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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Higher resolution .eps version here.

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

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

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

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

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

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

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