Habitable moon

The habitability of natural satellites is a measure of the potential of natural satellites to have environments hospitable to life. Habitable environments do not necessarily harbor life. Planetary habitability is an emerging study which is considered important to astrobiology for several reasons, foremost being that natural satellites are predicted to greatly outnumber planets and that it is hypothesized that habitability factors are likely to be similar to those of planets. There are, however, key environmental differences which have a bearing on moons as potential sites for extraterrestrial life.

The strongest candidates for natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn—Europa and Enceladus respectively, although if life exists in either place, it would probably be confined to subsurface habitats. Historically, life on Earth was thought to be strictly a surface phenomenon, but recent studies have shown that up to half of Earth's biomass could live below the surface. Europa and Enceladus exist outside the circumstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as liquid at the surface. In the Solar System's habitable zone, there are only three natural satellites—the Moon, and Mars's moons Phobos and Deimos (although some estimates show Mars and its moons to be slightly outside the habitable zone) —none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant a role providing heat as stellar radiation in the potential habitability of natural satellites.

Exomoons are not yet confirmed to exist. Detecting them is extremely difficult, because current methods are limited to transit timing. It is possible that some of their attributes could be determined by similar methods as those of transiting planets. Despite this, some scientists estimate that there are as many habitable exomoons as habitable exoplanets.

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