Monthly Archives: April 2007

Gl 581 — The Movie

Posted on by
9

Image Source.

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

Posted on by
2

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).

Image Source.

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.

Image Source.

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!)

Posted on by
19

Gl 581 c

Image Source.

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

Posted on by
6

null

Image Source.

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

Posted on by
3

Image Source.

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

Posted on by
2

Image Source

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:


Image Source

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,

Bali Hai

Posted on by
2

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.

GJ 6-7-4

Posted on by
3

Image Source.

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.

backlog

Posted on by
1

Image Source.

It was the end of the Winter quarter here at UCSC last week, and then I went on a trip, and then bam! More than a week with no posts… In the interim, there have been a number of interesting developments related to extrasolar planets. Here’s a brief run-down of some topics that I want to look at in more depth in the very near future:

Thanks to continuing efforts from the back-end user base, we’re accumulating a highly useful database of stable, low chi-square fits to the synthetic radial velocity data sets that comprise the Systemic Jr. catalog. Stefano has run a preliminary analysis and interpretation of the data. There are interesting implications for the overall eccentricity distribution of extrasolar planets, and there also appears to be a robust criterion for determining with confidence when you’ve extracted a real, previously unannounced planet from a given data set. We’re putting together a full report, which will appear quite soon. In the meantime, please keep submitting fits for systems that haven’t yet been adequately characterized.

The detection of another Neptune-mass planet orbiting a nearby red-dwarf was announced today. Yet more evidence for the core-accretion theory of planet formation! The discovery paper stops short of tabulating the radial velocities, but as I write this, Eugenio is busy dextering them onto the systemic back-end and onto the downloadable systemic console.

The theoretical case for the existence of Alpha Centauri B b is getting stronger by the day.

This year’s first ‘606 day is coming up next week, with a transit opportunity following on April 17th. I didn’t do enough to get the word out last December, but I’m hoping for good photometric coverage of the star during the upcoming window. A central transit for HD 80606b would last roughly 18 hours, so participation from observers worldwide will be required to definitively rule out transits.