© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

23.1  Theoretical Galactic Demography

The late Harlow Shapley was perhaps the first astronomer ever to seriously attempt quantitatively to estimate the frequency of extraterrestrial life in the universe.⁸¹⁶ It is said that Shapley was more optimistic than his published estimates indicate, but that he deliberately chose extremely conservative numbers to forestall possible criticism from the scientific quarter. His calculation went something like this.

Suppose that only one star out of every 1000 has any planets at all. Further, let us presume that out of every 1000 worlds, only a single one is located an appropriate distance from its sun (and thus has a climate suitable for life). Then assume that out of all these well-placed planets only one in 1000 is big enough to hold an atmosphere, and that of these worlds, only one in 1000 has a chemical composition appropriate for the origin of life. Combining all of these factors, we find that only 1 star in 10¹² is graced by life. There are at least 10²¹ stars in the universe so, according to Shapley, we may expect to find about 1,000,000,000 life sites in the cosmos.

At first blush this seems like a very large number, but upon further reflection we see it is actually quite pessimistic. Since the typical habitable galaxy has about 10¹¹ stars, only one galaxy out of ten will harbor any life at all. If this calculation is correct, humanity is very much alone.

It appears that Shapley selected his step-factors of 1000 largely out of cautious conservatism without any real basis in genuine, established scientific findings. Subsequent formulations have been devised in an attempt to deal with this major failing of the Shapley calculation. The most popular alternative among xenologists today is the so-called Drake Equation¹³¹⁷ (or "Green Bank Formula"²³⁵⁸), named for its originator Frank D. Drake, a well-known American radioastronomer currently associated with the Center for Radio-physics and Space Research at Cornell University.

Last updated on 6 December 2008