Orbital Evolution of Close-in Super-Earths Driven by Atmospheric Escape

Abstract

The increasing number of super-Earths close to their host stars have revealed a scarcity of close-in small planets with 1.5–2.0 R⊕ in the radius distribution of Kepler planets. The atmospheric escape of super-Earths by photoevaporation can explain the origin of the observed “radius gap.” Many theoretical studies have considered the in situ mass loss of a close-in planet. Planets that undergo atmospheric escape, however, move outward due to the change in the orbital angular momentum of their star–planet systems. In this study, we calculate the orbital evolution of an evaporating super-Earth with a H₂/He atmosphere around FGKM-type stars under stellar X-ray and extreme-UV irradiation (XUV). The rate of increase in the orbital radius of an evaporating planet is approximately proportional to that of the atmospheric mass loss during a high stellar XUV phase. We show that super-Earths with a rocky core of ≲10 M⊕ and a H₂/He atmosphere at ≲0.03–0.1 au (≲0.01–0.03 au) around G-type stars (M-type stars) are prone to outward migration driven by photoevaporation. Although the changes in the orbits of the planets would be small, they would rearrange the orbital configurations of compact, multiplanet systems, such as the TRAPPIST-1 system. We also find that the radius gap and the so-called “Neptune desert” in the observed population of close-in planets around FGK-type stars still appear in our simulations. On the other hand, the observed planet population around M-type stars can be reproduced only by a high stellar XUV luminosity model.