Introduction

The modern study of the ‘habitability’ of circumstellar environments started almost half a century ago (Huang, 1959). The concept of a circumstellar habitable zone (CHZ) is relatively well defined, being tightly related to the requirement of the presence of liquid water as a necessary condition for life-as-we-know-it; the corresponding temperature range is a function of the luminosity of the star and of the distance of the planet from it. An important amount of recent work, drawing on various disciplines (planetary dynamics, atmospheric physics, geology, biology etc.) has refined considerably our understanding of various factors that may affect the CHZ; despite that progress, we should still consider the subject to be in its infancy (e.g. Chyba and Hand, 2005; Gaidos and Selsis, 2007; and references therein).

Habitability on a larger scale was considered a few years ago by Gonzalez et al. (2001), who introduced the concept of the galactic habitable zone (GHZ). The underlying idea is that various physical processes, which may favour the development or the destruction of complex life, may depend strongly on the temporal and spatial position in the Milky Way (MW). For instance, the risk of a supernova (SN) explosion sufficiently close to represent a threat to life is, in general, larger in the inner Galaxy than in the outer one, and has been larger in the past than at present.