  The Kepler space telescope has detected transits of objects as small as the
Earth's Moon, and moons as small as 0.2 Earth masses can be detected in the
Kepler data by transit timing and transit duration variations of their host
planets. Such massive moons around giant planets in the stellar habitable zone
(HZ) could serve as habitats for extraterrestrial life. We here assess the
danger of exomoons to be in a runaway greenhouse (RG) state due to extensive
heating from the planet. We apply pre-computed evolution tracks for giant
planets to calculate the incident planetary radiation on the moon as a function
of time. The total energy budget of stellar flux, illumination from the planet,
and tidal heating in the satellite is compared to the critical flux for the
moon to experience an RG effect. Irradiation from a 13-Jupiter-mass planet onto
an Earth-sized moon at a distance of ten Jupiter radii can drive an RG state on
the moon for about 200 Myr. If stellar illumination equivalent to that received
by Earth from the Sun is added, the RG holds for about 500 Myr. After 1000 Myr,
the planet's habitable edge has moved inward to about 6 planetary radii.
Exomoons in orbits with eccentricities of 0.1 experience strong tidal heating;
they must orbit a 13-Jupiter-mass host beyond 29 or 18 Jupiter radii after 100
Myr (at the inner and outer boundaries of the stellar HZ, respectively), and
beyond 13 Jupiter radii (in both cases) after 1000 Myr to be habitable. If a
roughly Earth-sized moon would be detected in orbit around a giant planet, and
if this binary would orbit in the stellar HZ, then it will be crucial to
recover the moon's orbital history. If such a moon around a 13-Jupiter-mass
planet has been closer than 20 Jupiter radii to its host during the first few
hundred million years at least, then it might have lost substantial amounts of
its initial water reservoir and be uninhabitable today.

Earth and Planetary Astrophysics