Cold Ocean Planets or Rocky Worlds? Extrasolar Analogs of Our Solar System's Icy Moons

We have estimated total internal heating rates and depths to possible subsurface oceans for 17 planets that have been previously described as rocky worlds but that may be best described as Cold Ocean Planets. These low-mass exoplanets have equilibrium surface temperatures and/or densities that are consistent with icy surfaces and a substantial H₂O content. We have also investigated the potential for tidally-driven cryovolcanism and exosphere formation on these worlds. We find that estimated internal heating rates from tidal and radiogenic sources are large enough that all planets in our study may harbor subsurface oceans, and that their geological activity rates are likely to exceed geological activity rates on Jupiter's moon Europa. Several planets are likely to experience enhanced volcanic activity rates that exceed that of Io. Estimates for thin ice shells (<10 km) on Proxima Cen b, LHS 1140 b, Trappist-1f, and several Kepler planets suggest that any H₂O that is vented into space during explosive cryovolcanic eruptions could be sourced directly from their subsurface oceans. Conversely, estimates for relatively thick ice shells (>10 km) on the remainder of the planets in our study, including Trappist-1h, imply that any H₂O that is vented into space during explosive cryovolcanic eruptions may originate in near-surface water pockets that are perched within their icy shells. Owing to their extremely thin ice shells (60 m and 3 km, respectively) high rates of internal heating, and the potential for enhanced tidally-induced explosive cryovolcanism, next generation space telescopes may be able to detect active cryovolcanism in the transit spectra of Proxima Cen b and LHS 1140 b. Cold Ocean Planets may be astrobiologically significant worlds that harbor habitable environments beneath their icy surfaces. The presence of explosive cryovolcanism could serve as evidence for subsurface reservoirs of liquid water, possibly oceans, on these planets. Analyses of observational data from next-generation space telescopes, upcoming ground-based extremely large telescopes, and designs for future telescopic missions should be undertaken with these results in mind.