A habitable exomoon is a moon orbiting an extrasolar planet that has the ideal conditions to host life as we know it. A total of 21 exomoon candidates have been detected, but none of them have been confirmed. Habitability of extrasolar moons will depend on stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. Other factors such as tidal heating might play a role in a moon's habitability. Host planet Given the general planet-to-satellite mass ratio of 10000, large Saturn or Jupiter-sized gas planets in the habitable zone are believed to be the best candidates to harbor Earth-like moons, with more than 120 such planets discovered by 2018. Massive exoplanets known to be located within a habitable zone (such as Gliese 876 b, 55 Cancri f, Upsilon Andromedae d, 47 Ursae Majoris b, HD 28185 b and HD 37124 c) are of particular interest as they may potentially possess natural satellites with liquid water on the surface. It was also found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view. Habitable edge In 2012, scientists introduced a concept to define the habitable orbits of moons. The concept is similar to the circumstellar habitable zone for planets orbiting a star, but for moons orbiting a planet. This inner border, which they call the circumplanetary habitable edge, delimits the region in which a moon can be habitable around its planet. Moons closer to their planet than the habitable edge are uninhabitable. Magnetosphere The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another factor of exomoon habitability. Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability. Tidal-locking Earth-sized exoplanets in the habitable zone around red dwarfs are often tidally locked to the host star. This has the effect that one hemisphere always faces the star, while the other remains in darkness. Like an exoplanet, an exomoon can potentially become tidally locked to its primary. However, since the exomoon's primary is an exoplanet, it would continue to rotate relative to its