![Cartoon illustrating the effect of the runaway greenhouse transition on atmospheric composition for two rocky planets with identical He, H2O and CO2 inventories. With a thin He atmosphere, planets below the runaway greenhouse limit (right) will have He-rich upper atmospheres and may support surface liquid water. For planets inside the inner edge of the habitable zone (left), H2O from liquid water oceans and CO2 from carbonates are transported into the atmosphere, dramatically decreasing the scale height and making the transit spectrum of the planet flatter. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/07/Climates-of-temperate.png?fit=1200%2C669&ssl=1)
[astro-ph.EP] 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 N2 and H2 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 N2-dominated atmospheres but less than H2-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 H2-dominated atmospheres. Nonetheless, due to their large scale height compared to N2 or CO2-dominated atmospheres, temperate He-dominated atmospheres are favorable targets for characterization via transit spectroscopy with JWST.
Viviane Kuss, Robin Wordsworth, Collin Cherubim, Jessica Cmiel
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2607.06857 [astro-ph.EP](or arXiv:2607.06857v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2607.06857
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Submission history
From: Viviane Kuss
[v1] Tue, 7 Jul 2026 23:18:16 UTC (5,262 KB)
https://arxiv.org/abs/2607.06857
Astrobiology, Exoplanet,
