Summary

Galactic Habitable Zones (GHZs) are the regions of space and time in the Milky Way Galaxy most compatible with the existence of life-as-we-know-it. Such regions have main sequence stars with sufficient metallicity to form rocky planets, but not enough to produce hot Jupiters that move through circumstellar habitable zones. These stars should have main-sequence lifetimes long enough for life to emerge and evolve. And should exist in times and locations relatively far away from life-extinguishing supernovae. The occurrence rates of different types of planets depend on host metallicity, particularly for Earth-like planets. Because spiral galaxies like the Milky Way are the most common type of galaxy, many analogous GHZs exist among the 100 billion galaxies of the observable universe. Galactic habitability depends on the kind of life one is looking for: simple life? complex life? star-faring post-biological life? If one is searching for extraterrestrial intelligence (knowing that it took ~4 billion years for electrical engineers to evolve on Earth), attention should be focused on stars older than 4 billion years.