HAT-P-2b: SEVERE STORM WARNING

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Jonathan Langton’s hydrodynamics code has just finished a simulation of the atmospheric dynamics on HAT-P-2b. The short orbital period and the high orbital eccentricity conspire to make this world the stormiest exoplanet found to date. This planet should definitely be observed before Spitzer’s cryogen runs out.

I’ll post our more detailed analysis, along with the predicted light curves in the various Spitzer bands very shortly. In the meantime, however, here are two animations (HATa.mov and HATb.mov) showing the temperature over the planetary surface. The temperature scale runs from a (comparatively) mild 950K to a scorching-hot 2170K. The animation runs through two orbital periods of the planet, and thus covers ~6 rotation periods. The animations are shown from the point of view of a camera fixed above one spot on the planetary surface, one above the “eastern” hemisphere, the other above the “western” hemisphere. They work best when looped. If you’re a connoisseur, please click here for a .pdf-format description of our numerical model.

Corot-Exo-1b

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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.

Another HAT trick (plus XO-2b)

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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!

time series

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It’s remarkable how Keplerian fitting functions can be pushed to model a wide variety of time series. Anyone recognize this particular data stream?

It shows complicated behavior on timescales ranging from days to years, superimposed on an autoregressive tendency. The downloadable systemic console‘s periodogram points to significant power at low frequencies, reflecting the gradual overall decline during the duration of the time series. There are also a number of distinct peaks at higher frequencies.

A crazy (read eccentric) six-planet Keplerian system does a credible job of fitting the data.

largely because the periastron passages of eccentric planets are capable of producing peaks that ramp up and then decay. To fit a particular peak, the five keplerian parameters can be varied to produce an enormous variety of waveforms.

The Keplerian model can be evaluated at any forward time to make a prediction, albeit in this case, one with presumably zero physical justification…

Gl 581 — The Movie

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While talking to a reporter this morning, I ventured 1000:1 odds against Gl 581 “c” harboring a clement surface or a temperate ocean-atmospheric interface. Too bad we haven’t yet tapped into the galactic market — I’d like to hedge my bet with the purchase of an appropriate derivative security.

Habitable or not, Gl 581 c is pointing toward better worlds to come. As I remarked in the past two posts (1,2), we’re guessing that “c” formed beyond the snowline and migrated inward to its current position just outside the nebulous inner boundary of the habitable zone.

Here’s a 1.1 MB animation of Jonathan Langton’s simulation of the flow pattern on Gl 581 c. The clip shows 30 hours worth of weather on our model of the planet:

First a few technical details. We model the planet’s lower radiative stratosphere with a 2D compressible hydrodynamics code. We use a time-dependent model for radiative heating and cooling. The planet is assumed to be spin-synchronous, so that it rotates on its axis once every 12.9 days. The planetary mass is five-Earth masses (I’m holding out for a transit on May 7th!), and we take a radius of 1.7 Earth radii. The orbit is assumed circular, the luminosity of the star is 0.013 solar luminosities, and the planetary “Bond” albedo is assumed to be 55%. At the layer we’re modeling, we assume a molecular weight of 25, and an atmospheric column depth of 2500 kg/m^2. This corresponds to an atmospheric pressure at the troposphere-stratosphere interface of order 400 milli bar. We assume an equilibrium night-side temperature of 250K (as a result of heat welling up from beneath).

The animation shows the sub-stellar hemisphere. The weather on the planet rapidly reaches an equilbrium flow pattern with small windspeeds (of order 3-4 m/s). The temperature at the substellar point equilibrates at 330K.

In the deeper, convective layers of the atmosphere, we expect fierce thunderstorms to occur. In analogy with thunderstorms on Earth driving anvils into the stratosphere, we model the effect of the thunderstorms by supplying a random heating term to the stratospheric flow. We definitely welcome constructive criticism of this approach, since we’re neophytes in the exo-terrestrial planet climate business. For the technically inclined, here’s a .pdf write-up that details our radiation-hydrodynamical scheme (the example planet in the write-up is HD 80606b, rather than Gl 581c, but the numerical method is the same).

So what’s being plotted? We identified regions of higher wind speed with the formation of high water clouds (white) and regions of low wind speed with more transparent layers in which the spectrum of reflected starlight is controlled by Raleigh scattering (blue). The patterns in the atmospheric animation are thus controlled by atmospheric pressure waves and the random thermal variations driven by the thunderstorms, and not by actual advection of air.

