National Centre of Competence in Research PlanetS
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Project 4.3: Habitability under primordial atmospheres

Our conception of habitability usually builds on the fundamental assumption that exoplanetary habitability is similar to the one of Earth, meaning that the planetary atmosphere is of secondary nature with similar constituents as our atmosphere (nitrogen, oxygen, CO2) and that the energy source is stellar irradiation. In this project, we explore a very distinct potential habitat, namely the habitat below primordial H/He atmospheres that were accreted from the protoplanetary nebula and never lost subsequently.

It was previously understood (Stevenson 1999, Seager 2013) that such atmospheres can lead to conditions allowing liquid water on the surface and lead to potentially habitable free-floating rogue planets and/or to an extension of the habitable zone to orbital distances beyond the usual range. However, it was never demonstrated that such planets actually form with the necessary properties, like the appropriate amount of H/He and the orbital distance where the atmosphere is stable against atmospheric escape. It is also unknown how such a habitat would evolve in time.

In the project, we address these questions bringing together expertise from many different NCCR PlanetS projects. We study the formation and long-term evolution pathways of such potentially habitable planets, modelling their atmospheres and interiors, but also their possible observational characterisation. The project is executed by PhD student Marit Mol Lous under the joint supervision of C. Mordasini (UniBe) and R. Helled (UniZh).

The plot shows the atmospheric pressure-temperature profile of low-mass planets with a H/He envelope at three orbital distances from a solar-like star. Different lines correspond to different moments in time. Some yield to surface conditions that allow the presence of liquid water.

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