On The New York Times, Adam Frank says that for the longest time we questioned whether or not there were aliens out there; however, now that we know that "every star in the sky likely hosts at least one planet," even the worse case scenario still shows that "a trillion civilizations still would have appeared over the course of cosmic history." In other words, we may not know if there are aliens right now, but there's a good change that they definitely existed.
Or consider the average lifetime of a civilization. Humans have been using radio technology for only about 100 years. How much longer will our civilization last? A thousand more years? A hundred thousand more? Ten million more? If the average lifetime for a civilization is short, the galaxy is likely to be unpopulated most of the time. Once again, however, with only one example to draw from, it’s back to a battle between pessimists and optimists.
But our new planetary knowledge has removed some of the uncertainty from this debate. Three of the seven terms in Drake’s equation are now known. We know the number of stars born each year. We know that the percentage of stars hosting planets is about 100. And we also know that about 20 to 25 percent of those planets are in the right place for life to form. This puts us in a position, for the first time, to say something definitive about extraterrestrial civilizations — if we ask the right question.
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