Developed by Frank Drake in 1961 to determine the probable number of other technically advanced civilizations in a galaxy.
N = R * FP * NE * FL * FI * FC * L
N = number of technically advanced civilizations emitting some kind of detectable signal.
R = rate of star formation averaged over the lifetime of the galaxy.
FP = fraction of stars with planetary systems. NE = mean number of planets per system with environments suitable for the origin of life. FL = fraction of such planets on which life does develop.
FI = fraction of such planets on which intelligent life rises during the lifetime of the local sun.
FC = fraction of planets on which advanced technical civilizations rises. L = lifetime of this technical civilization.
According to Carl Sagan, this would mean that there are about 1 million planets in the Milky Way which have civilizations advanced enough for us to detect them.
So why haven’t we heard from any of them? That question is the Fermi Paradox.