Super-earths and mini-neptunes follow different orbital period-eccentricity relations.

Abstract

Many exoplanets have been observed with radius sizes between that of Earth and that of Neptune and are thus classified into two groups: super-earths (SEs) and mini-neptunes (MNs). There are no SEs or MNs in the Solar System, and the mechanisms responsible for their formation and evolution are debated. We investigated the relationships between the orbital period and eccentricity of SEs and MNs using both ensemble statistical analyses and individual measurements. We found that MNs follow an anticorrelation between orbital period and eccentricity, but SEs follow a different relation, possibly in the opposite direction. These trends imply that MNs and SEs are dynamically distinct populations. We suggest that SEs have been more strongly influenced by violent processes such as gravitational scattering and giant impacts, whereas MNs predominantly experienced quiescent secular evolution.