantalizing hint of the starspot activity that is known to characterize TrES-1. A more detailed analysis will be needed to see whether the small in-transit bump has statistical significance. As discussed in a previous post, HST has shown that starspot activity on TrES-1 can produce stange-looking features in the light curve:

Hentunen and his colleagues have constructed a very impressive facility at a dark (and from the look of things cold) site in the Finnish interior. Information in English regarding both their observatory and their scientific work can be found at the Taurus Hill Observatory Website.

Taurus Hill Observatory

Hentunen and friends will be able to kick back and take it easy for the next few months because it doesn’t really get dark at their location during the Summer. In the Winter, however, when the weather is clear, they’ll have the opportunity to make long-duration time-series photometric observations of stars near the polar cap. For example, they’ll be in awesome position to snag oklo.org favorite HD 80606 (+50 deg declination) during its Dec. 26, 2006 transit opportunity.

Mt. Hamilton Main Building at dusk, photo by Laurie Hatch

Last weekend, I wrote a post about the planet — stellar metallicity connection, and the small-scale Doppler-velocity planet search that Debra Fischer and I carried out at Lick Observatory as a precursor to the currently ongoing N2K survey.

We had eighteen candidate stars on our list, ordered in terms of increasing uvby-determined metallicity, and to keep track of them, we named them after heavy metal bands:

We started observations in September 2000. Several times a month, I would drive up the twisting road to Mt. Hamilton, windows open to the dry chaparral air. At the mountaintop, the observatory buildings on the ridgeline sleep in the hot quiet afternoon sunlight. The scene seems lost in a slower, less hectic time. E-mail, phone messages, deadlines, all-hands division meetings are far away and inconsequential.

After standing and staring for a long time at the sweeping expanse of ridges, mountains and valleys spreading out in all directions, I would pick up a set of keys, a flashlight, a thermous of coffee, and a night lunch from the diner, and walk to the dome.

Once inside, the lost-in-time feeling immediately gives way to the scramble to set up for the night. Open the spectrograph, fill the ccd dewar, focus the optics, set up the iodine cell, obtain the thorium-argon calibration and flat-field images, check the telescope pointing, and run through a number of other ordered tasks, all of which are required for a successful night of observations. We were using the then-undersubscribed 1-meter Coude Auxilliary Telescope, which has a certain Rube-Goldberg aspect to its inner workings. It took all (and more) of my clumsy theorist’s mechanical aptitude to make sure that I didn’t skip a step of the complicated set-up procedure.

Eventually, with the sky grading into deep blue twilight and the night’s first target star acquired, the stress associated with the set-up procedure would dissipate. Because of the small size of the telescope, the individual exposures were long and unhurried. I’d take two and sometimes even three separate half-hour exposures. With the shutter open, with starlight streaming through the iodine cell, reflecting off the spectrograph, and landing on the CCD, there was little to do aside from making sure that the autoguider was doing it’s job of keeping the star centered on the spectrograph entrance slit.

After three months, several of the stars were showing tantalizing hints of short-term radial velocity variablility. All of our main-sequence target stars have “F-type” spectra, meaning that they were somewhat more massive (and thus hotter) than the Sun. It was impossible to get the 3-5 m/s precision that can be obtained with slightly cooler solar-type “G” stars. For F-type stars, the metal lines that greatly aid the measurement of precise Doppler shifts are starting to fade, and, because F-type stars are generally quite young, they are often rapidly rotating. Unless the star is being viewed pole-on (which seems to be bad for planet detectibility) stellar rotation broadens the spectral lines and further degrades the velocity precision.

Mötley Crüe, in particular, was exhibiting radial velocity variations that seemed to strongly imply the presence of a short-period planet. This was mildly surprising, given its rather puny, barely super-solar “80’s hair metal” metallicity of [Fe/H] ~ 0.04 dex. We became more and more excited, as each velocity seemed to come in on target:

I have to go and teach a class now, so I’ll pick up the story sometime during the next few days. Also, when I get back from class, I’ll add the velocities shown in the figure above to the Systemic Console under the name “Mötley Crüe”. If you’re interested, you can then do a quick analysis which will show you why Debra and I were getting excited by the velocities.

Last week, we posted Kent Richardson’s light-curve for the nearby red dwarf star GL 581 (Sloan DSS image pictured above). Kent’s photometry was taken during a predicted transit window, and along with data from David Blank and collaborators in Australia, it contributed to rule out the possibility of planetary transits by the red dwarf’s steamy Neptune-mass companion.

Like Marlon Brando in On the Waterfront, GL 581 b “could’ve been a contender”, and connoisseurs of the might-have-been should be sure to read the oklo posts [1,2] that talk about what this planet would have taught us if only it was transiting…

No need to despair, however. There’s a whole slew of candidates on the transitsearch.org candidates list which remain entirely unexplored.

