# Carl Sagan explaining the Drake equation:

N = R*•fp•ne•fl•fi•fc•L

“In 1961, Carl’s close friend, fellow astronomer Frank Drake, created an equation for calculating the number of intelligent civilizations in the galaxy.”

The Drake equation is:

N = R*•fp•ne•fl•fi•fc•L

where:

N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone);
and

R∗ = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point

fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space