The Potential for Tidally Heated Icy and Temperate Moons around Exoplanets

Moons of giant planets may represent an alternative to the classical picture of habitable worlds. They may exist within the circumstellar habitable zone of a parent star, and through tidal energy dissipation they may also offer alternative habitable zones, where stellar insolation plays a secondary, or complementary, role. We investigate the potential extent of stable satellite orbits around a set of 74 known extrasolar giant planets located beyond 0.6 AU from their parent stars—where moons should be long-lived with respect to removal by stellar tides. For this sample, the typical stable satellite orbital radii span a band some ~0.02 AU in width, compared to the ~0.12-0.15 AU bands for the Jovian and Saturnian systems. Approximately 60% of these giant planets can sustain satellites or moons in bands up to ~0.04 AU in width. For comparison, the Galilean satellites extend to ~0.013 AU. We discuss how the actual number and characteristics of satellites will depend strongly on the formation pathways. We investigate the stellar insolation that moons would experience for these exoplanet systems and the implications for sublimation loss of volatiles. We find that between 15% and 27% of all known exoplanets may be capable of harboring small, icy moons. In addition, some 22%-28% of all known exoplanets could harbor moons within a "sublimation zone," with insolation temperatures between 273 and 170 K. A simplified energy-balance model is applied to the situation of temperate moons, maintained by a combination of stellar insolation and tidal heat flow. We demonstrate that large moons (>0.1 M_⊕), at orbital radii commensurate with those of the Galilean satellites, could maintain temperate, or habitable, surface conditions during episodes of tidal heat dissipation of the order 1-100 times that currently seen on Io.