The Feasibility of Asynchronous Rotation via Thermal Tides for Diverse Atmospheric Compositions

TL;DR

This study investigates how atmospheric properties influence the possibility of asynchronous rotation in planets, revealing that high-pressure, low-opacity atmospheres are necessary for such rotation around M stars, impacting habitability.

Contribution

The paper combines simulations and analytic theory to analyze how atmospheric optical depth and pressure affect thermal tides and planetary rotation states.

Findings

High-pressure, low-opacity atmospheres enable asynchronous rotation.

Asynchronous rotation is unlikely around low-mass stars due to atmospheric constraints.

Thermal tide strength depends on atmospheric optical depth and surface pressure.

Abstract

The equilibrium rotation rate of a planet is determined by the sum of torques acting on its solid body. For planets with atmospheres, the dominant torques are usually the gravitational tide, which acts to slow the planet's rotation rate, and the atmospheric thermal tide, which acts to spin up the planet. Previous work demonstrated that rocky planets with thick atmospheres may produce strong enough thermal tides to avoid tidal locking, but a study of how the strength of the thermal tide depends on atmospheric properties has not been done. In this work, we use a combination of simulations from a global climate model and analytic theory to explore how the thermal tide depends on the shortwave and longwave optical depth of the atmosphere, the surface pressure, and the absorbed stellar radiation. We find that for planets in the habitable zones of M stars only high-pressure but low-opacity atmospheres permit asynchronous rotation owing to the weakening of the thermal tide at high longwave and shortwave optical depths. We conclude that asynchronous rotation may be very unlikely around low-mass stars, which may limit the potential habitability of planets around M stars.