Kent obtained his data with a robotic telescope located at the San Diego Astronomy Association’s dark site at Tierra Del Sol, California, approximately 60 miles east of downtown San Diego:

Kent reports,

There are three major components of the installation: The dome and telescope, the data cabinet, and the satellite antenna. The details of each are as follows:

Dome
Robo Dome by Technical Inovations, Inc.
Meade 8″ LX-200 Classic f/10
Meade f3.3 Focal reducer
SBIG ST-7 CCD camera w/ CFW-8 filter wheel
Lumicon 80mm finder scope w/ Meade DSI Pro camera
Meade 8x50mm finder scope with Logitec web cam

Data cabinet
Compaq Presidio desktop computer
The Sky for telescope control
CCDSoft for ST-7 control
Meade Autostar Suite for DSI control
Logitec Image Studio for web cam control
Digital Dome Works for dome control
Tachyon Networks Inc. satellite network computer

Satellite Antenna
Tachyon Networks Inc. satellite dish

The photo shows the site. The data cabinet has been wrapped to protect it from the winter rains we have been experiencing. You can also see a second pier and electrical outlet which will enable us to install another dome and telescope when funding is available.

We’re definitely looking forward to seeing more data from this rig when the California weather finally improves.

In other news, Transitsearch.org and the American Association of Variable Star Observers have just announced a joint campaign to search for transits of the recently discovered planet orbiting HD 33283. There’s one last, fleeting window of opportunity this season before the star goes behind the Sun: April 26, 10:31 – April 27, 20:22 (UT). Details of the campaign can be found here.

Seize the moment!

51 Peg is an ordinary star in nearly every respect. It is, however, more metal-rich than the Sun by nearly a factor of two, which gives it a metallicity in excess of all but a few percent of the stars in the local galactic neighborhood.

When Astronomers talk about metallicity, they mean the fraction of a star’s mass that is contained in elements that are heavier than hydrogen and helium. To an astronomer, carbon, nitrogen, krypton, and radon are all “metals”. In the Sun, metals comprise a bit less than 2% of the total mass.

Nearly every parent star in the first wave of extrasolar planets (including 55 Cnc, Tau Boo, and Ups And) came in with considerably higher-than-average metallicity, and the strong connection between high stellar metallicity and the detectable presence of an extrasolar planet was on fairly secure footing by 1997. This connection has been quantified very clearly, and is perhaps the most important and dramatic result that come out of the first decade of investigation of extrasolar planets. The figure from Debra Fischer and Jeff Valenti’s recent paper is fast on its way to iconic status:

The planet-metallicity connection is telling us something important about the planet formation process, namely, that the core-accretion mechanism for forming giant planets is correct. (More on this coming up soon.) It also tells us how to efficiently find more planets. If you want to find planets, look at metal-rich stars.

Purists should definitely argue that by skimming the readily detectable planets from metal-rich stars, one is skewing the statistical properties of overall census of planets, while introducing additional systematic trends into a planetary catalog that is already shot through with biases both subtle and overt. “A targeted quick-look Doppler survey of metal-rich stars is the moral equivalent of eating candy for breakfast!”

I fully agree.

There are, however, scientifically compelling and unassailably selfless arguments for why it’s important to locate as many extrasolar planets as quickly as possible. I think the most important reason is that quick-look radial velocity surveys (such as N2K) are the best way to locate planets that transit bright parent stars. Once identified, objects like HD 149026b yield up a simply incredible amount of information.

But I’ll be straight with everyone. I’ve always wanted to be in on the discovery of new planets.

In 2000, I worked with Debra Fischer on a small-scale survey of metal-rich stars that wound up being the precursor project to N2K. It’s hard to imagine that the year 2000 once seemed like the distant future. At that time, it appeared that in addition to the planet-metallicity correlation, that there was also a planet-stellar mass correlation. The census of short-period planets known in 2000 was noticeably concentrated around stars somewhat more massive than the Sun, that is, early G and late F type stars.

I therefore drew up a list of 20 stars that we could observe using the then-undersubscribed CAT (Coude Auxilliary Telescope) on Mt. Hamilton. The criteria for inclusion were that a candidate star be (1) at least moderately metal-rich, (2) bright, (3) more massive than the Sun, and (4) not known to be on any other radial velocity survey lists. We needed stars brighter than about magnitude 6 because the CAT telescope has a mirror diameter of only one meter.

Rapidly, it became clear that Henry Draper catalog numbers are not a very effective way to mentally keep track of the stars. There was confusion, for example, one night when I asked Debra if I could add “HD Twenty –Six Seven Five” to the evening’s observing list. She thought that I meant “HD 2675 “(which had already set), when I actually had “HD 20675” in mind. We eventually realized that since we needed to keep both the stars and their metallicities straight, the best course of action would be to name the stars after heavy metal bands. At the high-metallicity end, we drew on speed-metal and death-metal outfits (e.g. Slayer, Sepultura), wheras at the lower-metallicity end, we resorted to hair-metal and even glam-metal bands (i.e. Warrant, Skid Row). Here’s our final list of stars in the survey:

(The original twenty star survey was reduced to eighteen after AC-DC turned out to be a spectroscopic binary, and W.A.S.P. turned out to be chromospherically active.)

In astronomically inclined households, the first wave of extrasolar planets to be discovered, 51 Peg, 70 Vir, Ups And, Tau Boo, are all still household names.