It’s interesting to compare this with the animation of the (rotating) Earth taken by the Galileo probe as it flew by to pick up a gravity assist.

The Gliese 581 system

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I’m still really jazzed that the systemic users detected Gl 581 c prior to its discovery announcement.

A dramatic ESO press release “Artist’s impression” of the Gl 581 system is all over the web today. It shows a planet that appears quite dry, clearly drawing on a model of in-situ formation from silicates and iron. In all likelihood, however, the planet migrated from beyond the snowline in Gl 581’s protostellar disk. It likely contains at least an Earth’s mass worth of water, and the view from space would show the upper layers of a deep and stormy atmosphere. Jonathan Langton is running hydrodynamical simulations to try to get a sense of what the weather is like on this world, and we’re hoping to have an animation up very shortly. (See this brief description of yesterday’s splash image).

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One of my pet peeves is that it’s possible to produce far more accurate and photo-realistic press release images of extrasolar planets than is usually done. Artist’s impressions generally veer toward being luxuriously long on depicting what we don’t know and rudely short when it comes to presenting what we do know.

At the JPL Cassini/Huygens website, there is a trove of photos taken by the orbiter showing Saturn and its moons from different vantages and illumination conditions. The photos below were taken from a location near the ring plane, and show Rhea and Enceladus. The two pictures were taken one minute apart as Enceladus (314 miles in diameter) is occulted by the larger Rhea (949 miles across) as seen from the spacecraft.

This sequence of photos makes the most of the kinds of information that we do know about extrasolar planets, namely the system geometry, the relative sizes, the orbital dynamics, and the illumination. Note how the night side of Enceladus is eerily lit by the unseen Saturn. These particular photos, furthermore, are effortlessly discrete with respect to what we don’t know about extrasolar planets, namely the geological details of the surfaces. In the absence of concrete information, the surface is perhaps better left either to the mind’s eye or to the moment when we get the real image. In Cassini’s glorious up-close view, Enceladus was revealed to be far more bizarre and interesting than anyone had imagined:

The lighting in the Gl 581 press release image is pretty weird. We’re looking straight at the parent star, and yet planet “c” is seen in quarter phase, illuminated by a source of white light placed to the right of the scene. The star, however, is thought to be single.

The dynamic range of illumination in the scene is way off as well. If we’re looking straight at a star, then the field of view is completely flooded, saturated with light, and replete with lens flares. Planets are always lost in the glare if you’re looking straight at a star. Since any view of a star is seen through an optical system, I think it should be possible to achieve a better sense of optical dynamical range by correctly applying lens flares. Over the next year, we’ll be looking into this in much more depth.

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This website has an interesting discussion of how to correctly render the colors of stars. Dynamic range aside, and assuming that the star is a 3000K blackbody radiator (which isn’t quite right, but is a reasonably good approximation) the color should be a lighter shade of orange. As drawn, the color is more appropriate to the night-side glow of a hot Jupiter.

What about the perspective in the scene? At first glance, it looks like Gl 581 “b” might have been drawn a little too large. Using the information in table 1 of the Udry et al. preprint, and adopting a 1.7 Earth-diameter size for “c”, a Neptune-size for “b”, and 0.3 solar diameters for Gl 581 itself, we can draw the orbits and sizes of the planets to scale and almost have it fit correctly in an image that fits on the blog. (You may want to make your browser window wider):

In reality, because of pixelation, the tiny dots showing the planets are a bit larger than they should be. Ellipses are circles seen from an angle, so by applying a 1-dimensional re-scale with Adobe Illustrator, we can view the system to scale from a long distance away:

When I’m looking at the ESO press release image on my computer screen, the planet measures 7.5 cm across, and is located 45 cm from my eye. It subtends an angle of 9.5 degrees at the vantage from which its being viewed. The point of view is thus located 11 planetary radii above the surface of the planet, and drawn to scale, the geometry in the image looks like this:

As viewed from the skies of planet “c”, planet “b” subtends an angle of 36 arc minutes, and remarkably, would appear just slightly larger than the Moon appears from Earth. The parent star, on the other hand would subtend 2.3 degrees of the sky, which is about ~4.6 times larger than the Sun appears in our sky. (Given that Gl 581 “c” is in a habitable orbit, and given that the star is a red dwarf, it’s absolutely necessary to have the star fill more of the sky.) With this information, we can draw the correct angular sizes of the star and the planet “b” as seen from the vantage of the drawing. The planet “b” should be somewhat smaller than drawn, and the star should be somewhat larger. On the balance, however, the angular sizes aren’t that far away from being correct.

