Orbital migration induced by anisotropic evaporation

Short-period planets are subject to intense energetic irradiations from their stars. It has been shown that this can lead to significant atmospheric mass loss and can create lower mass planets. Here, we analyse whether the evaporation mechanism can affect the orbit of planets. The orbital evolution of a planet undergoing evaporation is derived analytically in a very general way. Analytical results are then compared with the period distribution of two classes of inner exoplanets: Jupiter-mass planets and Neptune-mass planets. These two populations have very distinct period distributions, with a probability lower than 10-4 that they were derived from the same parent distribution. We show that mass ejection can generate significant migration with an increase in orbital period that matches the difference in distribution of the two populations very well. This would happen if the evaporation emanates from above the hottest region of planet surface. Thus, migration induced by evaporation is an important mechanism that cannot be neglected.