  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.
