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Last week, I wrote a post introducing HD 17156 b, a Jovian planet on a highly eccentric 21.2-day orbit around a V=8.17 solar-type star lying 250 light-years away in Cassiopeia (RA=2h 50m, Dec=72 deg).

A photometric check for transits by HD 17156 b was reported in the discovery paper, but due to the nearly three-week orbital period, it was only possible to rule out about 25% of the transit window. Given the highly favorable geometry of the planetary orbit, this means that there’s an impressive ~11% chance (8.25% if you take the discount) that the planet can be observed in transit. The expected transit depth is a very respectable 1%, and given the bright parent star, it’s a straightforward detection for small-telescope observers everywhere in the Northern Hemisphere.

What’s it worth to catch HD 17156 b in transit? From a crass cash-money standpoint, one can estimate a dollar value. Because the planet has a long period and an eccentric orbit, it would be the first transiting example of its kind, and would thus be expected to generate a fairly large number of citations. From a career standpoint, an ADS citation is worth at least $100 (see, e.g. here). Based on the citation count for the TrES-1 discovery paper (144 citations in three years) it’s reasonable to expect that at one decade out, a HD 17156 b transit would garner of order 200 citations, for a conservative total value of 20K. Given the 10% probability of the transit coming through, the resulting expectation value is equivalent to having twenty Benjamins floating down from the black velvet of the night sky.

I used the systemic console’s bootstrap utility to generate a set of orbital fits to the published radial velocities for HD 17156. Each orbital fit describes a unique sequence of central transit times. For a particular transit opportunity, the aggregate of predicted central transit times from the different fits can be plotted as a histogram. Here’s the resulting plot for the transit opportunity that’ll occur next Monday (HJD 2454353.68):

The uncertainty in the time of central transit is ~0.3 days. A window this narrow is rare for a planet that hasn’t yet been thoroughly checked. In fact, as far as Transitsearch.org opportunities are concerned, it doesn’t get much better than this. Extending our opportunity cost analysis, the expected monetary return for observations within the 1-sigma transit window is an impressive $114 per hour. (Only rarely does the expected return per hour exceed minimum wage for existing transit opportunities.)

Scientifically, a transit by HD 17156 b would certainly be very exciting. The planet should be heating up very rapidly during its periastron passage, which should spur the generation of hemispheric-scale vortices and an 8-micron light curve that’s detectable with the Spitzer telescope. Observation of the secondary eclipse (assuming it occurs) would allow for a measurement of the global planetary temperature near the orbital apastron.

The frame above is from a hydrodynamical study of HD 17156 b that Jonathan Langton has just finished computing. If all the talk of dollars, ephemerides, opportunity cost, and expectation value is leaving you stressed out, then just kick back with this fat 1.0 MB .mov of the simulation and get your groove on.

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August 1st marked the most recent ‘606 day, which came and went without wide remark. Perhaps this was because in late Summer, HD 80606 rises and sets in near-synch the Sun, and is thus lost from the Earth’s night skies.

At the moment, HD 80606b is headed back out toward apastron.

The global storms and shockwaves that were unleashed at the beginning of August are dissipating rapidly, and the flux of heat from the planet is likely fading back down to the sullen baseline glow that arises from tidal heating.

HD 80606’s next periastron passage occurs on November 20th, and the Spitzer Space Telescope is scheduled to observe the whole event (details here). It’s going to be a big deal. Spitzer can only observe HD 80606 during two three-week windows each year, and fortunately, the Nov. 20th Periastron passage occurs during one of these windows. It’s literally the only opportunity to catch HD 80606 b’s big swing before Spitzer’s cryogen runs out in 2009.

The orbital geometry of the periastron passage looks like this:

Each marker of the orbit is separated by one hour. The prediction for the pseudo-synchronous rotation of the planet is also indicated. The planet should be spinning with a period of 36.8 hours. Jonathan Langton’s hydrodynamics code predicts what the temperature distribution on the planet should look like at each moment from Spitzer’s viewpoint in our solar system:

Transitsearch.org observers have covered a number of the HD 80606 b transit opportunities, and it seems pretty certain that the planet doesn’t transit. This isn’t surprising. The geometry of the orbit is such that when the planet crosses the plane containing the line of sight to the Earth, it’s quite a distance away from the star. Not so, however, for the secondary transit. There’s a very respectable 15% chance that Spitzer will detect a secondary transit centered two hours prior to the periastron passage.

