Habitable Evaporated Cores and the Occurrence of Panspermia near the Galactic Center

Abstract

Black holes growing via the accretion of gas emit radiation that can photoevaporate the atmospheres of nearby planets. Here we couple planetary structural evolution models of sub-Neptune mass planets to the growth of the Milky way's central supermassive black-hole, Sgr A${}^{\ast }$ and investigate how planetary evolution is influenced by quasar activity. We find that, out to $\sim 20$ pc from Sgr A${}^{\ast }$, the XUV flux emitted during its quasar phase can remove several percent of a planet's H/He envelope by mass; in many cases, this removal results in bare rocky cores, many of which situated in the habitable zones (HZs) of G-type stars. The erosion of sub-Neptune sized planets may be one of the most prevalent channels by which terrestrial super-Earths are created near the Galactic Center. As such, the planet population demographics may be quite different close to Sgr A${}^{\ast }$ than in the Galaxy's outskirts. The high stellar densities in this region (about seven orders of magnitude greater than the solar neighborhood) imply that the distance between neighboring rocky worlds is short ($500-5000$~AU). The proximity between potentially habitable terrestrial planets may enable the onset of widespread interstellar panspermia near the nuclei of galaxies. More generally, we predict these phenomena to be ubiquitous for planets in nuclear star clusters and ultra-compact dwarfs. Globular clusters, on the other hand, are less affected by the black holes.

9 pages, 4 figures, accepted for publication in Astrophysical Journal Letters

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