Evaluating the habitability of exomoons

Jupiter’s moon Europa and Saturn’s moon Enceladus are among the most promising places to look for extraterrestrial life in the solar system. Despite their distance from the Sun, the moons qualify as habitable because their proximity to their massive planetary hosts engenders tidal heating that is strong enough to melt water ice below their frozen surfaces. No one has yet detected a moon around an exoplanet. The objects are hard to find, even with the most powerful telescopes. Nevertheless, astronomers are theorizing about the habitability of exomoons.

For their study, Rhett Zollinger of Southern Utah University, John Armstrong of Weber State University, and René Heller of the Max Planck Institute for Solar System Research chose to focus on Mars-sized moons that orbit gas giants around M dwarf stars. Their choice was not arbitrary. Big moons are more likely to harbor life than small moons. Theory predicts that big moons are more likely found around gas giants than around terrestrial planets. And M dwarfs are the most abundant type of star in the Milky Way.

Because M dwarfs are dimmer than the Sun, their habitable zones—that is, the orbital locations where an Earth-like planet or moon with an Earth-like atmosphere can sustain surface water—lie closer to the star. From their modeling, Zollinger, Armstrong, and Heller discovered that for a wide range of M dwarf masses and other parameters, extreme tidal heating rendered the exomoons uninhabitable. So if there are moons around giant planets that orbit the most abundant type of star, in many cases, those moons could be uninhabitable, even when they occupy the star’s habitable zone. (R. R. Zollinger, J. C. Armstrong, R. Heller, Mon. Not. R. Astron. Soc. 472, 8, 2017, doi:10.1093/mnras/stx1861 .)