With the later additions to the census, however, such as HD 33283 b et al., even the discoverers can have a hard time keeping the names in mind. In part, this is because it’s tough to keep a bunch of random Henry Draper Catalog numbers at the tip of the tongue. It’s also because planets #184, #185, and #186 don’t quite pack the same panache as planets #2, #3, and #4. Maybe it’s time to start naming these planets?

It’s easy to get the impression that the rate of discovery of extrasolar planets is increasing rapidly with time. Interestingly, however, this hasn’t been the case recently. The planet discovery rate peaked in 2002, with 34 planets detected, and the rate over the last four years has been flat, at about 25 planets per year. (The present year has brought us six new worlds during the span between New Years Day and Earth Day):

The detection rate has flattened for several reasons. After a decade’s worth of planet discoveries, the Doppler radial velocity method remains the most productive technique. The radial velocity method is most efficient when one has a bright parent star. Most of the suitable stars with V magnitudes brighter than 8 are already on the Doppler Surveys. The readily detectable short-period planets orbiting these stars have mostly been found. The much longer orbital periods of the outer planets mean that one must be increasingly patient as one waits for new discoveries from venerable stars. Indeed, the detection rate of planets over the past several years would be even lower, were it not for targeted Doppler surveys such as N2K, which are specifically designed to find new planets quickly by surveying metal-rich stars.

The transit and microlensing methods have a lot of promise for upping the planet detection rate, but to date, very few planets have been discovered with these techniques. In an upcoming post, we’ll look in more detail at the reasons why this has been the case.

It’s interesting to compare the planet detection rate with the history of minor planet detections. Ceres, the first minor planet to be discovered, was found in 1801, followed by Pallas in 1802, Juno in 1804, and Vesta in 1807. A thirty-eight year gap followed, until the discovery of Astraea in 1845. The 100th asteroid, Hecate, was found in Ann Arbor Michigan in 1868, and asteroid #188 (equal to the number of extrasolar planets currently known) Minippe was found ten years later in 1878. The rate has increased rapidly since then. As of last November, there were 120,437 numbered asteroids:

I think it will take about 15 more years to find 120,437 planets.

As I’ve mentioned earlier, Jean Schneider’s authoritative Extrasolar Planets Encyclopedia has introduced a slick .php-based approach that’s keeping the systemic team on their toes. At Schneider’s site, one can interactively produce correlation diagrams for the known extrasolar planets. As more planets are discovered, these diagrams (for example the a-e plot and the Msin(i)-a plot) are beginning to show a fascinating richness of detail.

The inclusion of date of discovery as one of the plottable parameters attracted my attention.

For example, a plot of the Log of the planetary period versus discovery date shows hints of interesting structure:

Over the past few years, the majority of newly detected planets can be divided into a population with P<10 days (the hot Jupiters), and a population with P>200 days (the eccentric giants). There is a statistically significant gap in the period distribution in the intermediate-period regime. This gap tells us something significant about the planet formation process. My interpretation is that the migration process is not readily halted until a planet reaches the region of the disk where the dyanamics of the protostellar disk gas are subject to the laws of ideal MHD. More on that later.

From a more practical standpoint, the paucity of intermediate period planets has made it tough going for the transitsearch.org collaboration. When planetary periods are less than about 10 days, the discovery team is usually able to complete a photometric search for transits before the planet is publicly announced. When a planetary period exceeds 200 days, it’s generally hopeless to mount an exhaustive transit search, even with a distributed network.

You’ll get another very interesting diagram if you plot the Log of the planetary mass as a function of discovery date. As usual, I’ve redone the axes and annotations with Adobe Illustrator to get that familiar oklo.org look-n-feel:

In this log-linear space, the lower envelope of detected planet mass is a linear function of time. This allows for an easy extrapolation to estimate the discovery date of the first Earth-mass planet orbiting a nearby main-sequence star…

Yesterday, John Johnson and the California-Carnegie Planet Search Team posted an astro-ph preprint announcing the discovery of three new extrasolar planets. All three radial velocity data sets have been added to the Systemic Console, and the transit predictions have been placed on the transitsearch.org candidates list.

HD 86081 b has an orbital period of 2.1375 days, which makes it a long-sought “missing link” between the weird, ultra-short period planets discovered by the OGLE survey and familiar hot Jupiters such as HD 209458 b and 51 Peg. HD 86081 b has been checked photometrically for transits, but unfortunately, they don’t occur. Because HD 86081 b orbits so close to its parent star, the a-priori transit probability was a healthy 17%.

HD 224693 b and HD 33283 b have longer periods of 26.73d and 18.179d, respectively. The parent stars are excellent targets for the transitsearch.org project, as neither one has been monitored photometrically during the centers of the transit windows. The a-priori transit probability for HD 33283 b is an impressive 6.2%, whereas HD 224693 tosses in a 3.2% chance. That’s a 9.4% chance that a small-telescope observer will be a world-famous astronomical hero sometime during the next year…

Dust off those CCD cameras!

Yesterday’s post talked about how Young, Binzel and Crane (2001) used high-cadence photometric observations of Charon transiting the disk of Pluto to construct a two-color image of Pluto’s surface. Transiting extrasolar planets can be employed in a similar way to create an image of the strip of stellar surface that lies beneath the path of an occulting planetary disk. Resolution-wise, this procedure is the effective equivalent of a satellite in low-Earth orbit making a detailed image of a stripe across a sand grain sitting on the Earth’s surface.

