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.

N = R ∗ ⋅ f p ⋅ n e ⋅ f ℓ ⋅ f i ⋅ f c ⋅ L

(i) the average rate of star formation, R*, in our galaxy,

(ii) the fraction of formed stars, fp, that have planets,

(iii) the average number of planets per star, ne, that can potentially support life,

(iv) the fraction of those planets, fl, that actually develop life,

(v) the fraction of planets bearing life on which intelligent, civilized life, fi, has developed,

(vi) the fraction of these civilizations that have developed communications, fc, 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 the first scientific meeting on the search for intelligent extraterrestrial life. The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.

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