the one armed scissor

The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence.
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 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 such extraterrestrial civilizations.
The Drake equation is closely related to the Fermi paradox.

The equation

The Drake equation states that:

$N\; =\; R$* x ƒp x ne x ƒℓ x ƒi x ƒc x L

where:

''N'' is the number of civilizations in our galaxy with which we might hope to be able to communicate;

and

''R*'' is the average rate of star formation in Milky Way;
''ƒ''_p'' is the fraction of those stars that have planets;
''n''_''e'' is the average number of planets that can potentially support life per star that has planets;
''f''_ℓ is the fraction of the above that actually go on to develop life at some point;
''ƒ''_''i'' is the fraction of the above that actually go on to develop intelligente life;
''ƒ''_''c'' is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space;
''L'' is the length of time such civilizations release detectable signals into space;