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Exoplanet's split personality
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 07 January 2010


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The most Earth-like exoplanet found to date, rocky CoRoT-7b, may have started life as a gas giant, say astronomers presenting research at the American Astronomical Society Meeting this week.

CoRoT-7b was discovered by the France-led Convection, Rotation and Planetary Transits (CoRoT) satellite in February 2009 and has a diameter and mass of just 1.7 times and 4.8 times that of the Earth, respectively. But residing 23 times closer to its star than Mercury is to the Sun it has an unforgiving environment that would see temperatures soar from a frigid -210 degrees on the night side to over 2,200 degrees on the day side.

Artist impression of sunrise over CoRoT-7b, a rocky exoplanet that is likely dominated by volcanism, but which once resembled a gas giant. Image: ESO/L. Calçada.

“With such a high dayside temperature, any rocky surface facing the star must be molten, and the planet cannot retain anything more than a tenuous atmosphere, even one of vaporized rock,” says Brian Jackson at NASA’s Goddard Space Flight Center. He estimates that solar heating may have already boiled off several Earth masses of material from CoRoT-7b.

Using computer simulations, Jackson's team tracked the mass loss and orbital changes of the planet through time and found that CoRoT-7b might have been more like Saturn when it first formed at a location fifty percent further away from the sun as it is now. “You could say that, one way or the other, this planet is disappearing before our eyes,” comments Jackson.

Jackson's team suggest that the same processes may have influenced other exoplanets that lie close to their parent stars, with many hot Jupiters undergoing this kind of mass-loss and migration in towards their stars, eventually leaving remnant cores like CoRoT-7b. “CoRoT-7b may be the first in a new class of planet – evaporated remnant cores,” says Jackson. “Studying the coupled processes of mass loss and migration may be crucial to unraveling the origins of the hundreds of hot, Earth-like planets space missions like CoRoT and NASA’s Kepler will soon uncover.”

In a different study of CoRoT-7b scientists at the University of Washington say that not only would the tiny planet's surface be molten, but that its slightly less-than-perfectly-circular orbit would exert different amounts of gravitational force at different points along the orbit, with the strongest gravitational pull when it is closest to the star and the weakest when it is most distant. This phenomenon causes the surface to flex which produces friction, heating the interior of the planet. The result: widespread volcanism on the surface – perhaps even more than Jupiter's moon Io experiences – an environment certainly not conducive to hosting life.

“If conditions are what we speculate, then CoRoT-7 b could have multiple volcanoes going off continuously and magma flowing all over the surface,” says Rory Barnes. A relatively tiny divergence of just 250 kilometres from a perfectly circular orbit would be enough to trigger volcanism, and in the case of CoRoT-7b, the deviation may in part be due to the next planet out in the system, 8.4 Earth-mass planet CoRoT-7c.

CoRoT-7b and -7c orbit the G-type main sequence star CoRoT 7, located nearly 500 light years away in the constellation Monoceros.