Drake equation

Description

is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy

N	the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone) (dimensionless)
R	the average number of star formation per year in our galaxy (year^-1)
f_p	the fraction of those stars that have planets (dimensionless)
n_e	the average number of planets that can potentially support life per star that has planets (dimensionless)
f_l	the fraction of planets that could support life that actually develop life at some point (dimensionless)
f_i	the fraction of planets with life that actually go on to develop intelligent life (civilizations) (dimensionless)
f_c	the fraction of civilizations that develop a technology that releases detectable signs of their existence into space (dimensionless)
L	the length of time for which such civilizations release detectable signals into space (year)