Various astrophysical effects distinguish investigations of exomoon habitability from studies on exoplanet habitability. We argue here that it will be possible to constrain their habitability on the data available at the time they will be discovered. Habitability of the moons around these planets has received little attention.
Among the 2312 exoplanet candidates detected with Kepler (Batalha et al., 2012), more than 50 are indeed in the IHZ (Borucki et al., 2011 Kaltenegger and Sasselov, 2011 Batalha et al., 2012), yet most of them are significantly larger and likely more massive than Earth. No such Earth analogue has been confirmed so far. Since the discovery of the first exoplanet almost two decades ago (Mayor and Queloz, 1995), roughly 800 more have been found, and research on exoplanet habitability has culminated in the targeted space mission Kepler, specifically designed to detect Earth-sized planets in the circumstellar irradiation habitable zones (IHZs Huang, 1959 Kasting et al., 1993 Selsis et al., 2007 Barnes et al., 2009) 1 around Sun-like stars. T he question whether life has evolved outside Earth has prompted scientists to consider habitability of the terrestrial planets in the Solar System, their moons, and planets outside the Solar System, that is, extrasolar planets. Key Words: Astrobiology-Extrasolar planets-Habitability-Habitable zone-Tides. If either planet hosted a satellite at a distance greater than 10 planetary radii, then this could indicate the presence of a habitable moon. 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. We identify combinations of physical and orbital parameters for which radiative and tidal heating are strong enough to trigger a runaway greenhouse. In addition to radiative heating, tidal heating can be very large on exomoons, possibly even large enough for sterilization. On the contrary, eclipses can significantly alter local climates on exomoons by reducing stellar illumination. These satellites can receive more illumination per area than their host planets, as the planet reflects stellar light and emits thermal photons. 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. Once they are discovered in the circumstellar habitable zone, questions about their habitability will emerge. (2013) AsBio, 13, 225.The detection of moons orbiting extrasolar planets (“exomoons”) has now become feasible. Finally, we will show preliminary results for the long-term climate of Ignan Earths by modeling the carbonate-silicate cycle with a vertical tectonic regime (known as heat-pipe tectonics, expected to dominate in such worlds) at varying amounts of tidal heating to define the upper limit of plausible tidal habitability References: Barnes, R., et al. We demonstrate how the mantle will remain largely solid despite high tidal heating, and how crustal thickness and stability is preserved despite higher mantle temperatures and the subsequent overall heat fluxes. We refer to these planets as Ignan Earths. We investigate the habitability of terrestrial planets with internal heating rates between these limits, many orders of magnitude higher than the Earths. Between Io and Tidal Venuses (combined stellar + tidal heating of > 300 W/m2), are a wide range of plausibly habitable tidal heating states. An ultimate upper limit to tidal heating is the internally driven runaway greenhouse, which may cause Tidal Venuses (Barnes et al. To be habitable, such a planet must maintain a stable crust and a temperate climate. However, terrestrial planets in the habitable zones around M-dwarf stars might experience tidal heating on par with Io or even higher. In our Solar System, the best example of such a world is Jupiters moon Io, which has a heat flux of 2 W/m2 compared to the Earths 60 mW/m2. While the primary heat sources of the Earth are the decay of radionuclides and core crystallization, extreme internal heating could be caused by tidal dissipation. The geological cycling is ultimately driven by internal heat. This causes negative feedbacks on climate, allowing temperate conditions to persist for geologic time.
Is it possible for a rocky planet to have too much internal heat to maintain a habitable surface? A prerequisite for long-term habitability is the geological cycling of material between the atmosphere and the crust/mantle.
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