Number of Civilizations

Number of Civilizations

It is possible to explore the great issue and make a crude estimate of N, the number of advanced technical 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:

N – the number of stars in the Milky Way Galaxy;

F(p) – the fraction of stars that have planetary systems;

N(e) – the number of planets in a given system that are ecologically suitable for life;

F(1) – the fraction of otherwise suitable planets on which life actually arises;

F(2) – the fraction of inhabited planets on which an intelligent form of life evolves;

F(3) – the fraction of planets inhabited by intelligent beings on which a communicative technical civilization develops; and

F(l) the fraction of a planetary lifetime graced by a technical civilization.

Written out, the equation reads N= N,f(p), n(e), f(1), f(2), f(3), f(l). All the f’s are fractions, having values between 0 and 1; they will park down the large value of N.

Stars. Much of the Cosmos is in the span of Drake equation. Image: © Megan Jorgensen (Elena)

To derive N we must estimate each of these quantities. We know a fair amount about the early factors in the equation, the numbers 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, to make your own choices and see what implications your alternative suggestions have for the number of advanced civilizations in our Galaxy. One of the great virtues of this equation, due originally 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 Drake equation.

We know N, the number of stars in the Milky Way Galaxy, fairly well, by careful counts of stars in small but representative regions of the sky. It is a few hundred billion; some recent estimates place it at 4 x 10(11). Very few of these stars are of the massive short-lived variety that squander their reserves of thermonuclear fuel. The great majority have lifetimes of billions or more years in which they are shining stably, providing a suitable energy source for the origin and evolution of life on nearby planets.