In 2001, Tim Brown and his collaborators used the STIS spectrograph on Hubble Space Telescope to obtain what has become an iconic composite light curve of the HD 209458 b transit. It’s probably fair to say that the majority of talks given by astronomers on the general topic of extrasolar planets have a powerpoint slide that shows the Brown et al. data. (The astro-ph version of their article is here).

Here’s a figure (done in Adobe Illustrator, like all of the other www.oklo.org diagrams) that shows their data:

The Brown et al. light curve contains 684 time samples spaced at an average cadence of 80 seconds with a relative precision of one part in 10,000 per photometric data point. (This photometric accuracy is easily good enough to detect the transit of an Earth-sized planet across the face of a Solar-size star if one knew when and where to look.) Because HST can only observe for about half of its 96.5 minute orbit, and because the transit lasts 184.25 minutes, the light curve was obtained by stitching together photometry from groups of observations obtained on four separate transits that took place between April 25 and May 13, 2000.

An interesting feature of the above diagram is that the transit light curve does not have a flat bottom. This results from brightness variations on the surface of the star itself. Stellar disks display a phenomenon known as limb darkening. If you can resolve a star (as one effectively does when one obtains a photometric light curve of a transit) you see that the center of the stellar disk is brighter than the edges. This effect occurs because when one looks at the stellar limb, the line of sight samples higher, cooler layers of the stellar atmosphere. When one looks straight at the middle of the star, one is seeing further in, to deeper, hotter layers. For a star like the Sun or HD 209458, this effect is quite significant. The intensity at the limb is only about 40% as much as that at the center of the stellar disk. The curved bottom of the time-series, then, could be inverted and processed to construct an image of the surface of the star beneath the planet. Additionally, if the transit is observed through different color filters, then one can build a colored image of the stellar strip.

More recently, Brown and Company have made similar HST observations of the TrES-1 transit. Their light curve in this case shows a bizarre uptick, which causes the transit to resemble a one-toothed grin:

The interpretation of this feature is that the planet covered up a starspot as it traversed the face of the star. Starspots — that is, sunspots on other stars — are cooler, and hence dimmer than their surroundings. When a starspot is occulted by the planet, the fraction of blocked starlight decreases. Photometric light curves really do give us an image of a strip of the stellar surface.

For those who prefer not to shave with Ockham’s razor, there’s a second, rather more exotic interpretation of the TrES-1 transit light curve. It’s possible (although highly unlikely!) that a second, longer-period, planet was also transiting TrES-1 at the time when the uptick in the light curve was recorded, and that the inner (known) planet happened to pass underneath the outer planet, as seen from Earth. According to Tim Brown (during a talk I heard him give in Japan) this model, while crazy, does just as good a job of fitting the photometric data!

TrES-1 is an 11.8 magnitude star, and the transits are thus highly suitable for measurement by amateur astronomers using the technique of differential aperture photometry. On the transitsearch.org website, there’s an extensive discussion of amateur observations that were made in the months following the discovery of the transit by Alonso et al. Many of these amateur light curves show strange asymmetric features. It’s likely that they were also observing the planet crossing over starspots. If this was indeed the case, then the 2-planet interpretation of the “tooth” can be safely ruled out.

I should emphasize that transit observations using HST are of blockbuster-level scientific value. The exquisite HST photometry allows a very accurate measurement of the planetary radius, which in turn puts strong constraints on our theoretical models of the planetary interior (see this post for more information). The transit also strongly constrains the elements of the planetary orbit, and the color-dependence of the light curves permits the measurement of atmospheric constituents such as sodium and carbon monoxide.

The above diagram for the TrES-1 transits is adapted from a review article entitled, When Extrasolar Planets Transit their Parent Stars that I co-authored with Dave Charbonneau (Harvard University), Tim Brown (The High Altitude Observatory), and Adam Burrows (The University of Arizona). It will be published in the forthcoming Protostars and Planets V Conference Proceedings.

Here at www.oklo.org we strive to keep things on the positive tip, but I do have one disappointing piece of news to report. I was a Co-I on Tim Brown’s recent HST Cycle 15 proposal to obtain a high-precision photometric time series of the HD 149026 b transit. The resulting light curve would have had higher photometric precision than both the TrES-1 and HD 209458 b time series shown above. The light curve would have had a higher cadence, the individual points would have been good to about one part in 20,000. (At magnitude 8.16, HD 149026 is about thirty times brighter than TrES-1, and the new ACS camera on HSTT is better-suited to the photometric transit-measurements that the defunct STIS spectrograph that was used by Brown et al. for HD 209458). Unfortunately, we learned yesterday that the proposal was not accepted. This is a bummer. An HST transit light curve would have dramatically improved our characterization of the planet that has already provided the first incontrovertible evidence for the core-accretion mechanism of giant planet formation. I think that the HD 149026 light curve would likely have been as informative as the Brown et al. HD 209458 light curve, which was recently shown as #4 in the list of Hubble’s top ten scientific achievements.

