The Astrobiological Copernican Weak and Strong Limits for
  Extraterrestrial Intelligent Life
  We present a cosmic perspective on the search for life and examine the likely
number of Communicating Extra-Terrestrial Intelligent civilizations (CETI) in
our Galaxy by utilizing the latest astrophysical information. Our calculation
involves Galactic star-formation histories, metallicity distributions, and the
likelihood of stars hosting Earth-like planets in Habitable Zones, under
specific assumptions which we describe as the Astrobiological Copernican Weak
and Strong conditions. These assumptions are based on the one situation in
which intelligent, communicative life is known to exist - on our own planet.
This type of life has developed in a metal-rich environment and has taken
roughly 5 Gyr to do so. We investigate the possible number of CETI based on
different scenarios. At one extreme is the Weak Astrobiological Copernican
principle - such that a planet forms intelligent life sometime after 5 Gyr, but
not earlier. The other is the Strong Condition in which life must form between
4.5 to 5.5 Gyr, as on Earth. In the Strong Condition (a strict set of
assumptions), there should be at least 36$_{-32}^{+175}$ civilizations within
our Galaxy: this is a lower limit, based on the assumption that the average
life-time, L, of a communicating civilization is 100 years (based on our own at
present). If spread uniformly throughout the Galaxy this would imply that the
nearest CETI is at most 17000$_{-10000}^{+33600}$ light-years away, and most
likely hosted by a low-mass M-dwarf star, far surpassing our ability to detect
it for the foreseeable future. Furthermore, the likelihood that the host stars
for this life are solar-type stars is extremely small and most would have to be
M-dwarfs, which may not be stable enough to host life over long timescales. We
furthermore explore other scenarios and explain the likely number of CETI there
are within our Galaxy based on variations of our assumptions.
