Asynchronous rotation of Earth-mass planets in the habitable zone of lower-mass stars

TL;DR

This paper demonstrates that Earth-like planets in the habitable zones of lower-mass stars can maintain asynchronous rotation states due to atmospheric tides, challenging the common assumption of tidal synchronization.

Contribution

It introduces a realistic atmospheric tide model predicting multiple equilibrium spin states, showing many habitable-zone planets may not be tidally locked.

Findings

Many Earth-like planets can have asynchronous rotation.

A thin atmosphere can drive non-synchronous spin states.

Stable asynchronous states are possible for planets around stars >0.5-0.7Msun.

Abstract

Planets in the habitable zone of lower-mass stars are often assumed to be in a state of tidally synchronized rotation, which would considerably affect their putative habitability. Although thermal tides cause Venus to rotate retrogradely, simple scaling arguments tend to attribute this peculiarity to the massive Venusian atmosphere. Using a global climate model, we show that even a relatively thin atmosphere can drive terrestrial planets' rotation away from synchronicity. We derive a more realistic atmospheric tide model that predicts four asynchronous equilibrium spin states, two being stable, when the amplitude of the thermal tide exceeds a threshold that is met for habitable Earth-like planets with a 1-bar atmosphere around stars more massive than 0.5-0.7Msun. Thus, many recently discovered terrestrial planets could exhibit asynchronous spin-orbit rotation, even with a thin atmosphere.