Habitable planets do have their drawbacks. For one, surface conditions near the triple point of water mean that the weather often interferes with differential photometry. That makes it hard for observers in the www.transitsearch.org collaboration to catch planet-bearing stars under clear dark skies during the time windows when transits are predicted to possibly occur!

Such was the case during the March 28, 2006 (06:59 UT) opportunity to check the low-mass red dwarf GL 581 for planetary transits. The planet orbiting GL 581 was announced by the Swiss Planet-hunting team last September (their discovery paper is here). GL 581b is one of the lowest-mass planets known outside our solar system. It’s likely similar in size and composition to Neptune or Uranus, with a minimum mass 17 times that of the Earth. The orbital period is 5.366 days, meaning that the surface temperature should be a bit under the boiling point of water. Tomorrow’s weather forecast for GL581b calls for cloudy skies, humidity near 100%, and afternoon highs near 180 F at the substellar point; the planet almost certainly spins once on its axis for every trip it makes around the star.

GL581 represents an ideal candidate for transitsearch.org observers, and there is no mention in the discovery paper that an attempt was made to check the star for planetary transits prior to the end of last year’s observing season. This lack of a transit check in the discovery paper makes sense, given the planet’s relatively low 3.6% a-priori transit probability, and the 5.366 day orbital period. Without a network of observers spread across the globe, it can take a very long time at a particular spot before one catches a transit window when the sky is (1) clear and (2) dark, and when the star is (3) high overhead. GL581b is a very exciting planet regardless of whether it transits, and so I’m sure Bonfils et al. just wanted to just get their discovery published in the literature. Papers “in prep.” garner no citations. Until Astronomy produces its first commercial killer apps, citations will remain the coin of the realm.

GL 581 is a springtime star, visible from both the Northern and Southern Hemispheres. There was an excellent opportunity last night for California observers to catch the transit, but the Golden State seems to have been clouded out from top to bottom. I have not gotten any reports of observations being made. The next windows of opportunity, and the best viewing sites are:

(1) April 2, 2006 19:04 UT — Japan, Australia
(2) April 8, 2006 00:33 UT — Europe, South Africa
(3) April 13, 2006 09:20 UT — North, South America

The transitsearch.org network has participants in all of these locations, so we should be set.

Boy oh boy would it be a big deal if GL 581b turns out to transit. The occurence of transits would fix the inclination of the planetary orbit, which would eliminate the sin(i) degeneracy that currently plagues the mass estimate. If the planet transits, we would know that it truly has a Neptune mass. The depth of the transit would give us the planetary size, which, coupled with the mass, would yield the density. The density would tell us what the planet is made of. If it is primarily water, like Uranus or Neptune, then we expect a radius of ~0.3 Jupiter radii. If the planet is made of rock and metal, however, like the terrestrial planets in our solar system, then the radius will be smaller, more in the neighborhood of ~0.22 Jupiter radii. A water-rich composition would tell us that the planet formed further away from the star, and then migrated inward to its steamy current location. This information, in turn, would give us valuable insight into the conditions that held sway in the disks surrounding low mass stars, and would help guide our hypotheses regarding the presence of habitable worlds orbiting the lowest mass stars.

Hopefully we’ll snag a transit on April 2nd and then confirm it on April 8th and April 13th. If that happens, I’ll mail a dollar to every registered user of oklo.org. With roulette wheel-like 3.6% odds, I’m not exactly betting the house, but nonetheless, hope springs eternal!

If you are interested in participating in transitsearch.org, feel free to subscribe to the (moderated) transitsearch.org observers list.

Nine extrasolar planets are known to transit their parent stars, but all of these planets have periods shorter than 5 days. They are frying beneath the brilliance of their parent stars. It would be nice to find a transiting planet with a longer period. Preferably, this would be a giant planet with towering thunderstorms and warm, drenching rains, and orbited by a habitable Earth-sized moon that we could detect with HST photometry.

Advanced observers can make the discovery of a transiting room-temperature Jupiter a reality by participating in the systemic team‘s distributed observing project: Transitsearch.org. A brief blurb on the transitsearch.org home page describes the basic strategy:

Transitsearch.org is a cooperative observational effort designed to allow experienced amateur astronomers and small college observatories to discover transiting extrasolar planets. In order to utilize the advantages of a network of small telescopes most effectively, our strategy is to observe known planet-bearing stars at the dates and times when transits are expected to occur.

At present, the majority of confirmed extrasolar planets have been discovered using the Doppler radial velocity technique (see the tutorials at www.oklo.org). The Doppler method, however, cannot determine the inclination of a planetary orbit to the line of sight from Earth. Therefore, each planet discovered by the Doppler method has an a-priori probability of transiting, which depends mainly on the orbital period of the planet. Short-period planets have relatively high transit probabilities, whereas long-period planets have low transit probabilities.

Transitsearch.org hasn’t found a new transiting planet. But if we can maintain the enthusiasm of the collaboration, then eventually it will. Every planet that is detected by the radial velocity technique has a finite a-priori probability of transiting. Hence we need to work systematically down through the list. Chances of success (among planets that have not yet been fully checked) range from a sporty 12.9% for HD 118203 “b” down to a depressingly low 0.1% for 55 Cancri “d” (for which our best-guess next opportunity to observe a transit center occurs, curiously enough, a week after the start of the next long count).

