The Habitability of our Evolving Galaxy

TL;DR

This paper explores the concept of the Galactic Habitable Zone (GHZ), analyzing models and observations to identify regions in the Milky Way most likely to support complex life, considering astrophysical hazards and temporal evolution.

Contribution

It compares existing GHZ models, discusses their differing predictions, and considers how astrophysical hazards and galaxy evolution influence galactic habitability.

Findings

Most models suggest the galactic disk is habitable.

Predicted habitable regions include 7-9 kpc ring and outskirts.

Gamma-Ray Bursts and other hazards impact habitability assessments.

Abstract

The notion of a Galactic Habitable Zone (GHZ), or regions of the Milky Way galaxy that preferentially maintain the conditions to sustain complex life, has recently gained attention due to the detection of numerous exoplanets and advances made in understanding habitability on the Earth and other environments. We discuss what a habitable environment means on large spatial and temporal scales, which necessarily requires an approximated definition of habitability to make an assessment of the astrophysical conditions that may sustain complex life. We discuss a few key exoplanet findings that directly relate to estimating the distribution of Earth-size planets in the Milky Way. With a broad notion of habitability defined and major observable properties of the Milky Way described, we compare selected literature on the GHZ and postulate why the models yield differing predictions of the most habitable regions at the present day, which include: (1) the majority of the galactic disk; (2) an annular ring between 7-9 kpc, and (3) the galactic outskirts. We briefly discuss the habitability of other galaxies as influenced by these studies. We note that the dangers to biospheres in the Galaxy taken into account in these studies may be incomplete and we discuss the possible role of Gamma-Ray Bursts and other dangers to life in the Milky Way. We speculate how changing astrophysical properties may affect the GHZ over time, including before the Earth formed, and describe how new observations and other related research may fit into the bigger picture of the habitability of the Galaxy.