Two Papers on Habitable ExoMoons

Capture of Terrestrial-Sized Moons by Gas Giant Planets

Author:

1. Darren M. Williams (a)

Affiliation:

a. Penn State Erie, The Behrend College, School of Science, Erie, Pennsylvania

Abstract:

Terrestrial moons with masses greater than 0.1 M possibly exist around extrasolar giant planets, and here we consider the energetics of how they might form. Binary-exchange capture can occur if a binary-terrestrial object (BTO) is tidally disrupted during a close encounter with a giant planet and one of the binary members is ejected while the other remains as a moon. Tidal disruption occurs readily in the deep gravity wells of giant planets; however, the large encounter velocities in the wells make binary exchange more difficult than for planets of lesser mass. In addition, successful capture favors massive binaries with large rotational velocities and small component mass ratios. Also, since the interaction tends to leave the captured moons on highly elliptical orbits, permanent capture is only possible around planets with sizable Hill spheres that are well separated from their host stars.

Exomoon Habitability Constrained by Illumination and Tidal Heating

Authors:

1. Rene´ Heller (a)
2. Rory Barnes (b,c)

Affiliations:

a. Leibniz-Institute for Astrophysics Potsdam (AIP), Potsdam, Germany

b. Astronomy Department, University of Washington, Seattle, Washington, USA

c. NASA Astrobiology Institute—Virtual Planetary Laboratory Lead Team, USA

Abstract:

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.