Galactic Effects on Habitability
  The galactic environment has been suspected to influence planetary
habitability in many ways. Very metal-poor regions of the Galaxy, or those
largely devoid of atoms more massive than H and He, are thought to be unable to
form habitable planets. Moreover, if such planets do form, the young system is
subjected to close stellar passages while it resides in its stellar birth
cluster. Various potential hazards remain after clusters disperse. For
instance, central galactic regions may present risks to habitability via nearby
supernovae, gamma ray bursts (GRBs), and frequent comet showers. In addition,
planets residing within very wide binary star systems are affected by the
Galaxy, as local gravitational perturbations from the Galaxy can increase the
binary's eccentricity until it destabilizes the planets it hosts. Here we
review the most recent work on the main galactic influences over planetary
habitability. Although there must be some metallicity limit below which rocky
planets cannot form, recent exoplanet surveys show that they form around stars
with a very large range of metallicities. Once formed, the probability of star
clusters destabilizing planetary systems only becomes high for rare, extremely
long-lived clusters. Regarding threats to habitability from supernovae, GRBs,
and comet showers, many recent studies suggest that their hazards are more
limited than originally thought. Finally, denser regions of the Galaxy enhance
the threat that very wide binary companions pose to planetary habitability, but
the probability that a very wide binary star disrupts habitability will always
be substantially below 100% for any environment. While some Milky Way regions
must be more hospitable to habitable planets than others, it is difficult to
state that habitable planets are confined to any well-defined region of the
Galaxy or that any other particular region of the Galaxy is uninhabitable.
