Drake equation: a standalone construct

The astronomer Frank Drake wrote this equation in 1961 as the agenda for the first scientific meeting on the search for extraterrestrial intelligence, held at the National Radio Astronomy Observatory in Green Bank, West Virginia, and he set it out in print a few years later (Drake, 1965), which is why it is also called the Green Bank equation. It estimates N, the number of civilizations in the Milky Way whose signals we might currently detect, as the product of seven factors: the rate at which suitable stars form, the fraction with planets, the number of habitable planets in each such system, the fraction on which life appears, the fraction of those on which intelligence evolves, the fraction that release signals into space, and the average length of time such a civilization stays detectable. Each factor turns one large unknown into a quantity that astronomy, biology, or the social sciences might in principle estimate, so the equation works less as a precise prediction and more as a shared accounting of every step between star formation and a detectable civilization. The rightmost factor, the lifetime of a communicating civilization, is the least constrained, and Frank and Sullivan (2016) recast the same terms to ask instead whether any technological species has ever arisen anywhere, a version that drops the lifetime factor. Its scope spans astrophysics, planetary science, origin-of-life chemistry, and the study of intelligence and technology, and it is the framework around which the SETI field has organized its questions (Burchell, 2006).[1][2][3]

Unit

N, a count of currently active, communicating civilizations in the Milky Way, written as the product of seven factors

Recovery means

Not applicable

Measured by

Estimated by assigning values to its seven factors from astronomical, biological, and social data, with the result reported as a wide range rather than a single figure because the later factors are poorly constrained