Michael Lemonick in National Geographic (photo by Mark Thiessen):
It's difficult to pin down when the search for life among the stars morphed from science fiction to science, but one key milestone was an astronomy meeting in November 1961. It was organized by Frank Drake, a young radio astronomer who was intrigued with the idea of searching for alien radio transmissions.
When he called the meeting, the search for extraterrestrial intelligence, or SETI, “was essentially taboo in astronomy,” Drake, now 84, remembers. But with his lab director's blessing, he brought in a handful of astronomers, chemists, biologists, and engineers, including a young planetary scientist named Carl Sagan, to discuss what is now called astrobiology, the science of life beyond Earth. In particular, Drake wanted some expert help in deciding how sensible it might be to devote significant radio telescope time to listening for alien broadcasts and what might be the most promising way to search. How many civilizations might reasonably be out there? he wondered. So before his guests arrived, he scribbled an equation on the blackboard.
That scribble, now famous as the Drake equation, lays out a process for answering his question. You start out with the formation rate of sunlike stars in the Milky Way, then multiply that by the fraction of such stars that have planetary systems. Take the resulting number and multiply that by the number of life-friendly planets on average in each such system—planets, that is, that are about the size of Earth and orbit at the right distance from their star to be hospitable to life. Multiply that by the fraction of those planets where life arises, then by the fraction of those where life evolves intelligence, and then by the fraction of those that might develop the technology to emit radio signals we could detect.
The final step: Multiply the number of radio-savvy civilizations by the average time they're likely to keep broadcasting or even to survive. If such advanced societies typically blow themselves up in a nuclear holocaust just a few decades after developing radio technology, for example, there would probably be very few to listen for at any given time.
The equation made perfect sense, but there was one problem. Nobody had a clue what any of those fractions or numbers were, except for the very first variable in the equation: the formation rate of sunlike stars. The rest was pure guesswork. If SETI scientists managed to snag an extraterrestrial radio signal, of course, these uncertainties wouldn't matter. But until that happened, experts on every item in the Drake equation would have to try to fill it in by nailing down the numbers—by finding the occurrence rate for planets around sunlike stars or by trying to solve the mystery of how life took root on Earth.
It would be a third of a century before scientists could even begin to put rough estimates into the equation.
More here.