2020ApJ...889L..20L

The presence of a liquid solvent is widely regarded as an essential prerequisite for habitability. We investigate the conditions under which worlds outside the habitable zones of stars are capable of supporting liquid solvents on their surface over geologically significant timescales via combined radiogenic and primordial heat. Our analysis suggests that super-Earths with radionuclide abundances that are >=10³ times higher than Earth can host long-lived water oceans. In contrast, the requirements for long-lived ethane oceans, which have been explored in the context of alternative biochemistries, are less restrictive: relative radionuclide abundances of >=10² could be sufficient. We find that this class of worlds might be detectable (10σ detection over ∼10 day integration time at 12.8 µm) in principle by the James Webb Space Telescope at distances of ∼10 pc if their ages are <=1 Gyr.