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Archive for May, 2007

Corot-Exo-1b

May 4th, 2007 1 comment

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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.

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Another HAT trick (plus XO-2b)

May 2nd, 2007 3 comments

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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!

Categories: detection Tags:

time series

May 1st, 2007 3 comments

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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…

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