Nature of the ocean beneath K2-18 b’s H2-rich atmosphere is unknown

Determine the physical nature of the ocean hypothesized beneath K2-18 b’s hydrogen-rich atmosphere, including whether a liquid water ocean exists and, if so, characterize its phase state and composition to evaluate potential habitability.

Background

The atmospheric composition retrieved for K2-18 b supports Hycean-like conditions and suggests a substantial interior water inventory. In Hycean scenarios, an ocean lies beneath an H2-rich atmosphere, but its properties cannot be directly inferred from current transmission spectra.

Establishing the ocean’s nature is central to assessing K2-18 b’s habitability, as liquid water requires specific thermal and radiative conditions. The authors note that, despite atmospheric evidence, the existence, phase, and characteristics of a putative ocean remain undetermined.