Preprint typeset using LATEX style emulateapj v. 5/2/11 KEPLER PLANETS: A TALE OF EVAPORATION

Inspired by the Kepler mission’s planet discoveries, we consider the thermal contraction of planets close to their parent star, under the influence of evaporation. The mass-loss rates are based on hydrodynamic models of evaporation that include both X-ray and EUV irradiation. We find that only low-mass planets with hydrogen envelopes are significantly affected by evaporation, with evaporation being able to remove massive hydrogen envelopes inward of ∼ 0.1 AU for Neptune-mass objects, while being negligible for Jupiter-mass objects. Moreover, most of the evaporation occurs in the first 100 Myr of the star’s lives when it is more choromospherically active. We construct a theoretical population of planets with varying core masses, envelope masses, orbital separations, and stellar spectral types, and compare these against the sizes and densities measured for low-mass planets, both in the Kepler mission and from radial velocity surveys. This exercise leads us to conclude that evaporation is the driving force of evolution for close-in Kepler planets. In fact, some 50 % of the Kepler planet candidates may have been significantly eroded. Evaporation explains two striking correlations observed in these objects: a lack of large radius/low density planets close to the stars, and a possible bimodal distribution in planet sizes with a deficit of planets around 2R⊕. Planet