Even if the planet doesn’t transit, we should be able to get a good sense of the orbital inclination from the shape of the light curve. If the orbit is nearly in the plane of the sky, then we should see a steady rise followed by a plateau in the 8-micron flux coming from the planet. For more nearly edge-on configurations, the flux peak should be clearly discernable. The observations are scheduled to start 20 hours prior to periastron and end 10 hours after.

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HAT-P-2b. The name doesn’t exactly ring of grandeur, but this planet — a product of Gáspár Bakos’ HAT Net transit survey — is poised to give the Spitzer Space Telescope its most dramatic glimpse to date of a hot Jupiter.

HAT-P-2b’s orbit is remarkably eccentric for a planet with an orbital period of only 5.6 days, and by a stroke of luck, periastron is located almost exactly midway between the primary and the secondary transits (as viewed from Earth). The strength of the stellar insolation at periastron is nine times as strong as at apastron, which more than guarantees that the planet will have disaster-movie-ready weather.

On June 6th, Josh Winn and his collaborators used the Keck telescope to obtain 97 radial velocities for HAT-P-2. The observations were timed to occur before, during, and after primary transit, and the Rossiter-McLaughlin effect is clearly visible in their data (preprint here):

The symmetry of the Rossitered points indicates that the angular momentum vector of the planetary orbit is aligned with the spin pole of the star:

This state of affairs also holds true for the other transiting planets — HD 209458b, HD 149026b, HD 189733b — for which the effect has been measured. The observed alignments are evidence in favor of disk migration as the mechanism for producing hot Jupiters.

With its apparent magnitude of V=8.7, the HAT-P-2b parent star is roughly ten times brighter than the average planet-bearing star discovered in a wide-field transit survey. The star is bright enough, in fact, to have earned an entry in both the Henry Draper Catalog (HD 147506) and the Hipparcos Database (HIP 80076), but with its surface temperature of 6300K (F8 spectral type) it was too hot to have been a sure-fire “add” to the ongoing radial velocity surveys. Prior to this May, it had been entirely ignored in the astronomical literature (save a brief mention in this paper from 1969).

HAT-P-2’s intrisic brightness and its planet’s orbital geometry mean that in a relatively compact 34-hour observation, Spitzer can collect on the most interesting features of the orbit with high signal-to-noise. In particular, there is an excellent opportunity to measure the rate at which the day-side atmosphere heats up during the close approach to the star. The planet, in fact, presents such a remarkable situation that a block of Director’s Discretionary time was awarded so that the observations can be made during the current GO-4 cycle. They’ll be occurring soon.

Both HAT-P-2b and HD 80606 b will provide a crucial ground truth for extrasolar planetary climate simulations. Jonathan Langton’s current model, for example, predicts that that the temperatures on HAT-P-2b will range over more than 1000K. At the four times shown in the above orbital diagram, the hemisphere facing Earth is predicted to show the following appearances:

Spitzer, of course, can’t resolve the planetary disk. It measures the total amount of light coming from the planet in chosen passband. At 8-microns, the planet’s light curve should look like this:

The temperature maps only hint at the complex dynamics of the surface flow. A better indication is given by the distribution of vorticity,

which we’ll pick up in the next post…

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Regular oklo readers will recall Gillon et al.’s discovery that the Neptune-mass planet orbiting the red dwarf star Gl 436 can be observed in transit. Transitsearch got scooped, and the whole eposide got me all worked up enough to neglect the exigencies of everyday academic life and reel off three straight posts on the detection and its consequences (see here, here, here, and also here). The transits of Gl 436 b are a big deal because they indicate that the planet is possibly composed largely of water. It’s not a bare rock and it’s not a Jupiter-like gas giant. Rather, it’s consistent with being a fully Neptune-like object, hauled in for inspection on a 2.64385 day orbit.

