The discovery of the first transiting Earth-like planet by the CoRoT satellite
Manne Siegbahn Memorial Lectures
Thursday 30 September 2010
Daniel Rouan (LESIA, Observatoire de Paris-Meudon, France )
In 2008, the photometric monitoring during 5 months by the French-European satellite CoRoT of thousands of stars, revealed for one of them 176 very shallow (ΔF/F = 3x10-4) but significant periodic decreases of brightness, every 0.854 day, with a duration of 1.3 h each. If the possible interpretation of those events as partial eclipses of the target, a solar-like star, by a transiting planet was correct, the planet should have a radius as small as 1.7 REarth, making it the smallest transiting planet discovered to date.
However, before publishing such an outstanding result, the team had to accomplish a thorough task of complementary observations from ground to assess this interpretation. Indeed, several alternative interpretations were possible, such as a background system of mutually eclipsing stars. Many techniques were used, on several among the most powerful telescopes: high resolution visible and infrared spectroscopy, on/off transit photometric observations, high resolution imaging with adaptive optics, colours of the transit, etc. None of them succeeded in invalidating the small planet hypothesis and the announcement of the discovery of Corot-7b was released in February 2009.
A firm confirmation came a few months later with the analysis of a long series of radial velocity measurements using HARPS, the best instrument in the world in this respect. The planet was there in the data, with a firm evaluation of its mass: 4.8 MEarth. The derived density, precisely equal to the Earth one, suggests a similar type - a rocky one - and a similar composition, dominated by silicates. In addition, it was shown that a second planet, of only twice the mass of the first one was present on a slightly larger orbit, but still extremely close from the star.
The discovery of the Corot-7 system is obviously an important milestone on the pathway to habitable planets and I will discuss several questions that have been worked out since then: Is there a third planet? What is the structure of Corot-7b? Its physical conditions? Are there clues on its formation? Etc.