Abstract EANA2024-124

From the discovery of low-mass exoplanets, it is known that the largest discovered population to date has a mass/radius between those of the Earth and Neptune demonstrating that the variety of planets existing in nature is much larger than previously thought. Although exoplanets with various densities/compositions certainly exist, this spread is tightly related to the existence of a large variety of low-mass planets that accreted within the gas disk and did not get rid of their accumulated primordial atmospheres. The discovery of exoplanets around the Earth-mass domain, including several planets within the habitable zone of their host stars, led to the question of whether terrestrial exoplanets can also possess significant hydrogen and/or even He-dominated atmospheres that must have been accreted from the protoplanetary disk. To model the mass loss evolution of such primordial atmospheres from terrestrial exoplanets within the habitable zone of solar-like stars, we apply a multispecies hydrodynamic upper atmosphere model. We find that depending on the planet`s host star`s high-energy flux between about 10 – 120 nm along its stellar evolution, and the accreted mass of the protoplanet within the gas disk, in some cases, Earth-like exoplanets accumulate primordial gas from the disk
that remains for more than one billion years or even until the whole lifetime of a planet. These Earth-mass planets end up as sub-Neptune`s, and no complex aerobic lifeforms can evolve. This can thus have important implications for the search for "Earth-like Habitats", and more in general for the habitability of terrestrial planets. From the results of our study, we conclude that thoroughly understanding the complex interplay between the lifetime of the gas disk, the accumulation of primordial atmospheres, and the EUV flux evolution of a planet`s host star are key to understanding how planets can develop to habitable worlds, inclduing the formation of N2/O2-dominated secondary atmospheres.