How stellar activity affects the size estimates of extrasolar planets

Lightcurves have long been used to study stellar activity and have more recently become a major tool in the field of exoplanet research. We discuss the various ways in which stellar activity can influence transit lightcurves, and study the effects using the outstanding photometric data of the CoRoT-2 exoplanet system. We report a relation between the `global' lightcurve and the transit profiles which turn out to be shallower during high spot coverage on the stellar surface. Furthermore, our analysis reveals a color dependence of the transit lightcurve compatible with a wavelength-dependent limb darkening law as observed on the Sun. Taking into account activity-related effects, we re-determine the orbit inclination and planetary radius and find the planet to be approximately 3% larger than reported previously. Our findings also show that exoplanet research cannot generally ignore the effects of stellar activity.