N = R* • fp • ne • fl • fi • fc • L Each variable symbolizes the following: N = the number of communicative civilizations in our Galaxy R* = the average rate of star formation in our Galaxy fp = the fraction of those stars that have planets ne = the average number of planets that can potentially support life fl = the fraction of planets that actually develop life fi = the fraction of planets with life on which intelligence arises (civilizations) fc = the fraction of civilizations that develop detectable signs of communication L = the length of time such civilizations send communicative signals
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