Following Gillon et al.’s announcement, it became clear that Gl 436 transits would fit into a window of observability during the June 24th – July 04 IRAC campaign on the Spitzer Space Telescope. The red dwarf parent star, furthermore, because of its proximity, is bright enough for Spitzer to achieve good photometric signal-to-noise at 8-microns. As a result, Joe Harrington’s Spitzer Target of Opportunity GO-4 proposal was triggered, and the Deep Space Network radioed instructions to the spacecraft to observe the primary transit on June 29th, as well as the secondary eclipse (when the planet passes behind the star) on June 30th, a bit more than half an orbit later. Joe, along with his students Sarah Navarro and William Bowman, and collaborators Drake Deming, Sara Seager, and Karen Horning asked me if I wanted to participate in the analysis. After watching all the ‘436 action from the sidelines in May, I was more than happy to sign up!

One of the most exciting aspects of being a scientist is the round-the-clock push to get a time-sensitive result in shape for publication. There’s a fantastic sense of camaraderie as e-mails, calculations, figures and drafts fly back and forth. On Monday afternoon PDT (shortly after midnight GMT) when Mike Valdez sent out his daily astro-ph summary, it was suddenly clear that we were under tremendous pressure to get our results analyzed and submitted. The Geneva team had swooped in and downloaded the data for the primary transit the moment it was released to the community! They had cranked out a reduction, an analysis, and a paper, all within 48 hours. Their light curve confirmed the ground-based observations. Spitzer’s high-quality photometry indicates that the planet is slightly larger than had been indicated by the ground-based transit observations. Drake submitted our paper yesterday afternoon.

Fortunately for us, the real prize from Spitzer is the secondary eclipse. Its timing is capable of independently confirming that the orbit is eccentric, and the depth gives an indication of the surface temperature on the planet itself.

The upper panel of the following figure shows the raw Spitzer photometry during the secondary eclipse window. IRAC photometry at 8 μm is known to exhibit a gradually increasing ramp-up in sensitivity, due to filling of charge traps in the detectors, but even before this effect is modeled and subtracted, the secondary transit is visible to the eye. The bottom panel shows the secondary transit in detail.

The secondary transit occurs 58.7% of an orbit later than the primary transit, which proves that the orbit is eccentric. A detailed fit to the transit times and to the radial velocities indicates that the orbital eccentricity is e=0.15 — halfway between that of Mars (e=0.1) and Mercury (e=0.2). The orbital geometry can be drawn to scale in a diagram that’s 440 pixels across:

The depth of the secondary eclipse is 0.057%, which allowed us to estimate a 712 ± 36K temperature for the planetary surface.

A temperature of 700+ K is hotter than expected. If we assume that the planet absorbs all the energy that it gets from the star and re-radiates its heat uniformly from the entire planetary surface, then the temperature should be T = 642 K. The higher temperature implied by the secondary eclipse depth could arise from inefficient transport of heat to the night side of the planet, from a non-“blackbody” planetary emission spectrum, from tidal heating, or from a combination of the three. If the excess heat is all coming from tidal dissipation, then the Q-value for the planet is 7000, suggesting that it’s a bit more dissipative inside than Uranus and Neptune.

What would Gl 436 b look like if we could go there? To dark adapted eyes, the night side is just barely hot enough to produce a faint reddish glow (as is the case on the surface of Venus, which has a similar temperature). The atmosphere is too hot for water clouds, and is likely transparent down to a fairly high atmospheric pressure level. The day-side probably reflects a #E0B0FF-colored hue that contrasts with the orange-yellow light of the star. The planet spins with a period of 2.32 days so that it can be as spin-synchronous as possible during the sector of its orbit closest to periastron. At a fixed longitude on the planet, the day drags on for 456 hours from high noon to high noon.

Jonathan Langton has been running atmospheric simulations with the latest parameters. On the phone, just a bit ago, he would only say that the preliminary results were “interesting”…

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There remain three blockbuster, front-page discoveries in exoplanetary science. The first is the identification of a potentially habitable Earth-mass planet around another star. The second is the detection of a life-bearing planet. The third is contact with extraterrestrial intelligence.

It’s hard to predict when (and in which order) discoveries #2 and #3 will take place. Discovery #1, on the other hand, is imminent. We’re currently 2±1 years away from the detection of the first habitable Earth-mass planet (which implies ~15% chance that the announcement will come within one year).

