First equation to estimate intelligent life in our galaxy

In 1961 US astronomer Frank Drake devised the Drake Equation, which is designed to give rough estimates of the number of planets inhabited by intelligent life in our Milky Way galaxy, based on a number of variables. The equation reads:

N=R* fp ne fl fi ft L

Where N is the number of civilisations capable of transmitting into space

R* = the birth rate of stars in our galaxy

fp = the fraction of stars with planet

ne = the number of planets per solar system that are suitable for life

fl = the fraction of such planets that actually spawn life

fi = the fraction of planets with life that evolve intelligent life

ft = the fraction of planets with intelligent life that produce technologically capable life

L = the average lifetime (in years) of a technological society

While some of these variables are extremely speculative, Drake himself estimated the number of advanced civilisations in our galaxy to be potentially as high as 10,000.