Abstract DGP2026-55

Recently, observations of sub-Neptunes with moderate instellation have raised the possibility of a large population of habitable planets with surface oceans, more amenable to atmospheric characterisation than smaller super-Earths. To assess the feasibility of ocean formation on planets with a thick hydrogen-water envelope, we develop a novel 1D model of sub-Neptune atmospheres. The model accounts for non-ideal properties of the hydrogen-water mixture near the critical point of water and for the inhibition of convection due to compositional gradients induced by water condensation.

Using this model, we determine, for different envelope water contents, the instellation at which a liquid ocean would form from the supercritical envelope. Assuming a fully convective atmosphere, liquid oceans can form with envelope water fractions >60% and moderate instellations. For example, at Earth’s instellation and assuming a Bond albedo of 0.4, ocean formation requires an envelope with >70% water. If convection is inhibited by mean molecular weight gradients, radiative layers form that heat the lower atmosphere and ocean formation is only possible under “ice giant” conditions with instellations and internal heat fluxes under 1 W/m². If diffusion of latent heat fluxes through the radiative layers is included in the model, intermediate scenarios are possible.

From our models, we conclude that the majority of the current sub-Neptunes receive too much instellation to host liquid water oceans. Our results also agree with other findings in the literature, that sub-Neptune ocean worlds can only form from extremely water-rich envelopes which may be difficult to justify from planetary formation models.