Drake equation

The Drake equation is a way to organize the factors that influence the number of technologically capable civilizations in the Milky Way whose emissions could be detected from Earth. Astronomer Frank Drake devised it in 1961 while planning a meeting at Green Bank Observatory in West Virginia about the scientific search for extraterrestrial intelligence (SETI). It served as a framework for discussion, not as a claim that the number of such civilizations was already known.

A commonly used form is N = R × fₚ × nₑ × fₗ × fᵢ × f꜀ × L. N is the number of detectable, communicating civilizations in the Milky Way. R is the rate at which suitable stars form; fₚ is the fraction of those stars with planets; nₑ is the average number of planets per planetary system that could support life; fₗ is the fraction of such planets where life develops; fᵢ is the fraction of life-bearing planets where intelligent life evolves; f꜀ is the fraction of intelligent civilizations that produce detectable signals; and L is the average length of time those signals are produced. Definitions of the terms can differ somewhat among presentations, but the factors trace a path from suitable stars to detectable technology.

The product can be understood as a rate of appearance of transmitting civilizations multiplied by the average time each remains detectable. Some factors, such as the abundance of planets around stars, have become more amenable to astronomical observation. Other quantities—especially the likelihood that life or technological intelligence emerges, and how long a civilization produces detectable signals—are not presently known well enough to calculate a definitive value for N.

The equation is not a directly solvable law or a prediction with a uniquely established answer: estimates depend on uncertain assumptions about its terms. Its enduring value is as a research and discussion framework, helping scientists identify which questions can be studied with astronomy and which require evidence about biology, intelligence or the history of civilizations. Related analyses sometimes change the question—for example, from how many civilizations may be detectable now to how many technological species may have arisen over cosmic history. Those are distinct questions and use different formulations.