Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs

TL;DR

This paper demonstrates that photoevaporation can strip mini-Neptunes of their gaseous envelopes around M dwarfs, potentially creating habitable, rocky planets called habitable evaporated cores, depending on stellar XUV evolution.

Contribution

It introduces a coupled model of atmospheric escape and orbital evolution showing how mini-Neptunes can become habitable cores around M dwarfs, a novel formation pathway.

Findings

HECs likely form from planets with ~1 Earth mass cores and up to 50% H/He.

Formation of HECs depends on early stellar XUV activity.

Evaporation could be a key mechanism for creating volatile-rich Earths.

Abstract

We show that photoevaporation of small gaseous exoplanets ("mini-Neptunes") in the habitable zones of M dwarfs can remove several Earth masses of hydrogen and helium from these planets and transform them into potentially habitable worlds. We couple X-ray/extreme ultraviolet (XUV)-driven escape, thermal evolution, tidal evolution and orbital migration to explore the types of systems that may harbor such "habitable evaporated cores" (HECs). We find that HECs are most likely to form from planets with $\sim 1 M_\oplus$ solid cores with up to about 50% H/He by mass, though whether or not a given mini-Neptune forms a HEC is highly dependent on the early XUV evolution of the host star. As terrestrial planet formation around M dwarfs by accumulation of local material is likely to form planets that are small and dry, evaporation of small migrating mini-Neptunes could be one of the dominant formation mechanisms for volatile-rich Earths around these stars.