Ignoring 500 billion galaxies: Mathematics vs Common sense in the debate about the probability of Extraterrestrial life

Carl Sagan said that "extraordinary claims, require extraordinary evidence." In a stunning display of mathematical logic vs common sense, David Spiegel of Princeton University and Edwin Turner from the University of Tokyo published a paper last summer that turns the Drake equation upside down using Bayesian reasoning to show that just because we evolved on Earth, doesn’t mean that the same occurrence would necessarily happen elsewhere; "using evidence of our own existence doesn’t show anything" they argue, "other than that we are here."

What Bayesian reasoning overlooks, of course, is the inconvenient fact that there are some one trillion galaxies in the known universe and some 50 billion planets estimated to exist in the Milky Way alone and some 500,000,000 predicted to exist in a habitable zone.
Spiegel and Turner point out that basing our expectations of life existing on other planets, for no better reason that it exists here, is really only proof that were are more than capable of deceiving ourselves into thinking that things are much more likely than they really are.
They argue that other unknown factors exist that could have contributed to us being here that we don’t yet understand. So, they conclude that, deriving numbers from an equation such as that put forth by Drake, only serves to underscore our belief in the existence of other alien life forms, rather than the actual chances of it being so.
We think evidence will be discovered in the next 20 years: The Kepler mission has discovered 1,235 exoplanets that revolve around a sun, in an area that represents around 1/400th of the Milky Way. By extrapolating these numbers, the Kepler team has estimated that there are at least 50 billion exoplanets in our galaxy — 500 million of which sit inside the habitable "Goldilocks" zones of their suns, the area that it is neither too hot nor too cold to support life.
Astronomers estimate that there are 100 billion galaxies in the universe. If you want to extrapolate those numbers, that means there are around 50,000,000,000,000,000,000 (50 quintillion) potentially habitable planets in the universe.
As Arthur C. Clarke, physicist and author of 2001: A Space Odyssey wrote, "The idea that we are the only intelligent creatures in a cosmos of a hundred billion galaxies is so preposterous that there are very few astronomers today who would take it seriously. It is safest to assume therefore, that they are out there and to consider the manner in which this may impinge upon human society."To an objectivist, empirical view, the rules of Bayesian statistics can be justified by requirements of rationality and consistency and interpreted as an extension of logic. Using a subjectivist view, however, the state of knowledge measures a "personal belief".
More information: "Life might be rare despite its early emergence on Earth: a Bayesian analysis of the probability of abiogenesis" http://arxiv.org/abs/1107.3835 and physorg.com