What is Natural satellite habitability ?

Natural satellite habitability is the measure of a natural satellite's potential to sustain life. It 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 of natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn—Europa  and Enceladus respectively—though if life existed 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 life 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 only three natural satellites may be found—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 as stellar radiation in the potential habitability of natural satellites.

Extrasolar moons are not yet confirmed to exist. Detecting them is extremely difficult, though 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. The identification of candidate moons such as KOI-433.02 m, which has an Earth Similarity Index (ESI) equal to that of the highest scoring exoplanet KOI-1686.01, and the categorising of a similar number of habitable exomoon candidates as habitable exoplanets, would appear to indicate that the distribution of planetary mass exomoons within the habitable zone is at least the same if not greater than that of planets.