Earths: Rare in Time, Not Space?

We compare two recent astrobiological approaches to the solution of the problem of the “Great Silence” (or Fermi’s paradox): the “rare Earth” hypothesis of Ward and Brownlee, and the phase-transition scenario of Annis. We show that they have important similarities, as far as self-selection of intelligent observers is concerned. In order to better illuminate the differences, we consider the duration of Galactic habitability and the notion of 4-dimensional Galactic Habitable Zone. The notion of “rarity” of complex biospheres can be made more precise in two different ways. We conclude that there are slight reasons to prefer the phasetransition picture, but more important still, the very possibility of such a discrimination between the rival hypotheses testifies that some progress has been made in resolving these difficult