Drake equation

The Drake equation (sometimes called the "Green Bank equation", the "Green Bank Formula" or – erroneously – the "Sagan equation") is an equation to calculate the potential number of extraterrestrial civilizations in our galaxy, the Milky Way. It is used in the fields of exobiology and the search for extraterrestrial intelligence (SETI).
This equation was devised by Dr. Frank Drake (now Professor Emeritus of Astronomy and Astrophysics at the University of California, Santa Cruz) in 1960, in an attempt to estimate the number of extraterrestrial civilizations in the Milky Way with which we might come into contact.