Continue reading →

Five radial velocity datasets (published last year by Marcy et al. 2005) have just been added to the systemic console: HD 183263, HD 117207, HD 188015, HD 45350, and HD 99492. Each of these more-or-less sunlike stars is too faint to be seen with the naked eye, and each is accompanied by (at least) one detectable planet. The periods range from 17 days to several years. None of these planets were extraordinary enough to warrant much fanfare in the popular press. (Ten years ago, however, the announcement of 5 planets would have been front page news. Ahh, those were the days!)

When you use the console to obtain orbital fits to these systems, you’ll notice that several of the stars have a long-term radial velocity trend superimposed on the variations that arise from the much more readily detectable shorter-period planet. These velocity trends are likely caused by as-yet undetected massive planets lying further out in the systems, and as these stars are monitored over the long term, the orbits of these distant, frigid giants will gradually reveal themselves.

In the meantime, the residual velocity trends underscore an interesting general property of extrasolar planets. The presence of a known planet is the best indicator that a given star harbors detectable (but as-yet undetected) planetary companions. That is, if you want to find new planets, then look at stars that already have known planets. Indeed, six of the first twelve planet-bearing stars that were monitored for more than two years at Lick Observatory were subsequently been found to harbor additional bodies. This impressive planetary six-pack includes luminaries such as Upsilon Andromedae, 55 Cancri, and 47 UMa, in addition to the more pedestrian Tau Boo, HD 217107, and HD 38529. (See Fischer et al. 2001).

In 1999, the sun-like star HD 209458 was discovered to harbor a transiting planet on a 3.52 day orbit. This was a big deal. The recurring occultations permitted, for the first time, an accurate measurement of both the radius and the mass of an extrasolar planet, and there have been a huge number of follow-up observations of the transits using a variety of telescopes and techniques. The most impressive result came from Brown et al. (2001), who used the (now defunct) STIS instrument on the Hubble Space Telescope to obtain a photometric light curve that has precision of about one part in ten thousand per 80-second sample:

The plot above can be found in the Astrophysical Journal , or, alternately, the paper containing the plot is available for free at the arXiv preprint server. A careful analysis of the photometric curve and the radial velocity data (which can be explored using the systemic console), combined with estimates for the size, mass and other properties of the parent star, indicates that the planet, HD 209458 “b”, has a radius about 1.35 times larger than the radius of Jupiter, and a mass of 0.69 times Jupiter’s mass. The temperature on the surface of the planet should be a toasty ~1200 K.

Various teams of scientists, including a group led by Peter Bodenheimer here at Santa Cruz, and independent groups led by Tristan Guillot and Gilles Chabrier in France, and Adam Burrows’ group in Arizona have all developed detailed computer programs that can predict how planets respond when placed in different physical environments. Everybody agrees that a gas giant planet with a standard hydrogen-helium composition and the mass and surface temperature of HD 209458 b should have a radius (corresponding to the 1-Atm pressure level) that is about 5-10% larger than Jupiter. The observed size of the planet is thus far out of agreement with the theoretical models. The planet is too large!

As soon the size problem became clear, a number of explanations for HD 209458 b’s large radius were put forward. The Burrows group (2003) pointed out that the planet may appear large during the transit because we are looking obliquely through long path lengths in the planetary atmosphere. Tristan Guillot and Adam Showman (2002) suggested that the ferocious winds on the planetary surface are transferring energy into the deeper layers of the planet, and that this extra source of energy is enough to bloat the planet to its observed size. These two phenomena don’t require anything special about HD 209458 b, and so both hypotheses predict that other planets with similar masses and temperatures should have similarly inflated radii. This doesn’t seem to be the case, however. In August 2004, a transiting planet of very similar mass and temperature was found in transit around an 11.8 magnitude star known as Tres-1 (Alonso et al. 2004). This planet has exactly the size (~1.05 Jupiter radii) predicted by the baseline theories. It thus appears that there is something unusual about HD 209458 b.

One intriguing possibility, suggested by Peter Bodenheimer, Doug Lin and Rosemary Mardling in 2001, is that another planet exists further out in the HD 209458 system. This planet would be exerting gravitational perturbations on HD 209458 b, which would cause its orbit to maintain a small eccentricity. If a planet like HD 209458 is in an eccentric (non-circular) orbit, then it experiences significant tidal stretching and squeezing which generate heat in the planetary interior. In a follow-up paper published in 2003, Bodenheimer et al. calculated that an orbital eccentricity, e=0.03 would likely be sufficient to generate enough tidal heating to inflate HD 209458 b to the observed size.

At that time, there were only 30 high-precision radial velocity measurements of HD 209458, and it was easily possible to find 2-planet fits to the radial velocity data which had (1) a small non-zero eccentricity for HD 209458 b, as well as (2) a second planet with a period of order 80 days, and a mass of ~0.12 Jupiter Masses. In the following diagram, the orbits are to scale, but the star and especially the planets are grossly too big.

