A common argument is that the universe is so vast there just has to be life out there somewhere. But what does that statement mean? If we restrict attention to the observable universe there are probably 1023 planets. Yes, that’s a big number. But it is dwarfed by the odds against forming even simple organic molecules by random chance alone. If the pathway from chemistry to biology is long and complicated, it may well be that less than one in a trillion trillion planets ever spawns life.
http://blogs.scientificamerican.com/guest-blog/maybe-life-in-the-cosmos-is-rare-after-all/
