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the last first look

October 5th, 2009

As is usually the case, there’s been little or no shortage of interesting developments in the field of extrasolar planets. The biggest recent news has been the announcement at the Barcelona conference of a definitive mass for the ultra-short period transiting planet CoRoT-7b. It weighs in at a mere 4.8 Earth Masses (copy of the Queloz et al. preprint here).

Recall that CoRoT-7b caused quite a stir earlier this year with its weird properties. The planet’s year is a fleeting twenty hours and twenty nine minutes, and it induces a tiny transit depth of 0.03%. Unfortunately, the parent star presents a less-than-ideal target for high-precision radial velocity work. It has spots that come and go, and its stellar activity produces frustratingly noisy Doppler measurements. As a result, at the time of CoRoT-7b’s initial announcement, there was no definitive measurement of the planet’s mass.

That’s changed, however, with an unprecedentedly all-out deployment of the HARPS spectrograph. From the Queloz et al. preprint:

A total of 106 measurements between 30 and 60 minute exposure time each were obtained over 4 months, and with sometimes 3 measurements being taken on the same night.

Now in my notoriously biased opinion, such observational enthusiasm is perhaps best reserved for stars such as Alpha Cen B, but a fair argument can be made that the massive investment of time did pay off. Remarkably, the radial velocity data set shows that there are two short-period planets in the CoRoT-7 system. The outer companion, which doesn’t transit, has a period of 3.7 days and at least eight Earth masses. Most dramatically, by combining the mass and radius measurements of CoRoT-7b, one arrives at a density of 5.5 grams per cubic centimeter, essentially identical to that of the Earth, suggesting that the planet is largely composed of refractory materials. (I hesitate to apply the term “rocky” to the CoRoT-7c landscape for the same reason that I’d refrain from describing the Amazon Delta as “icy”.)

In a very real sense, the HARPS campaign on CoRoT-7b has given us our last first look at a fundamentally new category of planet — that is, a world lying in the factor-of-fourteen mass gap spanned by Earth and Uranus. And, from exo-political point of view, the stakes surrounding this discovery were very high. The first density measurement of a planet in this category could just as easily have been made by teams combining high-precision Doppler measurements with either (1) Warm Spitzer, (2) ground-based photometry, (3) Kepler, (4) MOST, (5) HST, or (6) CoRoT. So I can imagine that there was a certain impetus underlying the scheduling of that huge block of HARPS time.

The discovery could, however, still be waiting to be made. Despite all the effort with HARPs, there remains a hefty 70% error on the density determination. This means that there’s a ~16% chance that CoRoT-7b is actually less dense than Neptune.

I’ll go out on a limb: CoRoT-7b’s density will turn out to be anomalously high. More than 90% of “super Earths” will turn out to be “sub-Neptunes” as far as their density is concerned.

  1. October 5th, 2009 at 10:45 | #1

    Wouldn’t an “Earth-like” density for a 5 M(e) planet be higher than 5.5, due to gravitational self-compression?

    And does your limb-going apply to 90% of existing super-earths, or 90% of measured super-earths? Short period planets are both more likely to transit and more likely to lose volatiles, so I’d say that even if Corot 7b type planets are unusual, they will make up most of our detected samples for the next 10 years or more.

  2. October 7th, 2009 at 21:59 | #2

    IIRC according to predictions of tidal theory it seems the HD 40307 planets are likely to be mini-Neptunes rather than super-Earths.

  3. October 9th, 2009 at 10:58 | #3

    As our exoplanet statistics improve in the coming years, I believe that there shall be a menagerie of objects in this range, perhaps with distributed characteristics from mini-neptunes to super-earths. A 2D scatter distribution could be used to show the distribution of properties of these models (rock, ice…) perhaps even extended to 3D…

  4. October 9th, 2009 at 21:30 | #4

    There’s a paper about attempting to characterise the inner planet of Gliese 876 on the arXiv today… some limits in inclination/albedo/radius parameter space… arxiv:0910.1505

  5. Starguy32
    October 11th, 2009 at 22:57 | #5

    I’m sorry to add in an unrelated comment, but I just recently re-downloaded the console and have absolutely no idea how to start it up or what I need to do to run it.

  6. benelson
    October 15th, 2009 at 19:23 | #6

    @Starguy32
    Windows: After extracting from the archive, go into the folder and simply double click the systemic.jar file. It’ll take a few seconds to load.

    Linux: Go into your command line, cd to the folder, and type “java -jar systemic.jar”.

    Make sure you have a relatively up-to-date Java Runtime Environment on as well.

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