Exomoon Habitability Constrained by Illumination and Tidal Heating

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.

Transits of extrasolar moons around luminous giant planets

“…This planet needs to orbit the star beyond several AU, where alternative energy sources are required on its putative moons to keep their surfaces habitable, i.e., to prevent freezing of H 2 O. This heat could be generated by (1) tides (Reynolds et al 1987;Scharf 2006;Cassidy et al 2009;Heller & Barnes 2013); (2) planetary illumination (Heller & Barnes 2015); (3) release of primordial heat from the moon's accretion (Kirk & Stevenson 1987); or (4) radiogenic decay in its mantle and/or core (Mueller & McKinnon 1988). All of these sources tend to subside on a Myr timescale, but (1) and (2) can compete with stellar illumination over hundreds of Myr in extreme, yet plausible, cases.…”

“…For the young 11 M Jmass planet, I take R = 1.65 R J (Snellen et al 2014) and T eff = 1700 K (Baudino et al 2014). For the 1 Gyr version, I resort to the Mordasini (2013) tracks, predicting R = 1.09 R J and T eff = 480 K. Tidal heating is computed as by Heller & Barnes (2013), following an earlier work on tidal theory by Hut (1981). Figure 2 shows the habitable zone (HZ) for exomoons around a Jupiter-like (dark green areas) and a β Pic b-like planet (light green areas) at ages of 10 Myr (left) and 1 Gyr (right).…”

Formation of exomoons: a solar system perspective

“…The mass and radius of a moon (or moons) of an extrasolar planet (exomoon) and its host planet can offer a unique window into the timing, duration, and dynamical environment of planet formation, just as the moons in our Solar System have yielded clues about the formation of our planets [1][2][3][4][5]. Exomoons may also provide conditions conducive to the presence of liquid water in planetary systems where it might not otherwise be present [6][7][8][9][10], expanding the number of possible abodes for life in the galaxy [11].…”

The Center of Light: Spectroastrometric Detection of Exomoons

“…Habitable, Earth-like moons orbiting gas giant planets in circumstellar habitable zones have been proposed (Williams et al 1997;Kaltenegger 2000) and discussed (Reynolds et al 1987;Scharf 2006;Kaltenegger 2010;Heller 2012;Forgan & Kipping 2013;Heller & Barnes 2013Hinkel & Kane 2013;Heller & Armstrong 2014;Heller & Pudritz 2015a). For rocky planets in the habitable zone, the undetected presence of a large satellite can confuse characterization attempts, as the moon is an additional source of thermal or reflected light (Robinson 2011), potentially with its own molecular features (Rein et al 2014).…”