Number Of Extraterrestrial Civilizations
Fundamental to all SETI approaches is the belief that there are a reasonable number of technological civilizations out there who might be trying to communicate with us.
The following formula for the number of technological civilizations in the galaxy is a modified form of one devised in 1961 by Frank Drake of Cornell, and is known as the "Drake Equation":
N = N* x fp x ne x fl x fi x fc x L
N* = number of stars in the Milky Way galaxy (400 X 109),
fp = fraction of stars that have planetary systems (0.1),
ne = average number of planets in such star systems that can support life (1),
fl = fraction of planets on which life actually occurs (0.1),
fi = fraction of such planets which intelligent life arises (0.01),
fc = fraction of intelligent beings knowing how to communicate with other civilizations (0.1),
L = average lifetime (fraction of the age of its star) of such technical civilizations (0.001).
Substituting the very conservative values given in parentheses for the entire Milky Way galaxy:
N = 4000
Thus, there could be 4,000 worlds waiting for us to detect in our galaxy. Of course, the real question is "If there are that many technical civilizations present, why haven't we been able to detect them?". Perhaps its because we have been using the wrong frequencies! It is the possibility that we have indeed been looking at the incorrect part of the electromagnetic spectrum that forms the heart of this investigation. Note that many ETI scientists give more optimistic values for these parameters, and thus yield much higher values for N.
Within 1000 light years of Sol there are 10 million stars, of which 1 million are solar-type.
Copyright (c), 1996