We propose the concept of a ” Galactic Habitable Zone ” ( GHZ ) .
Analogous to the Circumstellar Habitable Zone ( CHZ ) , the GHZ is that region in the Milky Way where an Earth-like planet can retain liquid water on its surface and provide a long-term habitat for animal-like aerobic life .
In this paper we examine the dependence of the GHZ on Galactic chemical evolution .
The single most important factor is likely the dependence of terrestrial planet mass on the metallicity of its birth cloud .
We estimate , very approximately , that a metallicity at least half that of the Sun is required to build a habitable terrestrial planet .
The mass of a terrestrial planet has important consequences for interior heat loss , volatile inventory , and loss of atmosphere .
A key issue is the production of planets that sustain plate tectonics , a critical recycling process that provides feedback to stabilize atmospheric temperatures on planets with oceans and atmospheres .
Due to the more recent decline from the early intense star formation activity in the Milky Way , the concentration in the interstellar medium of the geophysically important radioisotopes , ^ { 40 } K , ^ { 235 , 238 } U , ^ { 232 } Th , has been declining relative to Fe , an abundant element in the Earth .
Also likely important are the relative abundances of Si and Mg to Fe , which affects the mass of the core relative to the mantle in a terrestrial planet .
All these elements and isotopes vary with time and location in the Milky Way ; thus , planetary systems forming in other locations and times in the Milky Way with the same metallicity as the Sun will not necessarily form habitable Earth-like planets .
As a result of the radial Galactic metallicity gradient , the outer limit of the GHZ is set primarily by the minimum required metallicity to build large terrestrial planets .
Regions of the Milky Way least likely to contain Earth-mass planets are the halo ( including globular clusters ) , the thick disk , and the outer thin disk .
The bulge should contain Earth-mass planets , but stars in it have a mix of elements different from the Sun ’ s .
The existence of a luminosity-metallicity correlation among galaxies of all types means that many galaxies are too metal-poor to contain Earth-mass planets .
Based on the observed luminosity function of nearby galaxies in the visual passband , we estimate that : 1 ) the Milky Way is among the 1.3 % most luminous ( and hence most metal-rich ) galaxies , and 2 ) about 23 % of stars in a typical ensemble of galaxies are more metal-rich than the average star in the Milky Way .
The GHZ zone concept can be easily extrapolated to the universe as a whole , especially with regard to the changing star formation rate and its effect on metallicity and abundances of the long-lived radioisotopes .