Atmospheric Variety of Ocean Planets—a 1D Radiative-Convective Equilibrium Model

Abstract The potential diversity in atmospheric compositions of extrasolar planets with water oceans is investigated in light of the increasing number of observed transiting exoplanets. This study explores several atmospheric configurations, including those dominated by H$_2$, He, N$_2$, and mixed backgrounds containing water vapor. A critical aspect of these atmospheres is the Komabayashi–Ingersoll (KI) limit, which constrains the outgoing longwave radiation (OLR) from the stratosphere and establishes an upper boundary influenced by atmospheric pressure and molecular weight. Using a 1D radiative-convective equilibrium model for a gray atmosphere, we examine the relationship between OLR and surface temperature $T_s$. We find that the OLR initially increases with $T_s$, reaches a plateau, subsequently decreases, and then rises to a secondary threshold for H$_2$ and He atmospheres. Our analysis reveals that this distinct OLR behavior, observed across varying atmospheric compositions, is governed by two key limits: the KI limit and the Simpson–Nakajima (SN) limit. Whereas the KI limit depends on atmospheric pressure and molecular weight, the SN limit is primarily determined by water vapor concentration and is independent of other atmospheric components. Notably, the KI limit for He-dominated atmospheres is lower than the SN limit which is applicable to H$_2$-dominated atmospheres. We have derived the limiting value formulas for the KI limit and SN limit, respectively. This study underscores the critical roles of atmospheric composition and stratospheric OLR in determining the habitability of exoplanets.