Earth-mass planets with primordial hydrogen and helium atmospheres in the habitable zone of solar-like stars

The discovery of terrestrial exoplanets within the Earth-to-super-Earth mass domain led to the question of whether these bodies developed atmospheres and what kind of atmospheres surround them. Observations at some planets within the above-mentioned mass domain revealed the existence of a large population of planets with less than three Earth masses that possess huge hydrogen-dominated primordial atmospheres, which must be remnants from the protoplanetary gas disk. These discoveries stimulate questions about whether planets within the Earth-mass domain could be within the habitable zone of solar-like stars that could have accreted and kept hydrogen- and/or helium-dominated primordial atmospheres. Since the gas disk consists mainly of these elements, we study the possible enrichment of the primordial helium fraction in hydrodynamically escaping initially hydrogen-dominated atmospheres of terrestrial planets between 0.75 to 3.0 Earth masses inside the habitable zones of Sun-like G-type host stars, considering different stellar evolutionary tracks of their high-energy emission. Depending on the planet’s accreted mass during the gas disk phase and the stellar radiation, we show that through hydrodynamic escape, Earth-mass planets inside the HZ of solar-like stars with masses between about 0.95 and 1.25 Earth masses can end with helium-dominated primordial atmospheres.
Our finding has important implications for the evolution of "Earth-like Habitats", as these thick helium-enriched primordial atmospheres can inhibit the habitability of terrestrial planets. From our results we conclude that thoroughly understanding the complex interplay between a rocky planet's accretion speed and the lifetime of the gas disk, the accumulation of primordial atmospheres, and the evolution of the extreme ultraviolet radiation of a planet's host star is key to understanding how terrestrial planets can develop later into "Earth-like Habitats" where nitrogen-oxygen-dominated secondary atmospheres can evolve. The upcoming generation of giant telescopes, such as the Extremely Large Telescope, may enable us to observe and explore these atmospheres.