### Archive

Archive for June, 2007

## A miss is as good as a mile

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When I give a public lecture, I often start by trying to impart a sense of the extraordinarily rarefied character of the local galactic neighborhood. The known catalog of planet-bearing stars is akin to 200 small grains of sand dusting a volume more than 1000 kilometers on a side. It seems amazing to me that we’re able to see the stars at all with the naked eye. Even Sirius appears twelve billion times fainter than the Sun.

At the moment, the Alpha-Proxima trio is the closest group of stars to the Sun, and they are currently drawing closer still. In 27,000 years, they will pass at a minimum distance of 2.75 light years. Already, the Alpha-Proxima system is beginning to have an effect on the Oort Cloud, and as a result of the encounter, roughly half a million comets will be delivered into Earth-crossing orbits over the next several million years. This will generate something like a 10% increase in the arrival of new comets above the long-term average.

In 1999, Joan Garcia-Sanchez and collaborators filtered the known space motions of nearby stars in order to determine which systems are scheduled to make (or have already made) close encounters with the Solar System. The closest approach that they identified belongs to the currently inconspicuous red dwarf Gliese 710. In 1.4 million years, this half-solar mass star will skim by at a distance of ~1.09 light years, and will appear as bright and as red in the night sky as Betelgeuse. Like most low-mass stars in the galactic disk, Gliese 710 probably has a retinue of terrestrial planets. If the encounter were occurring now, Gliese 710 would likely have both an evocative Arabic name, as well as hundreds to thousands of high-precision radial velocity measurements.

The Gliese 710 encounter will produce a comet shower roughly six times more severe than what Alpha and Proxima will generate. It’s unlikely, however, that the increase in the number of comets will lead to an extinction-level impact. Nevertheless, the impending passage of a red dwarf at a distance of only 70,000 AU has a certain panache.

Given that encounters of the Alpha-Proxima and Gliese 710 variety are occurring on a million-year timescale, what is the most hair-raising encounter that one can one expect on a 4.5-billion year time scale? The mean encounter velocity between stars in the galactic disk is of order 40 kilometers per second, and the density of stars is ~0.1 systems per cubic parsec. Using these numbers, a simple n-sigma-v calculation yields an expected close-approach distance of only 770 AU. An encounter this close would literally thread the orbits of outer solar system bodies such as Sedna.

Imagine waking up to one of two headlines: (1) “Red Dwarf Discovered heading straight toward the Solar System at 400 meters per second!” and (2) “Red Dwarf Discovered heading straight toward the Solar System at 40 kilometers per second!”

Naively, one might expect that headline #2 bears much worse news, but surprisingly, that’s not the case. A red dwarf passing through the outer Solar System at 40 kilometers per second would barely deviate from a straight-line trajectory. Aside from any comets or Kuiper belt objects lying nearly directly in its path, it would barrel past us and produce only a minimal perturbation to the planetary orbits. Headline #2, on the other hand, could potentially be very bad news, as a close encounter with a slowly moving star can be far more damaging. The reason is that the interloping star is in the vicinity for much longer, and has time to build up a far stronger overall perturbation on solar system bodies. When the solar system was forming, the Sun very well could have belonged to an open cluster like the Orion Nebular Cluster. In a cluster environment, a close (several hundred AU) passage of a slowly moving brown dwarf or a low-mass star is a fairly common event, and indeed (as argued here by Morbidelli and Levison) the Sun may well have grabbed Sedna and a few hundred other as-yet undiscovered dwarf planets from an interloping star.

Asteroids that hit the Earth routinely kick clouds of debris into interplanetary space. Large rocks launched in this fashion can harbor hardy bacteria for nearly indefinite periods of time. The outer solar system, then, at any given moment, is often sparsely populated by viable dormant spore-forming bacteria that originated on Earth (see, e.g. here).

Odds are, that once-in-the-solar-lifetime (~770 AU) close encounter involved a red dwarf as the interloping star. A run-of-the-mill red dwarf has 0.3 solar masses and 1% of the solar luminosity. A habitable orbit around such a star lies at a distance of ~0.1 AU, and orbits at a speed of ~50 kilometers per second. This orbital velocity is quite close to the ~40 kilometer per second relative velocity that one would expect for an interloping star at our galactocentric radius. This means that the existence of a habitable terrestrial planet would have given the impinging parent red dwarf a dynamical mechanism for absconding with some of the material that belonged to our own outer solar system. Comets, rocks, and dwarf planets captured in this way would have stuck with the red dwarf, orbiting until they either collided with or were ejected by the red dwarf’s planets.

