Possible climate bistability on hycean planets

Hycean planets are exoplanets characterized by water oceans and hydrogen-rich atmospheres. These planets are high-priority targets for biosignature searches, thanks to their abundant surface liquid water combined with having easy-to-characterize hydrogen-rich atmospheres. The climates and potential habitability of hycean planets are still poorly understood, however. One of their most unusual climate features is moist convection inhibition. In a hydrogen-rich atmosphere the presence of H2O can suppress moist convection and dramatically alter a planet’s temperature structure, an effect which so far has largely been studied for gas giants in the Solar System. This work develops pen-and-paper theory to analyze the effects of moist convective inhibition on hycean planets. The theory is tested and verified against a one-dimensional radiative-convective model. We show that hycean planets with moderately thick atmospheres can exhibit climate bistability. The bistability arises because when the inhibition occurs, the cooling in the upper atmosphere can offset the radiative effect of surface warming, which is most effective when the total optical thickness is slightly greater than unity. In addition, our results show that climates of hycean planets are highly sensitive to small-scale vertical diffusion in the inhibition layer. This diffusion is determined by a wide range of processes which are difficult to resolve in 1D models, such as convective overshoot and large-scale horizontal shear. Our results suggest that hycean planets have unexpectedly rich climate dynamics, and highlight the importance of sophisticated 3D modeling for understanding the potential habitability of hycean worlds