The breakthrough detection of a habitable Earth will almost certainly stem from high-precision Doppler monitoring of a nearby red dwarf star, and already, both the Swiss team and the California-Carnegie team are coming tantalizingly close. The following table of notable planet detections around red dwarfs gives an interesting indication of how the situation is progressing:

The masses of the stars and planets are given in Solar and Earth masses respectively. The year of discovery for each planet is listed, along with the half-amplitude, K, of the stellar reflex velocity (in m/s), the number of RV observations on which the detection was based, the average reported instrumental error (sigma) associated with the discovery observations, and a statistic, “µ”, which is K/sigma multiplied by the square root of the number of observations at the time of announcement. The µ-statistic is related to the power in the periodogram, and gives an indication of the strength of the detection signal at the time of discovery. In essence, the lower the µ, the riskier (gutsier) the announcement.

What will it take to get a habitable Earth? Let’s assume that a 0.3 solar mass red dwarf has an Earth-mass planet in a habitable, circular, 14-day orbit. The radial velocity half-amplitude of such a planet would be K=0.62 m/s. Let’s say that you can operate at 1.5 m/s precision and are willing to announce at µ=20. The detection would require N=2,341 radial velocities. This could be accomplished with an all-out effort on a proprietary telescope, but would require a lot of confidence in your parent star. To put things in perspective, the detection would cost ~10 million dollars and would take ~2 years once the telescope was built.

Alternately, if the star and the instrument cooperate to give a HARPS-like precision of 1 m/s, and one is willing to call CNN at µ=14, then the detection comes after 500 radial velocities. The Swiss can do this within 2 years on a small number of favorable stars using HARPS, and California-Carnegie could do it on a handful of the very best candidate stars once APF comes on line. Another strategy would be to talk VLT or Keck into giving several weeks of dedicated time to survey a few top candidates. Keck time is worth ~$100K per night, meaning that we’re talking a several-million dollar gamble. Any retail investor focused hedge funds out there want to make a dramatic marketing impact? Or for that matter, with oil at $68 a barrel, a Texas Oil Man could write a check to commandeer HET for a full season and build another one in return. “A lone star for the Lone Star.”

If I had to bet on one specific headline for one specific star, though, here’s what I’d assign the single highest probability:

The Swiss Find a habitable Earth orbiting Proxima Centauri. Frequent visitors to oklo.org know about our preoccupation with the Alpha-Proxima Centauri triple system. We’ve looked in great detail at the prospects for detecting a habitable planet around Alpha Centauri B, and Debra Fischer and I are working to build a special-purpose telescope in South America to carry out this campaign (stay tuned for more on this fairly soon). Proxima b, on the other hand, might be ready to announce right now on the basis of a HARPS data set, and the case is alarmingly compelling.

Due to its proximity, Proxima is bright enough (V=11) for HARPS to achieve its best radial precision. For comparison, Gl 581 is just slightly brighter at V=10.6. Proxima is effortlessly old, adequately quiet, and metal-rich. If our understanding of planet formation is first-order correct, it has several significant terrestrial-mass planets. The only real questions in my mind are, the inclination of the system plane, the exact values of the orbital periods, and whether N_p = 2, 3, 4 or 5.

The habitable zone around Proxima is close-in. With an effective temperature of 2670K, and a radius 15% that of the Sun, one needs to be located at 0.03 AU from the star to receive the same amount of energy that the Earth receives from the Sun. (Feel free to post comments on tidal locking, x-ray flares, photosynthesis under red light conditions, etc. Like it or not, if the likes of Gl 581 c is able to generate habitability headlines and over-the-top artist’s impressions, just think what a 1 Earth-Mass, T=300 K Proxima Centauri b will do…) A best guess for Proxima’s mass is 12% that of the Sun. An Earth in the habitable zone thus produces a respectable K=1.5 m/s radial velocity half-amplitude. It’s likely that HARPS gets 1.2 m/s precision on Proxima. A µ=15 detection thus requires only 144 RV observations. Given that Proxima is observable for 10 months of the year at -30 South Latitude, there are presumably already more than 100 observations in the bag. We could thus get an announcement of Proxima Cen b as early as tomorrow.