We are often told that with so many stars in the galaxy – not to mention the universe – and so many Earth style planets being discovered it is impossible that there is no life on other worlds. The counter-argument to this is simple: where are they? Why haven’t they been in touch?
I recently read an enlightening article on this very subject in Perihelion SF by a chap called J. Richard Jacobs. In Alien Argument he gives one possible explanation as to why little green men (and women) have not yet appeared. It’s definitely worth a read and Perihelion SF is completely free, though donations are gratefully accepted.