The Drake Equation (sometimes called the Green Bank Equation), named for SETI pioneer Frank Drake (1930–), is a mathematical expression that encapsulates the conceptual framework of SETI. According to this equation, the number of alien civilizations in our Milky Way galaxy with which humans could communicate is the product of seven factors: (1) the average rate of star formation in the Milky Way; (2) the fraction of those stars that have planets; (3) the average number of habitable planets that each planet-hosting star has orbiting around it; (4) the fraction of habitable planets that actually have life forms on them; (5) the fraction of life-bearing planets that develop civilizations of intelligent life; (6) the fraction of civilizations that produce detectable signs of their existence, such as radio waves or atmospheric changes; and (7) the length of time that such civilizations emit those detectable signs.

The Drake Equation is a useful way to think about SETI scientifically. Each of the seven factors can be studied with the methods of science. Unfortunately, at this point in human history, we simply do not have enough information to know the actual value of most of the seven factors with any degree of accuracy. It is a sure bet, though, that astronomers will keep trying to find them out. Already, astronomers think that one Sun-like star is formed in the Milky Way every few years or so, although this star formation rate is still only an estimate.