N = R* × fp × ne × fl × fi × fc × L, where the terms are defined as: N: the number of species in our galaxy that possess the technology necessary for interstellar communication; R*: the rate of star formation in our galaxy; fp: the fraction of stars with planetary systems; ne: the number of planets in each system with environmental conditions amenable to life; fl: the fraction of planets on which life arises; fi: the fraction of planets on which intelligent life arises; fc: the fraction of intelligent life that develops sufficiently sophisticated technology to take part in interstellar
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