In the 1950s, astronomer Frank Drake developed a thought-provoking equation to predict how many civilizations might exist in our galaxy. The point of the exercise was to estimate the likelihood of our detecting radio signals sent from other technologically advanced civilizations. This was the beginning of sporadic attempts by Earthlings to detect intelligent life on other planets. Now called the Drake Equation in its creator's honor, it has had enormous influence in a (perhaps necessarily) qualitative field. The Drake Equation is simply a string of factors that, when multiplied together, give an estimate of the number of intelligent civilizations, N, in the Milky Way galaxy. As originally postulated, the Drake Equation is.
N* = stars in the Milky Way galaxy fs = fraction of sun-like stars fp = fraction of stars with planets Be = planets in a star's habitable zone fi = fraction of habitable planets where life does arise fc = fraction of planets inhabited by intelligent beings fl = percentage of a lifetime of a planet that is marked by the presence of a communicative civilization our ability to assign probable values to these terms varies enormously. When Drake first published his famous equation, there were great uncertainties in most of the factors. There did (and does) exist a good estimate for the number of stars in our galaxy (over 300 billion). The number of star systems with planets, however, was very poorly known in Drake's time. Although many astronomers believed that planets were common, there was no theory that proved star formation should include the creation of planets, and many believed that the formation of planetary systems was exceedingly rare. During the 1970s and later, however, it was assumed that planets were common,- in fact, Carl Sagan estimated that an average of ten planets would be found around each star. Even though no extra solar planets were found until the 1990s, their discovery seemed to vindicate those who believed planets were common. But is it so? A new look at this problem suggests that planets may indeed be quite rare—and thus the presence of animal life rarer still.
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