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$^*$ and investigate how planetary evolution is influenced by quasar activity. We find that, out to ${\sim} 20$ pc from Sgr A$^*$, 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$^*$ 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.

• The paper demonstrates that Sgr A* quasar phases can erode a planet's hydrogen-helium envelope, transforming sub-Neptunes into rocky, potentially habitable cores.
• The authors employ MESA simulations to couple thermal and mass-loss evolution, showing that planets within 20 pc may become habitable evaporated cores.
• The study highlights that dense stellar environments near the Galactic Center could enhance panspermia through close proximities among rocky worlds.

Habitable Evaporated Cores and Panspermia Near the Galactic Center

The paper entitled "Habitable Evaporated Cores and the Occurrence of Panspermia near the Galactic Center" authored by Chen, Forbes, and Loeb presents an investigation into how the quasar phase of the Milky Way’s central supermassive black hole (SMBH), Sgr A*, influences the evolution of planets within its vicinity, specifically focusing on habitable evaporated cores (HECs). The study extends known mechanisms of atmospheric erosion driven by stellar XUV/EUV flux to a galactic scale, illustrating potential pathways for creating terrestrial super-Earths from sub-Neptune exoplanets near the Galactic Center.

Main Findings

The authors employ planetary structural evolution models using the MESA code, coupling the thermal and mass-loss evolution of sub-Neptune-like planets with the XUV radiation emitted by Sgr A* during its active phases. The primary findings can be summarized as follows:

1. Atmospheric Erosion by SMBH Quasar Activity: The paper reveals that XUV radiation from SMBHs, specifically during their quasar phases, can strip away several percent of a planet's hydrogen-helium envelope. For planets situated within approximately 20 pc from Sgr A*, this erosion can lead to the transformation of gaseous planets into bare rocky cores. This transformation is contingent on the galactocentric distance and the initial mass and composition of the planet.
2. Creation of Habitable Evaporated Cores (HECs): It is posited that the regions close to the Galactic Center may possess a significant population of HECs, which lie in the habitable zones (HZs) of solar-type stars. These HECs are more likely to sustain conditions favorable for life as they lack a suffocating gaseous envelope, thus exposing potentially habitable rocky surfaces.
3. Implications for Planetary Population Demographics: The results imply a distinct compositional divergence in planet populations between the Galactic core and outskirts, suggesting that terrestrial planet formation near the core might heavily rely on the photoevaporation mechanism influenced by quasar activity.
4. Prospects for Widespread Panspermia: Due to the high stellar density near the Galactic Center, the study suggests enhanced potential for panspermia (interstellar transfer of life), as the separation between rocky worlds may be mere thousands of AU, facilitating the exchange of material capable of bearing life.

Implications

The theoretical implications of this study are noteworthy for understanding planet formation and atmospheric evolution on a galactic scale. Two key applications are highlighted:

• Astrobiology and Exoplanet Studies: The potential existence of numerous HECs enriches the discussion on the diversity of planetary systems and their habitability. Areas with higher stellar density might offer natural laboratories for studying panspermia, due to the frequent gravitational interactions among closely packed planetary systems.
• Galactic Astronomy: The findings provide a framework for predictions about how the Milky Way’s evolutionary history, especially its central black hole’s activity, impacts the architecture of proximal planetary systems. These insights might extend to other galactic centers hosting SMBHs.

Future Directions

Given the conclusions drawn from simulations, future work could involve observational campaigns targeting the Galactic Center for signatures consistent with the presence of HECs. High-angular-resolution instruments on future telescopes (such as the E-ELT) could resolve individual stars and potentially detect transiting planets, offering an empirical basis to test the theoretical predictions proposed in this paper. Additionally, refining models that account for other chemical and physical processes in planet atmospheres will enhance the robustness of predicting the evolution of planets under the influence of galactic phenomena.

In sum, the analysis provided in this study offers profound insights into the interplay between SMBH-induced radiation and planetary evolution, augmenting the scientific community's understanding of planetary diversity and the prospects of life beyond Earth.