Climates of temperate rocky planets with He-dominated atmospheres

Abstract

We present radiative-convective modeling of rocky exoplanets with $He$-dominated atmospheres and low envelope mass fractions. Helium has a steeper adiabatic temperature profile than $N_2$ and $H_2$ as it has fewer degrees of freedom. Line broadening differences are small, collision-induced absorption (CIA) is not important in $He$-dominated atmospheres, and Rayleigh scattering by $He$ is weak. The combined impact of these effects is that $He$-dominated atmospheres provide more warming than $N_2$-dominated atmospheres but less than $H_2$-dominated atmospheres, for the same surface pressure. For surface pressures in the range 1-20 bar, the habitable zone for rocky planets around M dwarfs with $He$-dominated atmospheres is narrower than for $H_2$-dominated atmospheres. Nonetheless, due to their large scale height compared to $N_2$ or $CO_2$-dominated atmospheres, temperate $He$-dominated atmospheres are favorable targets for characterization via transit spectroscopy with JWST.