National Geographic : 2014 Jul
32 national geographic • july 2014 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 es- sentially taboo in astronomy,” Drake, now 84, remembers. But with his lab director’s blessing, he brought in a handful of astronomers, chem- ists, 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 equa- tion, lays out a process for answering his ques- tion. 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 technol- ogy 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 sur- vive. 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 for- mation 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 hap- pened, experts on every item in the Drake equa- tion 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 scien- tists could even begin to put rough estimates into the equation. In 1995 Michel Mayor and Didier Queloz of the University of Geneva detected the first planet orbiting a sunlike star outside our solar system. That world, known as 51 Pegasi b, about 50 light-years from Earth, is a huge, gas- eous blob about half the size of Jupiter, with an orbit so tight that its “year” is only four days long and its surface temperature close to 2000°F. Nobody thought for a moment that life could ever take hold in such hellish conditions. But the discovery of even a single planet was an enormous breakthrough. Early the next year Geoffrey Marcy, then at San Francisco State Uni- versity and now at UC Berkeley, would lead his own team in finding a second extrasolar planet, then a third. After that, the floodgates opened. To date, astronomers have confirmed nearly two thousand so-called exoplanets, ranging in size from smaller than Earth to bigger than Jupiter; thousands more—most found by the exquisitely sensitive Kepler space telescope, which went into orbit in 2009—await confirmation. None of these planets is an exact match for Earth, but scientists are confident they’ll find one that is before too long. Based on the discov- eries of somewhat larger planets made to date, astronomers recently calculated that more than a fifth of stars like the sun harbor habitable, Earth- like planets. Statistically speaking, the nearest one could be a mere 12 light-years away, which is practically next door in cosmic terms. That’s good news for astrobiologists. But in recent years planet hunters have realized that there’s no reason to limit their search to stars just like our sun. “When I was in high school,” says David Charbonneau, an astronomer at Harvard, “we were taught that Earth orbits an average star.