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.[1][2] 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, fp, that have planets, (iii) the average number of planets per star that has planets, 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:
N = R* fp ne fl fi fc L
where,
N = The number of communicative civilizations
R* = The rate of formation of suitable stars (stars such as our Sun)
fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun.)
ne = The number of Earth-like worlds per planetary system
fl = The fraction of those Earth-like planets where life actually develops
fi = The fraction of life sites where intelligence develops
fc = The fraction of communicative planets (those on which electromagnetic communications technology develops)
L = The “lifetime” of communicating civilizations
Frank Drake’s own current solution to the Drake Equation estimates 10,000 communicative civilizations in the Milky Way. Dr. Drake, who serves on the SETI League’s advisory board, has personally endorsed SETI’s planned all-sky survey.
(from QST, August 1995, page 38)