Climates of temperate rocky planets with He-dominated atmospheres

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.