Carl Sagan on SETI Drake Equation

The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence.

This equation was devised by Dr Frank Drake (now Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz) in the 1960s in an attempt to estimate the number of extraterrestrial civilizations in our galaxy with which we might come in contact. The main purpose of the equation is to allow scientists to quantify the uncertainty of the factors which determine the number of extraterrestrial civilizations. In recent years, the Rare Earth hypothesis, which posits that conditions for intelligent life are quite rare, has been advanced as a set of arguments based on the Drake equation that the number of civilizations in the universe is small, and possibly unique to Earth.