Joel Achenbach — Nov 4, 2013
Roughly one in every five sunlike stars is orbited by a potentially habitable, Earth-size planet, meaning that the universe has abundant real estate that could be congenial to life, according to an analysis of observations by NASA’s Kepler space telescope.
Our Milky Way galaxy alone could harbor billions of rocky worlds where water might be liquid at the surface, according to the report, which was published Monday in the Proceedings of the National Academy of Sciences and discussed at a news conference in California.
If the estimate is correct, the nearest ocean planet might be just 12 light-years away, which, though extremely distant for all practical purposes (such as sending a robotic space probe), is just around the corner in our galactic neighborhood.
“When you look up at the stars in the night sky, how many of them have a planet like the Earth?” asked Erik Petigura, a graduate student at the University of California at Berkeley and the lead author of the paper. “We’re able to start answering this question.”
The best estimate is 22 percent of stars like our own, with an error margin of plus or minus eight percentage points.
“Earth-sized planets having the temperature of a cup of tea are common around sunlike stars,” said planet hunter Geoff Marcy, a Berkeley astronomer and a co-author of the study. He said the finding “represents one great leap toward the possibility of life, including intelligent life, in the universe.”
Kepler, launched in 2009, is no longer able to search for “exoplanets” — outside our solar system — because it has been unable to point with precision after the failure of a steering mechanism this year. But the telescope amassed more than three years of observations before going on the blink. Kepler mission scientist Natalie Batalha said there is still another full year of data to rummage through.
The telescope’s original mission was to obtain an estimate of the percentage of stars with potentially habitable planets, and this latest analysis comes close to meeting that goal. This is still an extrapolation of data and is not the same thing as taking a careful census of these Earth-size planets directly, said Sara Seager, an astrophysicist at MIT who was not directly involved in the new analysis.
“Earth-size” doesn’t necessarily mean “Earth-like,” Seager noted. But she said this result will boost efforts to build telescopes that could obtain direct imagery of one of these extremely distant worlds.
“Earth-sized planets are not rare, so we’ll know we’ll have stuff to look at,” Seager said. “It’s reassuring for us.”
Jill Tarter, a pioneer in “SETI,” the search for extraterrestrial intelligence, said in an e-mail: “We haven’t yet found Earth 2.0, but these statistics suggest that it should be forthcoming, and soon. When we can point to Earth 2.0 in the sky, it will seem completely natural to ask ‘Does anybody live there?’ and ‘Can we go there?’ I think Earth 2.0 will concretize SETI as nothing else has.”
Kepler studied 150,000 stars in a small patch of the sky in the constellation Cygnus. The planets surrounding distant stars cannot be seen directly, because their faint, reflected light is swamped by the much brighter starlight. Thus Kepler looked for the periodic dimming of a star, which could be the signal of a planet passing across the star’s face. Ground-based telescopes, such as the Keck I in Hawaii, helped produce an estimate of the size of these transiting planets.