Tidal Limits to Planetary Habitability

TL;DR

This paper explores how tidal heating influences planetary habitability, proposing a revised habitable zone that accounts for both stellar insolation and tidal effects, with implications for planets like GJ 581 d.

Contribution

It introduces a new habitable zone model that integrates tidal heating effects, refining the criteria for planetary habitability around low-mass stars.

Findings

GJ 581 d lies within the traditional habitable zone.

Tidal heating on GJ 581 d may be too weak to support plate tectonics.

Tidal heating can either hinder or help habitability depending on its magnitude.

Abstract

The habitable zones of main sequence stars have traditionally been defined as the range of orbits that intercept the appropriate amount of stellar flux to permit surface water on a planet. Terrestrial exoplanets discovered to orbit M stars in these zones, which are close-in due to decreased stellar luminosity, may also undergo significant tidal heating. Tidal heating may span a wide range for terrestrial exoplanets and may significantly affect conditions near the surface. For example, if heating rates on an exoplanet are near or greater than that on Io (where tides drive volcanism that resurface the planet at least every 1 Myr) and produce similar surface conditions, then the development of life seems unlikely. On the other hand, if the tidal heating rate is less than the minimum to initiate plate tectonics, then CO_2 may not be recycled through subduction, leading to a runaway greenhouse that sterilizes the planet. These two cases represent potential boundaries to habitability and are presented along with the range of the traditional habitable zone for main sequence, low-mass stars. We propose a revised habitable zone that incorporates both stellar insolation and tidal heating. We apply these criteria to GJ 581 d and find that it is in the traditional habitable zone, but its tidal heating alone may be insufficient for plate tectonics.