Astrobiology Lecture Series - 'Exomoon Habitability'
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. First, speaker Rory Barnes of the University of Washington will review formation mechanisms for habitable exomoons and show that they are likely to orbit within a few tens of planetary radii. Hence, exomoons are likely to be tidally locked to their planet and experience days much shorter than their orbital period around the star and have seasons, both of which increase the likelihood for habitability. These satellites can receive more illumination per area than their host planets, as the planet reflects stellar light and emits thermal photons (both from reradiating stellar energy and its own gravitational contraction). On the other hand, eclipses may cool local climates on exomoons by reducing stellar illumination. In addition to radiative heating, tidal heating can be very large on Earth-sized 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". Finally, Barnes will discuss how Kepler observations of exomoons can be used to characterize exomoons.