Hycean worlds can only form under extremely cold and water-rich conditions

In the past five years, there has been increasing interest in sub-Neptune exoplanets in the habitable zone of their stars. These planets are abundant and easier to observe via transmission spectroscopy owing to their hydrogen-rich atmospheres.  Claimed biosignature detections on the sub-Neptune K2-18 b also motivate further studies into whether their climates are suitable for life. Whether it is possible for sub-Neptunes in the traditional Earth-like habitable zone to have liquid water oceans (and be a “Hycean” planet) is debated. One possible limiting factor is the inhibition of convection in which radiative layers forming in condensing regions (due to mean molecular weight gradients) make planetary interiors too hot for oceans to condense. In this work we aim to determine the conditions under which ocean formation on a sub-Neptune can occur, assuming that the hydrogen-water envelope evolves from a hot, well-mixed initial state.

 

To determine the conditions under which oceans will form on a sub-Neptune, we calculate the atmospheric structure at which an interior adiabat passes through the critical point of water (for a given deep water composition). Temperature-pressure profiles warmer than this “critical profile” will form water clouds in the upper atmosphere; profiles colder than this will bubble out hydrogen from a water rich ocean state. To calculate the critical profile, we use a non-ideal Peng-Robinson equation of state to calculate the moist adiabatic gradient in adiabatic regions, and Rosseland mean opacities to calculate the temperature gradient in the deeper regions where convection is inhibited. We explore the parameter space of sub-Neptunes by varying the temperature at 1 bar, the internal heat flux and the background composition. 

 

We find that the critical ocean-forming state must have an extremely cold upper atmosphere if there are convectively inhibited layers. At 1 bar, the temperature in most simulations is <100 K (Figure 1), which would correspond with orbital distances well beyond the traditional habitable zone. At the upper end of this temperature range, a high internal water content approaching 100% is required for ocean formation. With reduced internal heat fluxes (<0.01 W/m^2), the presence of radiative layers depends on the composition of the atmosphere, with the inclusion of high-opacity greenhouse gases such as CH4 and CO2 sustaining high temperature gradients at lower internal heat fluxes. Our results suggest that the current population of detected sub-Neptunes are unlikely to host liquid water oceans.

Figure 1: A range of temperature-pressure profiles which pass through the critical point of water for a stated internal water content (annotated on lines). For the given internal water content, this profile represents the state in which any colder profile can form a liquid water ocean. The critical curve of water is shown in black, and regions with convective inhibition are highlighted with thicker lines. For all deep internal water contents, the atmosphere at 1 bar must be extremely cold for liquid water oceans to form.