The recent report of the ubiquity of extrasolar planets coupled with the fact that it bodes well for searches for life friendly Earth twins, brought me back to my reading of Carl Sagan's 1966 Intelligent Life In The Universe and later article in Sky & Telescope where he came up with a remarkable number in favor of such.
Searching for extraterrestrial intelligence has long been a hot topic among astronomers, biologists, and the general public. But not many recall how the subject was jump-started 50-years ago.
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published a landmark article in the British weekly journal Nature with the provocative title, "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars.
Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. The project was well designed, cheap, simple by today's standards, and unsuccessful.
Following the experiment, Drake organized a meeting with a select group of scientists to discuss the prospects and pitfalls of the search for extraterrestrial intelligence nowadays abbreviated SETI.
It was in preparing for this meeting that Drake came up with what soon became known as the Drake Equation. Today this string of letters and symbols can be found on T-shirts, coffee mugs, and bumper stickers. It expresses the number of "observable civilizations" that currently exist in our Milky Way Galaxy as a simple multiplication of several, more approachable unknowns (and I will avoid that discussion for now).
Some 50-years of SETI have failed to find anything, even through radio telescopes, receiver techniques, and computational abilities have improved enormously since the early 1960's. But we have discovered, at least, that our galaxy is not teeming with very powerful alien transmitters continuously broadcasting near the 21-centimeter hydrogen frequency. No one could say this in 1961.
In July 1996, at the Fifth International Bio-Astronomy Conference in Italy, Drake confessed: "Maybe I was a little bit too optimistic. Success can't be predicted."
Cocconi and Morrison already told him so in their 1959 Nature article: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."