Potential long-term habitable conditions on planets with primordial H-He atmospheres

TL;DR

This study explores the potential for long-term habitability on cold super-Earths with primordial H-He atmospheres, showing they could sustain liquid water and habitable conditions for billions of years at certain distances from their stars.

Contribution

It demonstrates that planets with primordial H-He atmospheres can remain habitable for billions of years, challenging traditional habitability criteria and expanding the scope of potentially habitable exoplanets.

Findings

Planets with 1-10 Earth masses can stay temperate for 5-8 Gyr beyond 2 AU.

Required envelope masses are around 10^-4 Earth masses, larger than Earth's atmosphere.

Habitability duration depends on core and envelope mass, and orbital distance.

Abstract

Cold super-Earths which retain their primordial, H-He dominated atmosphere could have surfaces that are warm enough to host liquid water. This would be due to the collision induced absorption (CIA) of infra-red light by hydrogen, which increases with pressure. However, the long-term potential for habitability of such planet has not been explored yet. Here we investigate the duration of this potential exotic habitability by simulating planets of different core masses, envelope masses and semi-major axes. We find that terrestrial and super-Earth planets with masses of $\sim$1 - 10$M_{\oplus}$ can maintain temperate surface conditions up to 5 - 8 Gyr at radial distances larger than $\sim $2 AU. The required envelope masses are $\sim 10^{-4} \, M_{\oplus}$ (which is 2 orders of magnitude more massive than Earth's), but can be an order of magnitude smaller (when close-in) or larger (when far out). This result suggests that the concept of planetary habitability should be revisited and made more inclusive with respect to the classical definition.