When that first SETI signal gets picked up, it’s unlikely, but not impossible that it’ll be coming from my trillionth cousin five hundred billion times removed.

Categories: worlds Tags:

## Win — Place — Show

June 25th, 2007 1 comment

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After several late nights of work, Jonathan Langton and I submitted our new paper that predicts the weather conditions on unevenly heated (read eccentric) short-period planets. We’re hoping that by observing these worlds in the infrared, we’ll be able to learn about the atmospheric dynamics that characterize all of the hot Jupiters.

One of our main results is a head-to-head comparison of the expected 8-micron light curves for the six most promising short-period eccentric planets. HAT-P-2b (in turquoise) comes up the winner in terms of observability, with HD 80606 b (in black) running second, and HD 118203 b (red) in third place:

In a concluding paragraph, we took the liberty to wax slightly-more-than-scientific enthusiastic about home-town favorite HD 80606b:

A short-period Jovian planet on an eccentric orbit likely presents one of the Galaxyâ€™s most thrilling sights. One can imagine, for example, how HD 80606 b appears during the interval surrounding its hair-rising encounter with its parent star. The blast of periastron heating drives global shock waves that reverberate several times around the globe. From Earthâ€™s line of sight, the hours and days following periastron are characterized by a gradually dimming crescent of reflected starlight, accompanied by planet-wide vortical storms that fade like swirling embers as the planet recedes from the star. Itâ€™s remarkable that we now have the ability to watch this scene (albeit at one-pixel and two-frequency resolution) from a vantage several hundred light years away.

We’ll post the paper online after it makes it through the refereeing process. And stay tuned as we get HAT-P-2b and HD 80606b ready for their multi-frequency screen tests…

Categories: worlds Tags:

## “Visitable” Planets

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Two evenings ago, Venus and the Moon hung close together in the deep blue twilight. Their alignment, along with the location of the fading sunset glow, gave a suggestion of the sweep of the ecliptic plane.

Zooming in on the pixels of the above photograph, it’s just possible to see that Venus is not a point source. The hint of a half-illuminated world indicates that the planet is now fairly near maximum elongation. In our lifetimes, I think we’ll likely see images of habitable extrasolar terrestrial planets that harbor something like this level of detail.

Venus gets a bad rap because the surface is so unpleasant. The Venera landers were built like submarines, and yet they still managed only an hour or two on the ground before expiring. The coke-bottle lens panoramas of basaltic slabs that they radioed back do little to fire the imagination.

My attitude toward Venus was transformed by David Grinspoon’s Venus Revealed, which I think is probably the best trade book ever written on planetary science. The text is filled with gems of insight. One passage that sticks is:

There is a level in the clouds (about 33 miles up), where the atmospheric pressure is about 70% of the pressure at sea level on Earth, and the temperature is a balmy 107 degrees Fahrenheit. For ballooning at this altitude on Venus, you would need only a thin, acid resistant suit, and oxygen tank and a large supply of cold lemonade. It’s cool enough for liquid water, and small amounts of it exist there (in a strong sulfuric acid solution).

By contrast there’s no place on Mars that could be explored using gear from an Army Surplus store.

Are there other similarly “visitable” environments in the Solar System? Surprisingly, the answer is yes. On Jupiter, at a level where the atmospheric pressure is ~6 times that at sea level, the temperature is chilly, and yet still comfortably above freezing. This level (at which hot tea might be preferable to lemonade) lies in the midst of the Jovian water cloud deck, and is subject to torrential downpours accompanied by lightning and thunder. If one were ballooning at this level, you would see an misty gray expanse, stabbed by lightning discharges, with the rotten-egg smell of hydrogen sulfide seeping in through your uncomfortably heavy scuba-store face mask.

