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55 At an informal meeting of scientists in November 1961, Frank Drake pro- posed his now famous equation as a way to estimate the number of com- municating civilizations in our galaxy. Drake, who was trying to quantify SETI, thought that the equation could be used to justify and optimize searches. His formula also gave the subject a credibility that allowed other people to enter the field.* Astronomy writer Govert Schilling later commented that the Drake equation, by breaking down a great unknown into a series of smaller, more addressable questions, made the search for alien civilizations more realistic and promising. The formula focused our attention on the really important issues.° Others put it more modestly, describing the equation as a way of organizing our ignorance. An Evolving Equation The original Drake equation read as follows, with N representing the number of communicating civilizations now in existence: N=R fp) ne) FOO FO) L The first factors were physical. R is the rate of star formation in our galaxy; f(p) is the fraction of stars that have planets; n(e) is the number of planets per star suitable for life. Later factors had more to do with biological evolution: f(/) is the fraction of those planets on which life develops; f(i) is the fraction of life-bearing planets with intelligent life. Then came the cultural factors. f(c) is the fraction of intelligent cultures that develop radio communication we can detect; L is the average time spent by civilizations in a communicative state. The Drake equation has been modified since then. Shostak, describ- ing the formula as it existed in 1998, stated that f(c) is the fraction of civilizations that have the technology and the incentive to communicate over interstellar distances.” A version available from the SETI Institute in 1999 showed N as the number of communicative civilizations in the Galaxy whose radio emis- sions are detectable. R here means the rate of formation of suitable stars; f(p) means the fraction of those stars with planets; n(e) now means the number of “Earths” per planetary system. f(/) is the fraction of those planets where life develops; f(i) now means the fraction of life sites where intelligence develops; f(c) now means the fraction of planets where technology develops. The Drake Equation The Drake Equation N=R fp) ne) FOO FO) L