A formula for estimating the possible number of intelligent civilizations in our galaxy. Most of the terms in these equations are unknown. In the equation, N is the number of civilizations, R is the average rate of star formation in the galaxy (about 20 stars per year ), fs is the fraction of stars that are suitable (about 0.1), fp is the fraction of stars with planets (about 0.5), ne is the mean number of planets that are located in a "habitable zone," where water exists in liquid form, fl is the fraction of these planets on which any life form evolves, fi is the fraction of places where some the life becomes intelligent, fc is the is the fraction of intelligent species who could communicate with us, and L is the lifetime (in years) of a civilization (this is quite uncertain). The equation was formulated by radio astronomer Frank Drake in 1961.

An approach to estimating some of the factors in guessing at the number of communicating civilizations in the galaxy.

Expression that gives an estimate of the probability that intelligence exists elsewhere in the galaxy, based on a number of supposedly necessary conditions for intelligent life to develop.