Categories: worlds Tags:

## A Habitable Earth

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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 2Â±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:

Planet

M star

M sin(i)

date K #obs sig Âµ
Gl 876 b 0.32 615 1998 210 13 6.0 247
Gl 876 c 0.32 178 2001 90 50 5.0 127
Gl 436 b 0.44 22.6 2004 18.1 42 4.5 26
Gl 581 b 0.31 15.7 2005 13.2 20 2.5 23
Gl 876 d 0.32 5.7 2005 6.5 155 4.0 20
Gl 674 b 0.35 11.8 2007 8.7 32 0.82 60
Gl 581 d 0.31 7.5 2007 2.7 50 1.23 16
Gl 581 c 0.31 5.0 2007 2.4 50 1.23 14

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, “Âµ”, which is K/sigma multiplied by the square root of the number of observations at the time of announcement. The Âµ-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 Âµ, 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 Âµ=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 Âµ=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 Âµ=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.

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## Relaunch

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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.

Categories: detection Tags:

## When morning comes twice a day (or not at all).

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From all accounts, it looks like Jonathan Langton’s talk at last week’s AAS meeting in Honolulu went quite well. Here’s a link to a gzipped tar file of his Keynote presentation. It weighs in at 10.5 MB, and includes a number of cool animations. The following frames have been grabbed from the 2-orbit animation of HD 185269b:

We’re putting the finishing touches on a paper that we hope to submit this weekend. It shows that there’s a remarkable range in weather patterns and predicted infrared light curves among the short-period planets with non-zero eccentricity. The bottom line is that HAT-P-2b and HD 80606b are the best prospects for Spitzer observations, whereas HD 185269 b seems to produce the most complex and photogenic weather (see the three frames above).

HD 185269 b was discovered by John Johnson during the course of his radial velocity survey of slightly evolved high-mass stars. The orbital eccentricity is a modest yet still significant e=0.3, which leads to a 344% increase in the amount of energy received by the planet between apastron and periastron. This seasonal variation is strong, but not crazy enough to drive the shock waves that show up on HD 80606 b or HAT -P-2b. The combination of Coriolis deflection, periodic heating, eddy formation and Kelvin-Helmholtz instabilities on a global scale lead to a mesmerizing, endlessly evolving flow.

Categories: worlds Tags:

## transitsearch dot org

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Gl 436 b was the first planet to be detected in transit after the radial velocity detection of the planet itself was publicly announced. Gillon et al.’s discovery shows that the basic strategy of checking known Doppler wobble stars for transits can pay off dramatically, and indeed it’s recharged my interest in keeping transitsearch.org up and running.

Successful transit predictions depend on having accurate ephemerides, which in turn depend on fits to the most recent radial velocities available. The period error in an old fit builds up to the point where the predicted transit window is longer than the orbital period itself. Indeed, relying on a published fit that’s five, six, or even eight years old, is akin to showing up at the 2007 Grammy Awards in a 2001 Escalade.

We’ve thus started the job of making sure that the transitsearch.org candidate tables are as up to date as possible. I’ve committed to spending a bit of time each day checking and updating the master orbit.data and star.data files that are used as input to the cron job that runs every night to update the prediction tables. In each case, we’ll use the most recent published orbital data for a given planet.

In addition, the eighteen known transiting planets have all had their ephemeris tables updated using the latest literature values for the orbital parameters. I got the most of these data from Frederic Pont’s useful summary table, and took the radial velocity half-amplitudes from exoplanet.eu and exoplanets.org. At the moment, the occultations are all treated as central transits by my code, which means that the predicted transit durations will in general be longer than the actual observed events. This discrepancy will be patched shortly, but in the meantime, the predicted transit midpoint times in the ephemeris tables should be extremely accurate for all 18 planets. (See the candidates faq for more information).

We’ve made the decision to base the main transitsearch.org candidates table only on published orbital fits that have appeared in the refereed literature. In many cases, however, one finds a need to go beyond predictions based on published fits. There are two main circumstances under which this can occur. (1) The systemic console provides the ability to obtain fits to all existing radial velocity data for any given system. For many systems, one thus has the opportunity to obtain orbital parameters for the planet that are more accurate than published values that are based on fewer data sets. (2) You may have used the console to locate a candidate planet that is not yet published. If this planet can be observed in transit, then you’ve got dramatic confirmation of your discovery.

Eugenio has written an extension to the bootstrap window of the most recent version of the console that allows anyone to make transit predictions for any planet produced by the console. In an upcoming post, we’ll look in detail at how this new feature works.

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