Tidal obliquity evolution of potentially habitable planets

TL;DR

This paper investigates how tidal forces influence the obliquity, rotation, and habitability of Earth-like planets around low-mass stars, revealing rapid tilt erosion and significant tidal heating in certain conditions.

Contribution

It introduces a detailed analysis of obliquity evolution using two tide theories, highlighting the importance of tides in assessing planetary habitability around low-mass stars.

Findings

Earth-like obliquities erode in less than 0.1 Gyr around stars with <0.25 solar masses.

Tilt erosion times exceed 1 Gyr only for stars with >0.9 solar masses.

Tidal heating can be significant for planets orbiting very low-mass stars.

Abstract

Stellar insolation has been used as the main constraint on a planet's habitability. However, as more Earth-like planets are discovered around low-mass stars (LMSs), a re-examination of the role of tides on the habitability of exoplanets has begun. Those studies have yet to consider the misalignment between a planet's rotational axis and the orbital plane normal, i.e. the planetary obliquity. We apply two equilibrium tide theories to compute the obliquity evolution of terrestrial planets orbiting in the habitable zones around LMSs. The time for the obliquity to decrease from an Earth-like obliquity of 23.5 deg to 5 deg, the 'tilt erosion time', is compared to the traditional insolation habitable zone (IHZ) as a function of semi-major axis, eccentricity, and stellar mass. We also compute tidal heating and equilibrium rotation caused by obliquity tides. The Super-Earth Gl581d and the planet candidate Gl581g are studied as examples for tidal processes. Earth-like obliquities of terrestrial planets in the IHZ around <0.25 solar mass (M_sun) stars are eroded in <0.1 Gyr. Only terrestrial planets orbiting stars with masses >0.9 M_sun experience tilt erosion times larger than 1 Gyr throughout the IHZ. Terrestrial planets in the IHZ of stars with masses <0.25 M_sun undergo significant tidal heating due to obliquity tides. The predictions of the two tidal models diverge significantly for e>0.3. In our two-body simulations, Gl581d's obliquity is eroded to 0 and its rotation period reached its equilibrium state of half its orbital period in <0.1 Gyr. Tidal surface heating on the putative Gl581g is <150 mW/m^2 as long as its eccentricity is <0.3. Obliquity tides modify the concept of the habitable zone. Tilt erosion of terrestrial planets orbiting LMSs should be included by atmospheric modelers. Tidal heating needs to be considered by geologists.