In 1961, Frank Drake encapsulated his thoughts about the search for extraterrestrials in what would become the Drake equation:

where N is the number of extraterrestrial civilizations that we could communicate with.

Let's look at the pieces:

R_star -- the rate of star formation (in the galaxy)

f_planets -- the fraction of stars with planets

n_hab -- the number of those planets in the habitable zone

f_life -- the fraction of those planets on which life exists

f_intel -- the fraction of those planets on which intelligent life emerges

f_techn -- the fraction of those planets on which technological civilizations arise

L -- the length of time such civilizations emit electromagnetic signals into space

There are many (slightly) different forms of this equation. This version is close to the original equation Frank Drake wrote in 1960. In any case, all the variations have the same general theme.

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