Gliese 581 c (confirmed!)

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Gl 581 c

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Big news today from the Geneva extrasolar planet search team. Using the HARPS instrument at La Silla, they have announced the detection of an Msin(i)=5 Earth Mass planet orbiting the nearby red dwarf Gliese 581. The planet has an orbital period of 12.9 days, which places it squarely within the habitable zone of the parent star.

The planet probably migrated inward to its current location from beyond the “snowline” in GL 581’s protostellar disk, and so its composition likely includes a deep ocean, probably containing more than an Earth’s mass worth of water. Atmospheric water vapor is an excellent greenhouse gas, so the conditions at the planet’s atmosphere-ocean boundary are probably pretty steamy. It’s also possible, however, that the planet formed more or less in-situ. If this is the case, it would be made from iron and silicates and would be fairly dry. It’s unlikely, but not outside the realm of possibility, that this could be a genuinely habitable world. There’s no other exoplanet for which one can make this claim. In short, it’s a landmark detection.

In 2005, the Geneva team announced the detection of a Neptune-mass planet in a 5.366-day orbit around the star, and they published 20 high-precision radial velocities in support of their detection. These radial velocities have been in the systemic backend database since last summer, and so naturally, when today’s detection was announced, I was eager to see the models that our users have submitted for the Gl 581 planetary system.

The six submitted fits with the lowest chi-square for the system — by flanker (fits 1,2), EricFDiaz (fits 3,5), eugenio (fit 4), and bruce01 (fit 6) — all contain both the known 5.366 day planet as well as a planet with properties (Msin(i)~5 Mearth, P~12.2 days) that are a near-match to the newly announced planet. In the following screenshot, I’ve highlighted Gl 581 b in blue and the newly confirmed Gl 581 c in light orange.

Eureka!

Congratulations, Gentlemen. You made the first public-record characterizations of the first potentially habitable planet detected from Earth.

I’ve gone on record a number of times to emphasize that I have no interest whatsoever in priority disputes regarding who discovered what. It’s a forgone conclusion that the Swiss should receive all of the credit for their detection. The F-test false alarm probability for the Gl 581 c signal based on the 20 originally published velocities is ~25%, and there are thousands of planets that have been submitted to the systemic backend that don’t actually exist. Nevertheless, the systemic users can take a genuine pride in knowing that they were among the first on Earth to sense the existence of this extraordinary new world. I can’t resist dusting off Sir John Herschel’s ringing exhortation to the British Association of the Advancement of Science on Sept. 15, 1846, two weeks prior to the discovery of Neptune.

“The past year has given to us the new [minor] planet Astraea; it has done more – it has given us the probable prospect of another […] Its movements have been felt, trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration”

The Perfect Storm

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Most of the hot Jupiters with periods that last less than a week have orbits that are nearly circular. Tidal dissipation in a body on a short-period eccentric orbit is very strong. The net result of tidal dissipation is that energy of orbital motion is turned into heat. Io is the poster-world example of this phenomenon in our solar system.

There are, however, two hot Jupiters — HD 118203b and HD 185269b — that have orbital periods of less than a week, and eccentricities, e~0.3. Indeed, a quick glance at the radial velocities for HD 185269 phased at 6.838 days shows that the variation is not a perfect sinusoid.

With its eccentricity of 0.3, HD 185269b should have long since been delivered into a state of spin pseudosynchronization, in which it spins roughly three times on its axis for every two trips around the parent star. This state of affairs prevents a steady state flow pattern from developing, and hence the weather on this world is likely to be much more interesting than on your standard-issue tidally circularized hot Jupiter. Furthermore, the amount of energy absorbed by the planet is 345% greater at periastron than at apastron, which will also contribute to a strong “seasonal” variation during the planet’s 6.838-day year.

