Habitability of Exomoons at the Hill or Tidal Locking Radius

TL;DR

This study investigates the habitability potential of exomoons orbiting giant exoplanets, analyzing flux variations, tidal locking, and planetary eccentricity effects to identify conditions conducive to life.

Contribution

It provides new insights into how orbital distance, planetary eccentricity, and heat redistribution influence exomoon habitability, expanding understanding beyond previous models.

Findings

Exomoons at the Hill radius experience maximum flux gradients.

Moons around planets partially in the habitable zone need high heat redistribution.

Exomoons can be habitable even if their host planet is only partially in the habitable zone.

Abstract

Moons orbiting extrasolar planets are the next class of object to be observed and characterized for possible habitability. Like the host-planets to their host-star, exomoons have a limiting radius at which they may be gravitationally bound, or the Hill radius. In addition, they also have a distance at which they will become tidally locked and therefore in synchronous rotation with the planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets mu Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on a moon at it's furthest, stable distance from the planet achieves it's largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in orbit to the planet. We have also analyzed the effect of planetary eccentricity on the flux on the moon, examining planets that traverse the habitable zone either fully or partially during their orbit. Looking solely at the stellar contributions, we find that moons around planets that are totally within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. Meaning, a planet does not need to spend its entire orbit within the habitable zone in order for the exomoon to be habitable. Because the applied systems are comprised of giant planets around bright stars, we believe that the transit detection method is most likely to yield an exomoon discovery.