@Psyx: Re: @Nissemus & MdB: : ((
"Why, as a matter of interest?"
That's a very fair question but I can't really give you a short synopsis or overview.
But I will point you to a book, "Where Is Everybody?" by Stephen Webb. (The full title of which is actually "If The Universe Is Teeming With Aliens, Where Is Everybody: Fifty Solutions To The Fermi Paradox And The Problem Of Extraterrestrial Life".) I would consider this to be required reading for anyone interested in the subject. (As you can easily imagine, Google will find you reviews; Amazon will be happy to sell you a copy.)
Not only are the astrophysical reasons for skepticism powerful, but once having found out about the biological reasons for skepticism, the effect is only reinforced. Abiogenesis is at this point, and maybe forever, an insoluble problem, but certain occurrences necessary for the emergence of life on earth can be reasonably (but not, of course, definitively) shown to have probabilities that are "indistinguishable from zero".
Here are a few of the influences, some hypothetical, on the genesis of life on earth: the sun and its specific characteristics, the earth and its specific characteristics (what if there was even less molybdenum?), the moon and its specific characteristics, the location of the earth in the Continuously Habitable Zone, tides, plate tectonics, Snowball Earth, Jupiter, being effected (or not!) by gamma ray bursters or supernovae or supervolcanoes, and more! Note that these factors only concern the physical and environmental conditions that were necessary for the origin of life on earth. Some of these conditions are quite common: stars like the sun abound, obviously. Some of these conditions might in fact have no bearing on the matter at all. But as the you look for planetary systems that satisfy the conditions, probabilities diminish precipitously. And that is even before we look at the biological considerations:
The biological considerations: the genesis of prokaryotes (eubacteria and archaea) and especially eukaryotes, the origin of proteins, polypeptides and amino acids, enzymes, nucleic acids (RNA and DNA)... Again, the probabilities against some of these things having arisen have been estimated by some to involve "probabilities indistinguishable from zero". (And when two necessary conditions have "probabilities indistinguishable from zero" - well, it's possible to end up dealing with numbers that exceed the number of atoms in the universe, let alone stars or planets.)
Think about panspermia for a moment. It's a respectable theory even if not accepted by all knowledgeable parties. I consider panspermia to be a theory meant to show that there can be life elsewhere in the universe while evading the incredibly long odds of life arising from non-life in the first place. I.e Life is so hard to get from non-life, that even if it only happened *once* we can still be not alone - we have cousins... somewhere!
( I certainly do not want this post to convey the impression that I have any great knowledge of the matters under discussion - especially the biology of it. My knowledge is at best that of a layman. But some of the numbers involved really do make skeptical conclusions inescapable.)