According to a paper cited here (found via Instapundit), extraterrestrial life must be rare in our galaxy. As far as the numbers the two Spanish authors came up with are concerned, I think this comment left by “dnivie” about nails it:
This seems like an excellent example of “If I’m allowed to pick any numbers I like, and multiply them with eachothers, I can arrive at any answer I want”
Be that as it may, life should exist elsewhere. After all, it seems unlikely that all life would eventually evolve into intelligent life. It took about 3.5 billion years of relatively benign conditions, or at least benign enough not to wipe out all life, for us to evolve. That’s more likely to be the exception than the rule. On the other hand, intelligent life must be extremely rare, if not unique. I’m not so sure about the probes mentioned in the paper, but it seems if it were otherwise we should have detected some electromagnetic signal in all the years we’ve been listening. Then again, maybe Carl Sagan’s conjecture was right. Maybe intelligent life forms do tend to self-destruct shortly after they evolve intelligence.