An analysis of the CoRoT-2 system: a young spotted star and its inflated giant planet

Context. CoRoT-2b is one of the most anomalously large exoplanet known. Given its high mass, its large radius cannot be explained by standard evolution models. Interestingly, the planet's parent star is an active, rapidly rotating solar-like star with with spots covering a large fraction (7-20%) of its visible surface. Aims. We attempt to constrain the properties of the star-planet system and understand whether the planet's inferred large size may be caused a systematic error in the inferred parameters, and if not, how it can be explained. Methods. We combine stellar and planetary evolution codes based on all available spectroscopic and photometric data to obtain self-consistent constraints on the system parameters. Results. We find no systematic error in the stellar modeling ( including spots and stellar activity) that would cause a similar to 10% reduction in size of the star and thus the planet. Two classes of solutions are found: the usual main-sequence solution for the star yields for the planet a mass of 3.67 +/- 0.13 M-J, a radius of 1.55 +/- 0.03 R-J for an age that is at least 130 Ma and should be younger than 500 Ma given the star's rapid rotation and significant activity. We identify another class of solutions on the pre-main sequence, for which the planet's mass is 3.45 +/- 0.27 M-J and its radius is 1.50 +/- 0.06 R-J for an age of 30 to 40 Ma. These extremely young solutions provide the simplest explanation of the planet's size that can then be matched by a simple contraction from an initially hot, expanded state, if the atmospheric opacities are larger by a factor of similar to 3 than usually assumed for solar composition atmospheres. Other solutions imply that the present inflated radius of CoRoT-2b is transient and the result of an event that occurred less than 20 Ma ago, i.e., a giant impact with another Jupiter-mass planet, or interactions with another object in the system that caused a significant rise in the eccentricity followed by the rapid circularization of its orbit. Conclusions. Additional observations of CoRoT-2 that could help us to understand this system include searches for an infrared excess, a debris disk, and additional companions. The determination of a complete infrared lightcurve including both the primary and secondary transits would also be extremely valuable to constrain the planet's atmospheric properties and determine the planet-to-star radius ratio in a manner less vulnerable to systematic errors caused by stellar activity.