Habital Cold Ocean Planets Could Be Vastly Outnumber Earth-Like Worlds

Ocean planets are a proposed class of low-density, terrestrial exoplanets with substantial liquid water layers. They may exist in a variety of climactic states including ice free, partially ice covered, or completely frozen over at their surfaces. Researchers call ocean-bearing planets with ice-covered surfaces as cold ocean planets. Cold ocean planets may be plentiful in extrasolar planetary systems and common throughout the Galaxy. Their interior structures and compositions may resemble those of the large icy moons in our outer solar system. Despite their ice-covered surfaces, these planets may be astrobiologically significant worlds, with habitable environments in subsurface oceans.

Above- Explosive cryovolcanism in the form of geyser-like plumes on two of our solar system’s ocean worlds. Left: Cryovolcanic eruptions at the south pole of Saturn’s moon Enceladus (credit: NASA/JPL-Caltech/SSI). Right: Hubble Space Telescope UV observations of cryovolcanic eruptions at the south pole of Jupiter’s moon Europa (credit: NASA/L. Roth). By facilitating the transport of liquid water and energy between their interiors and surfaces, similar activity could create habitable environments on cold ocean planets.

Researchers considered conditions on 17 confirmed exoplanets that are roughly Earth-sized but less dense, suggesting that they could have substantial amounts of ice and water instead of denser rock. Although the planets’ exact compositions remain unknown, initial estimates of their surface temperatures from previous studies all indicate that they are much colder than Earth, suggesting that their surfaces could be covered in ice. They have also determined they likely have activity to heat internally to cause liquid oceans.

The Astrophysics Journal – Prospects for Cryovolcanic Activity on Cold Ocean Planets

Researchers have estimated total internal heating rates and depths to possible subsurface oceans for 17 planets that may be cold ocean planets, low-mass exoplanets with 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. Estimated internal heating rates from tidal and radiogenic sources are large enough that all planets in our study may harbor subsurface oceans, and their geological activity rates are likely to exceed the geological activity rates on Jupiter’s moon Europa. Several planets are likely to experience enhanced volcanic activity rates that exceed that of Io. Owing to their relatively thin ice shells and high rates of internal heating, Proxima Cen b and LHS 1140 b are the most favorable candidates for telescopic detection of explosive, tidally driven cryovolcanism. Estimates for thin ice shells on Proxima Cen b, LHS 1140 b, Trappist-1f, and several Kepler planets suggest that any H2O vented into space during explosive cryovolcanic eruptions on these worlds could be sourced directly from their subsurface oceans. Like the icy moons in our outer solar system, cold ocean planets may be astrobiologically significant worlds that harbor habitable environments beneath their icy surfaces. These possibilities should be considered during analyses of observational data for small exoplanets from current and upcoming telescopes and during planning for a future space telescope mission aimed at characterization of potentially habitable exoplanets (e.g., Habitable Worlds Observatory).