Letter to the Editor

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

¹ Departamento de Física da Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal e-mail: correia@ua.pt
² Astronomie et Systèmes Dynamiques, IMCCE-CNRS UMR 8028, 77 avenue Denfert-Rochereau, 75014 Paris, France
³ Université de Lyon 1, CRAL, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France

Received: 13 June 2008
Accepted: 22 July 2008

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 12 . 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.