Defense Intelligence Reference Document An Introduction To The Statistical Drake Equation

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It is very hard to be sure. There may be several impediments to the evolution of a
technical civilization. Planets may be rarer than we think. Perhaps the origin of life is
not so easy as our laboratory experiments suggest. Perhaps the evolution of advanced
life forms is improbable. Or it may be that complex life forms evolve more readily, but
intelligence and technical societies require an unlikely set of coincidences - just as the
evolution of the human species depended on the demise of the dinosaurs and the ice-
age recession of the forests in whose trees our ancestors screeched and dimly
wondered. Or perhaps civilizations arise repeatedly, inexorably, on innumerable planets
in the Milky Way, but are generally unstable; so all but a tiny fraction are unable to
survive their technology and succumb to greed and ignorance, pollution and nuclear
war.
It is possible to explore this great issue further and make a crude estimate of N, the
number of advanced civilizations in the galaxy. We define an advanced civilization as
one capable of radio astronomy. This is, of course, a parochial if essential definition.
There may be countless worlds on which the inhabitants are accomplished linguists or
superb poets but indifferent radio astronomers. We will not hear from them. N can be
written as the product or multiplication of a number of factors, each a kind of filter,
every one of which must be sizable for there to be a large number of civilizations:
• Ns, the number of stars in the Milky Way galaxy.
• fp, the fraction of stars that have planetary systems.
• ne, the number of planets in a given system that are ecologically suitable for life.
• fl, the fraction of otherwise suitable planets on which life actually arises.
• fi, the fraction of inhabited planets on which an intelligent form of life evolves.
• fc, the fraction of planets inhabited by intelligent beings on which a communicative
technical civilization develops.
• fl, the fraction of planetary lifetime graced by a technical civilization.
Written out, the equation reads
N=N.1··.fi1-11e-_tl-_fi-_fc·.fL (7)
All of the fs are fractions, having values between 0 and 1; they will pare down the
large value of Ns.
To derive N we must estimate each of these quantities. We know a fair amount about
the early factors in the equation, the number of stars and planetary systems. We know
very little about the later factors, concerning the evolution of intelligence or the lifetime
of technical societies. In these cases our estimates will be little better than guesses. I
invite you, if you disagree with my estimates below, make your own choices and see
what implications your alternative suggestions have for the number of advanced
civilizations in the galaxy. One of the great virtues of this equation, due to Frank Drake
of Cornell, is that it involves subjects ranging from stellar and planetary astronomy to
organic chemistry, evolutionary biology, history, politics and abnormal psychology.
Much of the Cosmos is in the span of the Drake equation.
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