N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R* = The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space. Estimates Original estimates There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were: R* = 1/year (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative) fp = 0.2-0.5 (one fifth to one half of all stars formed will have planets) ne = 1-5 (stars with planets will have between 1 and 5 planets capable of developing life) fl = 1 (100% of these planets will develop life) fi = 1 (100% of which will develop intelligent life) fc = 0.1-0.2 (10-20% of which will be able to communicate) L = 1000-100,000,000 years (which will last somewhere between 1000 and 100,000,000 years) Inserting the above minimum numbers into the equation gives a minimum N of 20. Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 civilizations in the Milky Way galaxy.