On the equilibrium rotation of Earth-like extra-solar planets

TL;DR

This paper explores the possible equilibrium rotation states of Earth-like exoplanets, showing that most are unlikely to be tidally locked due to their eccentric orbits and atmospheric effects.

Contribution

It provides a general framework for understanding the equilibrium rotation states of eccentric, Earth-like exoplanets, extending beyond the synchronous assumption.

Findings

Most Earth-like exoplanets with eccentric orbits are unlikely to be in synchronous rotation.

Up to four equilibrium rotation states are possible, including retrograde rotation.

The analysis applies to planets with masses less than twelve Earth-masses.

Abstract

The equilibrium rotation of tidally evolved "Earth-like" extra-solar planets is often assumed to be synchronous with their orbital mean motion. The same assumption persisted for Mercury and Venus until radar observations revealed their true spin rates. As many of these planets follow eccentric orbits and are believed to host dense atmospheres, we expect the equilibrium rotation to differ from the synchronous motion. Here we provide a general description of the allowed final equilibrium rotation states of these planets, and apply this to already discovered cases in which the mass is lower than twelve Earth-masses. At low obliquity and moderate eccentricity, it is shown that there are at most four distinct equilibrium possibilities, one of which can be retrograde. Because most presently known "Earth-like" planets present eccentric orbits, their equilibrium rotation is unlikely to be synchronous.