HD 185269b was discovered by John Johnson, who has been carrying out a radial velocity survey of luminous Hertzsprung-gap stars (discovery paper here). The stars in his survey are more massive than the Sun, and are in the midst of ending the core hydrogen-burning phase of their life cycles. They’re in the process of turning into red giants, and are thus cool enough to be profitably observed with the Doppler radial velocity technique. (See this post for more on John’s survey and its implications). HD 189269 is about four times more luminous than the Sun, and so the surface of the planet should average out at ~1300 K, which is quite hot, even for a hot Jupiter.

UCSC graduate student Jonathan Langton has been making great progress in his hydrodynamical calculations of the global surface flows on extrasolar planets. His code (which he’s written from scratch during the past year) now has a more sophisticated scheme for time-dependant radiative transfer, and is ideal for simulating the weather on planets like HD 185269b, and HD 80606b that are subject to strongly varying fluxes of radiation. We’re getting close to submitting a paper on his research, which will have predicted light curves for all of the known planets that are potentially bright enough to be observed with the Spitzer Space Telescope.

Here’s a sequence of images (each spaced by a bit more than a day) which show the global weather map for HD 185269 b as computed by Jonathan’s code. The view is from a camera that hovers above a fixed spot on the surface, and thus rotates with the planet. The color-scale is chosen to roughly approximate what the eye might see in the absence of clouds in the atmosphere. The brightest yellow regions have a temperature of ~1500K, and the coolest regions are down at ~900K. In this approximation, it’s best to think of the planet as a gigantic transparent molten marble.

In the third frame, we’re getting a good view of the heating that occurs on the hemisphere of the planet that is subject to the brunt of the insolation delivered during the periastron passage. The rapid heating of the atmosphere drives an intense global storm that is still shedding vortices and dissipating when the next wave of heating begins to hit.







It’s quite a fascinating flow, and it’s best visualized if you take the time to download the animations. Here are links to the movies: The first movie animates the temperature of the flow pattern for a full 6.838-day orbital period as viewed from a camera placed above the eastern hemisphere, and the second movie animates the temperature of the flow pattern for the same period from a camera placed above the western (opposite) hemisphere. These are 1.2 MB .avi format files. Run them on loop for a groovy lava lamp effect, and better yet, place them near a copy of the downloadable systemic console to make your desktop look like self-contained Institute for Exoplanetary Studies.

If the above .avi files don’t play on your machine, you’ll likely need to download the Xvid component for QuickTime (or an appropriate player for your OS). They are available here, and are trivial to install on Mac OSX 10.4 (Thanks for pointing me to the link, Andy!) If you can’t get the animations to play, here are links to the original .avi files for the first movie and the second movie. These are 41 MB .avi format files. I’ve put them on the UCO/Lick Server in order to keep our friends at Bluehost from wigging out and going into overload mode…

In the zone

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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.

Walker Lake

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It’s hard to get a more profound sense of physical remoteness and isolation in the United States than to drive east from Walker Lake, Nevada as the Sun sinks below the western horizon. It’s like Mars.

On a transcontinental flight last month, I had a window seat away from the wing. The sky was clear over Nevada, and the sun angle was low. It was an ideal situation for high-resolution imaging of a habitable terrestrial planet. The airplane view provides an interesting link between the experience of driving across the landscape and examining the satellite photos. The area just east of Walker Lake imparts an impression of a planet that’s very different from the global idea of the “pale blue dot.” The lake itself is salty, alkaline.

Source: Google Maps

The satellite and aerial photographs show that Walker Lake seems to be an evaporating remnant of what was once a much larger body of water.

Four billion years ago, Gusev crater on Mars probably looked very similar, with a sour central lake receeding with bathtub-ring clockwork.

Image: NASA

On Mars, there are only a few spots where a high-level of zoom will reveal artificial features:


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On Earth, in the region to the East of Walker Lake, there’s very little that can’t be ascribed to natural processes. This smooth black curve seems to be a wave cut bench of the vanished shoreline:

This feature, however, would be more challenging for a planetary geologist to explain. It’s obviously younger than the channels that it cuts across. Perhaps it’s fresh material that welled up from a crack in the Earth’s crust? There are volcanos dotted across the Basin and Range province.

Just south of the region shown in the splash image for this post, there are some extremely strange landforms…

And as is often the case in planetary exploration, when one wants to see even more detail,