Exomoon habitability constrained by illumination and tidal heating

TL;DR

This paper explores the conditions under which exomoons could be habitable, considering illumination, eclipses, and tidal heating, and introduces the concept of a circumplanetary habitable edge.

Contribution

It models the combined effects of radiative and tidal heating on exomoon habitability and defines a habitable zone around planets, applying it to specific exoplanets.

Findings

Exomoons can be habitable if within a certain distance from their planet.

Eclipses can significantly reduce local illumination on exomoons.

Tidal heating can cause sterilization if too strong.

Abstract

The detection of moons orbiting extrasolar planets ("exomoons") has now become feasible. Once they are discovered in the circumstellar habitable zone, questions about their habitability will emerge. Exomoons are likely to be tidally locked to their planet and hence experience days much shorter than their orbital period around the star and have seasons, all of which works in favor of habitability. These satellites can receive more illumination per area than their host planets, as the planet reflects stellar light and emits thermal photons. On the contrary, eclipses can significantly alter local climates on exomoons by reducing stellar illumination. In addition to radiative heating, tidal heating can be very large on exomoons, possibly even large enough for sterilization. We identify combinations of physical and orbital parameters for which radiative and tidal heating are strong enough to trigger a runaway greenhouse. By analogy with the circumstellar habitable zone, these constraints define a circumplanetary "habitable edge". We apply our model to hypothetical moons around the recently discovered exoplanet Kepler-22b and the giant planet candidate KOI211.01 and describe, for the first time, the orbits of habitable exomoons. If either planet hosted a satellite at a distance greater than 10 planetary radii, then this could indicate the presence of a habitable moon.