Abstract

Sub-Neptunes—planets intermediate in size between Earth and Neptune—are a major focus in exoplanet science. Despite having no Solar System counterparts, they are the largest known class of exoplanets. They exhibit an unexpected bimodal size distribution, the larger portion of which has a density inconsistent with an Earth-like rocky composition. This has led to hypotheses about their internal composition, including “gas dwarfs” (rocky planets with hydrogen-rich envelopes) and “volatile-rich” scenarios (water-rich planets with thin hydrogen atmospheres). Interest has grown with suggestions that some volatile-rich planets could be potentially habitable “hycean” worlds. Notably, JWST observations of the temperate sub-Neptune K2-18 b have been variously interpreted: as confirming its hycean candidate status, supporting a gas-dwarf scenario with a magma ocean, or classifying it as a mini-Neptune. In this seminar, I will present an integrated framework we have developed to model gas dwarf scenarios—with and without magma oceans—on temperate sub-Neptunes and to predict their unique observable features. I will then discuss how we applied this framework to K2-18 b, and compare our predictions with available observations to draw conclusions about its likely internal structure.