The Drake equation is a  probabilistic argument used to arrive at an estimate of the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The number of such civilizations, N, is assumed to be equal to the mathematical product of (i) the average rate of star formation, R_*, in our galaxy, (ii) the fraction of formed stars, f_p, that have planets, (iii) the average number of planets per star that has planets, n_e, that can potentially support life, (iv) the fraction of those planets, f_l, that actually develop life, (v) the fraction of planets bearing life on which intelligent, civilized life, f_i, has developed, (vi) the fraction of these civilizations that have developed communications, f_c, i.e., technologies that release detectable signs into space, and (vii) the length of time, L, over which such civilizations release detectable signals, for a combined expression of:

The equation was written in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. Criticism towards the Drake equation follows from the fact that several of its terms are conjectural, the net result being that the error associated with any derived value is very large such that the equation cannot be used to draw firm conclusions.