Galactic scale phenomena relevant to life on a terrestrial planet are reviewed. The habitability of the Earth for complex life is surprisingly dependent on a diverse collection of processes ranging from Galactic chemical evolution to Galactic nuclear activity to comet impacts. The combined effect of these is to restrict the time and space that complex life can exist on a terrestrial planet. That region in the Milky Way is termed the Galactic Habitable Zone.

Introduction

The introduction of the Circumstellar Habitable Zone (CHZ) concept in the late 1950s (Huang 1959) and later refinements (Hart 1979; Kasting et al. 1993; Franck et al. 2000) have permitted the study of life in the universe to be systematized to some degree. However, discussion of habitability on the scale of the Milky Way Galaxy has received less attention. Trimble (1997a) considered habitability in the context of Galactic chemical evolution. Clarke (1981) discussed the possible effects on habitability of a Seyfert-like outburst in the Galactic center. In addition, many papers have been written about the possible threats to life by nearby supernovae (e.g. Ellis & Schramm 1995). While these studies have been helpful studies, they do not attempt to systematize the concept of habitability on the Galactic scale.

Before beginning any discussion about habitability, it is important to be up front about assumptions regarding life. As in CHZ studies, we assume Earth-like life in exploring Galactic-scale habitability constraints.