We've been conditioned by television and movies to accept the likelihood of intelligent life elsewhere in the Universe. "Of course there's intelligent life out there; I saw it last week on Star Trek." We've seen it all, from the cute and cuddly ET to the fanged monstrosity of Alien.
But is it likely that we're not alone in the universe? And if intelligent life is out there, why haven't they contacted us yet?
The first person to address this question in a systematic way was Frank Drake, who invented the Drake equation to predict the number of extraterrestrial civilizations in the Galaxy. His equation is rather complicated, but here's a simple version of his argument.
First, let's count how many stars are in the galaxy. To quote one of my predecessors, "Billions upon billions!" And how many of those stars have planets? Until recently, we really didn't know. But over the past 20 years, astronomers have made remarkable progress in discovering planets around other stars. We now know that many stars have planets orbiting them.
Could creatures actually live on any of those planets? Many of them are just giant balls of gas, or else too hot or too cold to contain liquid water, which is the basis of all life on Earth. But a few of them do seem to be at the right temperature. These are the Goldilocks planets: not too hot, and not too cold for liquid water. (And that's without even considering the possibility that exotic forms of life could survive without water).
Now we enter murkier territory. How likely is it that life will develop on a potentially habitable planet? We don't know the answer, but life on Earth got going very shortly after the formation of our solar system, and it has wedged itself into every available niche, no matter how hostile.
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