An equation used to estimate the number of technological civilizations that may exist in the galaxy. There is no exact answer to Frank Drake's famous equation, but it nonetheless a tool for the scientific community, and the force "behind every project at the SETI Institute."

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


N = The number of civilizations in the Milky Way whose radio emissions are detectable.

R* = 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 Sun-like stars with planets is currently unknown, but evidence indicates that planetary systems may be common for stars like the Sun.

ne = 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 length of time that communicative civilizations release detectable signals into space.

     Source: SETI