Over the past two years, the California-Carnegie Planet Search Team used the Keck telescope to obtain a number of additional radial velocity measurements of HD 209458, and these have been published in a new paper. The full set of (out-of-transit) measurements have been loaded into the system menu of the systemic console. In our paper, our conclusion was that HD 209458 b is likely the only RV-detectable planet in the system, and that its orbit is most likely circular (more on this in a future post). See, however, if you can use the console to find viable 2-planet fits that have the correct period for the inner planet P=3.52474541 d, and which have a required RMS jitter for the star of less than 5 m/s. (Technically, you should also apply a simultaneous constraint on Mean Anomaly and eccentricity that arises because the time of central transit is known very accurately, but the console doesn’t yet have this capability. If you find a good fit, and post it here, we can likely fold in the additional timing constraint without greatly changing the basic orbital parameters).

The number of known transiting planets has been increasing steadily, and the total now stands at nine. Using the results of Peter Bodenheimer’s planetary structure code, we can compare the planets predicted sizes with their observed sizes:

(Here’s a larger-size .pdf of the above table, which will appear in an upcoming PPV review article). Three of the planets in the table, HD 209458b, HD 149026b, and HD 189733b, have radii that do not agree at all with the predictions. HD 209458b (and to a slightly lesser extent) HD 189733b are both larger than predicted, whereas HD 149026b is too small, likely because it has a huge rocky core:

These discrepancies indicate that the bulk properties of the transiting planets must depend significantly on factors other than their mass and estimated effective temperatures. Like the planets of our solar system, the extrasolar planets are imbued with interesting individual personalities.

Of all the known extrasolar planets, HD 80606b — in both the technical and the colloquial sense — is the most eccentric. This world has at least five times the mass of Jupiter, and it circles its parent star on an extremely elongated 111.4 day orbit:

Today (as seen from Earth!) HD 80606b is still near the far point of its orbit, at a distance of about 0.85 AU from the central star. The temperature in the upper atmospheric layers of the night-side has possibly dipped low enough so that torrential rains and violent thunderstorms are rumbling across its vast billowing horizons. During the rest of December and through most of January, the planet will fall in almost the full distance to the star, eventually swooping within 6 stellar radii as it whips through periastron. On January 26th, at the moment of closest approach, the temperature at the cloud tops will exceed 1000 Kelvin. The auroral displays will be dramatic beyond compare, and indeed, during the days to either side of periastron passage, it might be worth tuning in to the planet on the decameter band.

The discovery of the planet and its orbital solution were announced by the Geneva Observatory Planet Search Team in an April 04, 2001 ESO press release, and the radial velocities have since been made publicly available (right on!) at the CDS repository (see Naef et al 2001). You can therefore use the systemic console to fit this system and examine how radial velocity curves behave for extremely eccentric orbits.

The star HD 80606 is accompanied by a visual binary companion, HD 80607. The projected separation of the two stars is 2000 AU (fifty times the Sun-Pluto distance). When the HD 80606 b travels through the segment of its orbit that lies between the two stars, the night-side cloud tops of the planet are lit by the distant binary companion to ambient brightness that is very similar to a fully moonlit night on Earth. The two stars have similar masses, sizes, and temperatures to the Sun, but, like many of hosts of short-period massive planets, they are enriched in “metals” (gold, chromium, iron, carbon, oxygen, etc. etc.) by a factor of more than two relative to the solar value.

How did the planet get into its weird orbit?

Wu and Murray (2003) have suggested that HD 80606b’s extreme eccentricity is the result of a three-body interaction known as the “Kozai effect” between the planet and the two stars.

We know that planets aren’t rare, and by now, with the tally over at the extrasolar planet encyclopedia poised to blast past 200, the announcement of a newly discovered run-of-the-mill Jupiter-sized planet barely raises the collective eyebrow.

The headline that everyone is anticipating is the discovery, or better yet, the characterization of a truly habitable world — a wet, Earth-sized terrestrial planet orbiting in the habitable zone of a nearby star. Who is going to get to this news first, and when?

299 million dollars of smart money says that Kepler, a NASA-funded Discovery mission currently scheduled for launch in June 2008, will take the honors. The Kepler spacecraft will fly in an Earth-trailing 377.5 day orbit, and will employ a 1-meter telescope to stare continuously (for at least four years straight) at a patchwork of 21 five-square-degree fields of the Milky Way in the direction of the constellation Cygnus. Every 15 minutes, the spacecraft will produce integrated photometric brightness measurements for ~100,000 stars, and for most of these stars, the photometric accuracy will be better than one part in 10,000. These specs should allow Kepler to detect transits of Earth-sized planets in front of Solar-type stars.

Kepler has a dedicated team, a solid strategy, and more than a decade of development work completed. It’s definitely going to be tough to cut ahead of Bill Borucki in line. Does anyone else stand a chance?

Practitioners of the microlensing technique have a reasonably good shot at detecting an Earth-mass planet before Kepler, but microlensing-detected planets are maddeningly ephemeral. There are no satisfying possibilities for follow-up and characterization. Doppler RV has been making tremendous progress in detecting ever-lower mass planets, but it seems a stretch that (even with sub-1 meter per second precision) the RV teams will uncover a truly habitable world prior to Kepler, although they may well detect a hot Earth-mass planet.

