At the first international conference on the search for extraterrestrial intelligence, Frank Drake presents in Green Bank, West Virginia, the formula N = R * fp * ne * fl * fi * ft * L with N = communicative civilizations, R = solar stars born each year, fp = fraction with planets, ne = planets in the habitable zone per system, fl = fraction of planets with life, fi = fraction of planets with intelligence, ft = fraction of planets with communicative technology, L = average lifespan of civilizations in years
