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, 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 life there if existed, would likely be confined to sub-surface habitats, in contrast to the primarily surface biosphere of Earth. These candidates 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 - Earth's Moon, and Mars' moons Phobos and Deimos - none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant role as stellar radiation in the potential habitability of natural satellites.
Extrasolar moons are not yet confirmed to exist. Detecting them is difficult if not impossible, 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-433.02m, which has an Earth Similarity Index equal (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.
Read more about Natural Satellite Habitability: Presumed Conditions, Possible Origins, In The Solar System, Extrasolar, See Also
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