There is one possibility, however, whereby just about anyone could detect a habitable planet (1) from the ground, (2) within a year, and (3) on the cheap. Stay tuned…

Paul Shankland has been visiting Santa Cruz this week, and everyone agrees that it’s about time to get the GJ 876 transit situation sewed up once and for all. Aquarius is still up in the early evening, and a planet “c” transit opportunity is bearing down with 30.1 day semi-clockwork precision. So out went the following alert to the transitsearch.org e-mail list:

Thursday Afternoon, Nov. 17, 2005

Dear Transitsearch Observers,

We’d like to alert you to an opportunity to check the GJ 876 system for planetary transits. Photometry is desired during a twelve-hour window centered on JD2453693.491 (Friday Nov. 18, 23:47 UT).

As you have likely heard, the GJ 876 system was recently found to harbor a low-mass (7.5 Earth Mass) planet on a 1.94 day orbit. The new planet is referred to (rather prosaically) as GJ 876 “d”, and is the third planet detected in the GJ 876 system. The discovery paper is scheduled for an upcoming issue of the Astrophysical Journal, and is also available on the astro-ph preprint server: http://arxiv.org/abs/astro-ph/0510508

Sadly, transits for planet “d” have been ruled out to high confidence.

As a result, however, of (1) inclusion of the third planet in the dynamical model for the system, and (2) a large number of new high-precision radial velocities, Eugenio Rivera has produced new transit ephemeris predictions for the outer two planets in the GJ 876 system. These differ by several hours from the dynamical predictions that are currently posted on the transitsearch.org candidates site, e.g.:

http://www.ucolick.org/~laugh/GJ876____c.transits.txt

We’re working through an extensive analysis which shows that neither “b” nor “c” is transiting, but this analysis is nevertheless in great need of observational verification. There is a conflict between dynamical fits to the radial velocities (which indicate that the system is inclined by 50 degrees to the plane of the sky) and the results of Benedict et al (2002, ApJL 581, 115), who used HST to get astrometric measurements that suggest a nearly edge-on configuration.

We’d thus like to request photometry of the star to six hours on either side of JD2453693.491 (Friday Nov. 18, 23:47 UT).

Information regarding observing GJ 876 and photometry submission instructions are at:

http://www.aavso.org/news/ilaqr.shtml

Additional background is on the transitsearch GJ 876 results page:

http://www.ucolick.org/%7elaugh/GJ876____c.results.html

Note that this page states that the photometric campaign is over, but the new dynamical model indicates that more photometry is desirable.

Other observing opportunities are (were) as follows:

For planet c (the middle one):

predicted central transit (UT)
————————————
2005 Aug. 20 15:40
2005 Sep. 19 18:41
2005 Oct. 19 20:53
2005 Nov. 18 23:47
2005 Dec. 19 01:36

For planet b (the outer one):
————————–
2005 Aug. 22 17:28
2005 Oct. 22 17:34
2005 Dec. 22 18:05

Finally, we’d like to thank everyone for being patient over the 8 months, during which we have not been running coordinated campaigns. With Shankland of USNO “on the bridge”, we’re now ramping up for a more active phase. Stay tuned!

After using the console for a while, you’ll notice that it’s often easy to find a reasonably good (say, chi-square of 3-5) multiple-planet fit to a given radial velocity data set. This rule of thumb tends to be especially true if you allow the planets to have large eccentricities. But how does one know whether the fit is likely to be correct?

This is one of the questions that the systemic simulation is designed to answer.

Most of the time, however, if a fit contains large enough eccentricities for the planet orbits to cross, then the trial system will be dynamically unstable. That is, the planets in the model will suffer a close encounter, which is generally followed (or directly accompanied) by a disaster. The planets collide, or one or more of them is ejected, or one of them is thrown into the central star.

While it is certainly true that such catastrophes have been reasonably common throughout galactic history, it is exceedingly unlikely that any particular planetary system that we observe will be on the verge of a dramatic instability. The stars that can be observed using the Doppler radial velocity method are billions of years old. If a star had an unstable planetary system, it is likely that the instability either occurred long ago, or that won’t happen for a long time to come.

As a result, an important requirement for any radial velocity fit is that it correspond to a dynamically stable system. Traditionally, this can be checked either by integrating the system forward in time, or by applying a technique which checks for the presence of chaos in the orbits. (Indeed, all of the planetary systems that underlie the systemic database have been integrated for one million orbits prior to being “observed”. These pre-integrations establish a strong likelihood of short-term dynamical stability for all the systemic systems.)

Here’s an idea that sounds possibly promising. If the radial velocity waveform of a planetary system is converted into an audio signal, is it possible for the human ear to rapidly detect whether a system is likely to be unstable? To test this, we’re working on bringing an audio generator into the systemic console.

More generally, what do the extrasolar planetary radial velocity reflex waveforms sound like? The short answer is, they sound terrible. There are interesting reasons for this, which we’ll pick up in a future post. For now, have a listen to these .wav’s (created by Aaron Wolf) of two of the best-known multiple planet systems: GJ 876 and Upsilon Andromedae

And try to listen for the (heavily processed voice of the better-voice-of-the-two GJ876 in the forthcoming James Alley Remix).