Are we alone?

Stephen Hawking has recently weighed in on the question of whether we are alone in the universe. His logic is that there are a hundred billion galaxies, each containing hundreds of millions of stars, so it's unlikely that Earth is the only planet where life originated. Sounds plausible, but here's a counter-argument: suppose a phenomenon occurs once in 10^100 times. We know that there are 10^20 samples, which is a large enough number, and erroneously conclude that it is large enough for the phenomenon to occur more than once. The moot point here is the actual probability of the event, i.e., the 10^100 figure -- there's no reliable and/or scientific way to validate this.