With hundreds of known exoplanets and thousands more that will soon be confirmed, it is a natural consequence that moons will also exist around many of these exoplanets. Such a moon is known as an exomoon. The search for exomoons is already well underway for the Kepler space telescope and the first detected exomoon is expected to be roughly Earth-sized. A large number of Neptune-sized to Jupiter-sized exoplanets are known to orbit their host stars at the right distance where any Earth-sized exomoons orbiting such exoplanets could be potentially habitable.
This is an artist’s impression of Kepler-47c which is a Neptune-sized planet that orbits a binary star at a comfortable distance where an Earth-sized moon can be potentially habitable. (Credit: NASA/JPL-Caltech/T. Pyle)
Exomoons are attractive with regards to habitability for a number of reasons. An exomoon is expected to be tidally locked to its host planet and this ensures that exomoons in the stellar irradiation habitable zone (IHZ) have days that are shorter than their stellar year. This is advantageous for the habitability of Earth-sized exomoons in the IHZ of M-dwarf stars since an Earth-sized planet orbiting independently within the IHZ of an M-dwarf star is expected to be tidally locked to the star where the same hemisphere of the planet perpetually faces the star. Neptune-sized and Jupiter-sized exoplanets are likely to maintain their original spin-orbit misalignment than smaller planets. For this reason, an Earth-sized exomoon orbiting in the equatorial plane of such a planet is more likely to experience seasons than a single Earth-sized planet orbiting independently at the same distance from the star. Given the large number of Neptune-sized and Jupiter-sized exoplanets orbiting within the “Goldilocks” distance from their host stars, there is a possibility that habitable exomoons may outnumber habitable exoplanets.
Besides illumination from its host star, the habitability of an exomoon also depends on illumination from the host planet, tidal heating, constraints from orbital stability and eclipses when passing through the shadow of the host planet. There is an outer and inner limit to the range of distance where a habitable exomoon can orbit its host planet. The outer limit is defined by the host planet’s sphere of gravitational influence, beyond which the orbit of the exomoon becomes unstable to perturbations from the host star. The inner limit is defined by the minimum distance an exomoon can be from its host planet before tidal heating becomes significant enough to trigger a runaway greenhouse effect.
For a given planet-moon system, the distance between the outer and inner limit shrinks when the planet-moon system is moved from the IHZ of a G-dwarf star to a K-dwarf star and finally to an M-dwarf star. Our Sun is a G-dwarf star while M-dwarf stars are the smallest and most abundant class of stars. The range of habitable orbits ultimately vanishes for M-dwarf stars below 0.2 times the Sun’s mass. In the solar system, there is no moon with a mass that is in the range for habitable exomoons and the most massive moon, Ganymede, is only 0.025 times the Earth’s mass. As a result, it is unclear if exomoons as massive as Mars (0.107 times the Earth’s mass) or 10 times the mass of Ganymede can easily exist. However, given the unexpected diversity of known exoplanets, it is hard to not expect the existence of Earth-mass exomoons.
René Heller and Rory Barnes (2012), “Constraints on the Habitability of Extrasolar Moons”, arXiv:1210.5172 [astro-ph.EP]