An Etymological Dictionary of Astronomy and Astrophysics

A probabilistic argument used to estimate the number of → intelligent, communicating → extraterrestrial civilizations in the → Milky Way galaxy. The Drake equation is:
N = R . f_p . n_e . f_l . f_i . f_c . L, where:
N = the number of → civilizations in our Galaxy with which → communication might be possible,
R = the average rate of → star formation in our Galaxy,
f_p = the fraction of those stars that have → planets,
n_e = the average number of planets that can potentially support → life per star that has planets,
f_l = the fraction of planets that could develop life at some point,
f_i = the fraction of planets with life that actually go on to develop intelligent life,
f_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space,
L = the length of time for which such civilizations release detectable signals into space.

The first three terms of the equation have been successfully investigated by astronomers and are to some extent known. In contrast, values for the last four are very speculative. Drake himself estimates that N might be as high as 10,000. Carl Sagan was more optimistic, and came up with the value of a million or more for N. These estimates may be too optimistic. A pessimistic choice of parameters leads to N smaller than 1, which means that we might be the only technically sophisticated civilization in the Galaxy.

See also: Frank Donald Drake (1930-); → equation.