  The detection of moons orbiting extrasolar planets ("exomoons") has now
become feasible. Once they are discovered in the circumstellar habitable zone,
questions about their habitability will emerge. Exomoons are likely to be
tidally locked to their planet and hence experience days much shorter than
their orbital period around the star and have seasons, all of which works in
favor of habitability. These satellites can receive more illumination per area
than their host planets, as the planet reflects stellar light and emits thermal
photons. On the contrary, eclipses can significantly alter local climates on
exomoons by reducing stellar illumination. In addition to radiative heating,
tidal heating can be very large on exomoons, possibly even large enough for
sterilization. We identify combinations of physical and orbital parameters for
which radiative and tidal heating are strong enough to trigger a runaway
greenhouse. By analogy with the circumstellar habitable zone, these constraints
define a circumplanetary "habitable edge". We apply our model to hypothetical
moons around the recently discovered exoplanet Kepler-22b and the giant planet
candidate KOI211.01 and describe, for the first time, the orbits of habitable
exomoons. If either planet hosted a satellite at a distance greater than 10
planetary radii, then this could indicate the presence of a habitable moon.
