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Transiting Gas Giant a ‘Rosetta Stone’

Claire Moutou, one of an international team of astronomers behind the discovery of the planet CoRoT-9b, says the distant world will become a ‘Rosetta stone in exoplanet research.’ And perhaps it will, for this is a transiting gas giant, but not a ‘hot Jupiter.’ In an orbit not dissimilar to that of Mercury, CoRoT-9b transits its star every 95 days, each transit lasting about eight hours. We’ve identified approximately 70 planets by transit methods, but this one is ten times farther from its host star than most gas giants previously discovered by this technique.

We may be jumping the gun a bit to call the climate here ‘temperate,’ as this European Southern Observatory news release does, because temperatures here will depend on layers of highly reflective clouds that may or may not exist on CoRoT-9b. ESO cites temperatures between 160 degrees and minus twenty degrees Celsius beneath those assumed clouds, but we should be able to learn much more because of the lengthy transit periods. Didier Queloz, a co-author of the Nature paper announcing the find, sees it this way:

“Our analysis has provided more information on CoRoT-9B than for other exoplanets of the same type. It may open up a new field of research to understand the atmospheres of moderate- and low-temperature planets, and in particular a completely new window in our understanding of low-temperature chemistry.”

The planet is about eighty percent the mass of Jupiter and was identified by CoRoT after 145 days of observations in 2008, with follow-up observations confirming the find at the 3.6-meter ESO telescope at La Silla (Chile). Team member Tristan Guillot says that it is mostly hydrogen and helium, with as much as 20 Earth masses of other elements, including water and rock at high-temperatures and pressures. The host star is similar to the Sun and is located some 1500 light years away from Earth in the direction of the constellation Serpens.

From the paper:

…Corot-9b is the first transiting planet among those with longer periods that does not represent a case of extreme eccentricity with associated extreme temperature changes (e.g. HD80806b’s temperature is estimated to rise from ~800K to ~1,500K over a six-hour period near periastron). On the contrary, it is the first transiting planet whose general properties coincide with the largest known population of planets, those of longer periods and low- to-moderate eccentricities, but which previously were known only from RV surveys. Our results on Corot-9b show that these planets may be expected to be rather similar to the giants of our Solar System.

Image: The orbital parameters of CoRoT-9b among extrasolar planets. Shown is the eccentricity and period of all 339 exoplanets for which both values are known as of Nov 1, 2009. Solid dots are the 58 transiting planets among them – most of them have short periods of < ~5 days and zero or low eccentricities. Only two further transiting planets have orbital periods longer than 10 days; they are HD17156b with 21.2 and HD80806b with 111.4 days. However, both of them also have the highest eccentricity among planets of similar periods. Open dots are the remaining exoplanets, known only from radial velocity observations. Credit: www.exoplanet.eu/Deeg et al. So we have a transiting gas giant that is not a hot Jupiter, allowing us to study a category of 'temperate' gas giants in much greater depth now that we can apply both transit observations (revealing the planet's diameter) and radial velocity measurements, which can help us figure out its mass and hence its density. That should produce information that will illuminate the large number of temperate gas giants found thus far by non-transit methods. The paper is Deeg et al., "A Transiting Giant Planet with a Temperature Between 250 K and 430 K," Nature 464 (18 March 2010), pp. 384-387. tzf_img_post

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  • Edward March 17, 2010, 14:56

    The ESO just released some nice media on this: http://www.eso.org/public/news/eso1011/

  • Zen Blade March 17, 2010, 15:47

    I first saw this as a brief write up on some news site and my reaction was “… okay… so what”…

    I think the brief write up here is much better at highlighting the significance of this find. It is interesting that it took more than a year, perhaps a year and a half to confirm and publish this data.
    Does this suggest that an orbit of ~300-400 days might take several years (After initial discovery) to confirm/generate sufficient data for publication?

    I hope we are recording our observations throughout space (perhaps this is not possible considering what may be required… but it seems like it should be) so that we can always go BACK in time to confirm newly-generated data. It strikes me that recorded data from previous years is really one of the most powerful ways to speed up and enhance exoplanet discovery and characterization.

    -Zen Blade

  • Administrator March 17, 2010, 15:52

    Good point, Zen Blade, and it’s certainly true that there are exoplanets lurking in older data that we’ll uncover with time. Proper data storage gets to be a huge issue when we’re culling as much information as we are. In any case, longer orbital periods do indeed take more time to confirm, which is one reason why we’ll have an anxious few years as we wait for Kepler’s discoveries — they’ll be coming in, to be sure, but these worlds will need multiple transits before they can be confirmed.

