From HomoExcelsior

Proposed equation to estimate the number of technological civilizations that might exist in our galaxy. The Drake Equation was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors.

N = R* x fp x ne x fl x fi x fc x L

N = The number of communicative civilizations The number of civilizations in the Milky Way Galaxy whose radio emissions are detectable. R* = The rate of formation of "suitable" stars The rate of formation of stars with a large enough "habitable zone" and long enough lifetime to be suitable for the development of intelligent life. fp = The fraction of those stars with planets The fraction of Sun-like stars with planets is currently unknown, but evidence indicates that planetary systems may be common for stars like the Sun. ne = The number of "earths" per planetary system All stars have a habitable zone where a planet would be able to maintain a temperature that would allow liquid water. A planet in the habitable zone could have the basic conditions for life as we know it. fl = The fraction of those planets where life develops Although a planet orbits in the habitable zone of a suitable star, other factors are necessary for life to arise. Thus, only a fraction of suitable planets will actually develop life. fi = The fraction life sites where intelligence develops Life on Earth began over 3.5 billion years ago. Intelligence took a long time to develop. On other life-bearing planets it may happen faster, it may take longer, or it may not develop at all. fc = The fraction of planets where technology develops The fraction of planets with intelligent life that develop technological civilizations, i.e., technology that releases detectable signs of their existence into space.

L = The "Lifetime" of communicating civilizations The length of time such civilizations release detectable signals into space.