Towards equilibrium climate states of potential ocean worlds through radiative-convective-photochemical modeling

The sheer existence of the radius gap, a.k.a. Fulton gap, poses significant scientific questions: Can planets exist — and remain — in such in-between atmospheric states? Neither gas planet, nor traditionally terrestrial? Are these merely planets transitioning from one to the other regime? Or could this hint to the existence of ocean worlds? Analytical models used to simulate these possible climate states in the gap-region have evolved from either Earth or Jupiter models, trying to close the gap — so to speak. We present our newest developments towards modeling planets on both sides of the gap with one single model. In these efforts we present our newly developed 1D-radiative-convective model, which is coupled to our well established photochemistry model (Hu+2012, 2020, 2021). We discuss the equations of radiative transfer used, and our method of choice for solving directly for atmospheric steady-state together with convective regions in the atmosphere. This is in contrast to recent moves in the community to time-stepping towards an equilibrium atmosphere (e.g. Kopparapu+2013, Malik+2017, Lincowksi+2018, Scheucher+2020). Multiple planets and planet candidates testing the upper boundary of the radius-gap, similar to the sub-Neptune K2-18b and cooler (e.g. TOI-712d, TOI-2257b, TOI 4310.01) have been discovered with many more expected to follow. We show first applications to such temperate exoplanets and candidates and discuss further steps.