It's an idea that's had a long run, dating back to those distant days when Aristotle strolled Athens with his students: namely, that Earth is unique. True, we no longer believe that our planet's astronomical position is special, that we sit like a queen bee in the central court of the cosmos. But serious people still contend that our terrestrial environment might be singular — that Earth is one of the few places in the universe where life could arise.
Well, I'll bet you cash to crullers that — within a thousand days — this idea, too, will be deposed. If so, are we prepared to take the next step — to explore newly discovered cosmic real estate for living things?
The prospects aren't encouraging. NASA has postponed building new telescopes that could find methane or oxygen in the atmospheres of distant planets, clues to the presence of extraterrestrial pond scum. At the other end of the evolutionary scale, some of these proximate worlds might have evolved thinking beings — inhabitants that might notice our TV and radar signals and possibly attempt to get in touch. But projects for eavesdropping on transmissions from advanced societies are hobbled by lack of funding.
Meanwhile, researchers continue to find new planets, and with increasing efficacy. A decade ago, only 5 percent of stars examined by these planet hunters actually showed evidence of orbiting worlds. Today, thanks to improved instruments, it's nearly 50 percent.
The implication is astounding: Planets are stupefyingly plentiful. The number of solar systems in our galaxy must be a few hundred billion, more than all the trees in America. And since planets, like kittens, are born in litters, they will tally a trillion or more.
But how many are gentle enough for life? NASA's Kepler telescope, launched in March, is searching for cousins of Earth in an attempt to answer that question. But Kepler now competes with terrestrial telescopes.
Just this month, researchers using telescopes in California and Australia reported the discovery of a handful of planets orbiting stars that are neighbors of the sun. Unlike most planets found to date, several of these are thought to be small and rocky, somewhat like Earth or Mars.
Three of them pirouette around a star known as 61 Virginis, a doppelganger of our sun, and a mere 28 light-years distant. One has an egg-shaped orbit that would put it somewhere between Mercury and Venus in our own solar system, suggesting that it's probably too toasty for life — but just barely.
Astronomer Steven Vogt of the University of California-Santa Cruz, one of the leaders in making these discoveries, notes the significance of finding livable worlds in our backyard:
"If you walk onto a beach with a metal detector and find a gold coin only 2 feet from your starting point, then either you're very lucky or there are a lot of gold coins on the beach. If we search among the nearest stars and find one or two of these habitable planets, well, that suggests there must be lots of them."
But even if salubrious worlds are as common as crickets, can we be sure any have life? On Earth, biology began early, suggesting that — given a suitable planetary Petri dish — life is neither improbable nor long in coming.
SETH SHOSTAK is senior astronomer at the SETI Institute based in Mountain View. He wrote this article for the Mercury News.