Formation of hot Neptunes by evaporation of hot Jupiters

Context. Hot Jupiters are subject to intense energetic irradiations from their stars. It has been shown that this can lead to signi ficant atmospheric mass-loss and create a population of smaller mass planets. Aims. We analyse whether the observed hot Neptunes can be the outcome of the partial evaporation of hot Jupiters. Methods. 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 the two classes of inner exoplanets: Jupiter-mass planets and Neptune-mass planets. Results. Hot Jupiters and hot Neptunes have a very distinct period distribution, with a probability lower than 10 −4 that they were derived from the same parent distribution. This difference can be perfectly explained by the presented migration mechanism if hot Neptunes are partially evaporated hot Jupiters, where matter is ejected from the hottest region of the planet surface. Conclusions. We show that hot Neptunes and lower-mass planets are likely to be partially evaporated hot Jupiters.