  • andy March 17, 2010, 16:02

    Looks like you’ve left some reference numbers in there, the planets in question are HD 17156b and HD 80606b…

    95 days puts it near the outer edge of the intermediate period planet desert: there seems to be a lack of gas giant planets between about 10 and 100 days around solar-type stars. I guess this is the first transiting giant planet which has a reasonable chance of hosting a system of massive satellites. Unfortunately since the Nature paper is behind a paywall, I’m unable to find out what kind of limits the CoRoT photometry sets on this.

  • Administrator March 17, 2010, 16:47

    I assume those are extra reference numbers; in any case, I’m quoting the caption directly from the paper and didn’t want to change it without checking. But I think you’re right and will alter the copy slightly.

  • Adam March 17, 2010, 18:00

    Makes me wonder how long a habitable moon might remain habitable around such a planet. With a tidally pumped magnetosphere it might hang on to atmosphere for aeons, though whether any could accrete in the first place is an open question – consider the differences between the Galilean moons and Titan.

  • Thomas March 17, 2010, 22:20

    @ Zen Blade,

    Yes, it takes at least three transits to fully confirm the orbital period of a transiting planet through photometry. First is the initial detection. Second likely tells you the orbital period. Third confirms the orbital period (because there may be multiple transiting planets in the system, or the transits may be artifacts of other phenomena, one needs to check for a consistent period).

    Radial velocity is also needed to constrain the mass (and it can also constrain the orbital period).

    Thus, a planet in an Earth-like orbit will need three years of continuous monitoring to really say for sure that you have detected it photometrically (assuming no pre-emptive RV follow-up, which might be costly if there are many targets and they are so dim that it takes a lot of telescope time). Kepler was built with this in mind, thus the three-year (minimum) mission.

  • Zen Blade March 18, 2010, 8:20


    Do you know anything about whether Kepler or other space telescopes have the ability to record data/light of coming from star/planetary systems that are not currently the focus of investigation/data processing.

    Essentially, can Kepler or multitask (if so, how much) such that data can be built up and that past data can be processed in the future to confirm new findings?

  • andy March 18, 2010, 19:14

    Adam: in the case of CoRoT-9b, the planet is probably too close to the star for habitable moons. As for whether such moons would accrete, it is somewhat discouraging that instead of forming a “super-Titan”, the Jovian system formed multiple satellites with masses comparable to the Earth’s moon. Then again, if exoplanet research has taught us anything, it is to expect suprises.

  • Ronald March 19, 2010, 6:09

    I was surprised that the media called this a rather ‘normal’ planet: the star is a G3, probably about 0.80 – 0.85 of solar luminosity.
    Since the planet is at about 0.4 AU, the it receives between 4 and 5 times as much light as Earth. Conditions must be between Mercurian and Venusian.
    Not a place you want to be.

  • Administrator March 19, 2010, 8:20

    Ronald, another ESO news release just came out, including the following:

    Corot-9b is the first transiting planet to have both a longer period and a near-circular orbit. Its orbit is slightly elliptical but at closest approach to its parent star it reaches a distance of 54 million kilometres. Although that is only about the distance of Mercury in our Solar System, it is by far the largest orbit of any transiting planet found so far. Because it orbits a star cooler than our Sun, calculations estimate that Corot-9b’s temperature could lie somewhere between –23°C and 157°C.

  • andy March 20, 2010, 9:48

    Defining “normal” based on the planets in our system may not be a very good way of doing things. Given the currently-known populations of gas giant planets, this is pretty normal: the majority of known giant planets are members of the “eccentric Jupiters”, i.e. planets with periods of more than about 100 days and a wide eccentricity distributions. This is despite the strong bias towards finding hot Jupiters (which are easy to find but apparently quite rare). CoRoT-9b is at the short-period, low-eccentricity end of the main giant planets population.

  • Thomas March 24, 2010, 9:28

    @ Zen Blade,

    Kepler can detect the phase curve for non-transiting planets. The difficulty would probably be recognizing that it has been discovered (instead of, say, intrinsic stellar variability), and getting the resources to confirm it.

    Kepler is observing several stars that are not expected to host transiting planets (or at least planets that may be seen as transiting) to get in extra science as part of a guest observer program or something.