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.
The Kepler team has found amid the space telescope’s data 10 “candidate” planets that, while not confirmed, appear to be roughly one to two times the mass of Earth and orbiting at “Goldilocks” distances, neither too close nor too far from the stars.
The scientists extrapolated in two ways. They know that most planets are unlikely to be in orbits that, just by chance, cause them to pass in front of the face of the star as seen from Kepler. That’s a 1-in-100 long shot. For every planet seen, multiply by 100.
The scientists also knew that some planets would remain difficult to detect because of natural fluctuations in starlight — the “noise” in the signal. They found a way to test the accuracy of the algorithms for detecting planets by inserting 40,000 “synthetic” planets into the computer programs and seeing how many would be accurately retrieved by those programs.
The analysis does not prove that any of these “habitable zone” planets resemble Earth. The report states only that they are roughly the size of our planet and are not too close or too far from the star for water — if it is present — to be liquid at the surface.
Moreover, being in a star’s habitable zone does not ensure that life will spring forth. Scientists have limited understanding of the origin of life on Earth. The Earth has a number of features that are amenable to life and that might not be common, including a nearly circular orbit, a large companion moon and tectonic activity that recycles the planet’s carbon.
Kepler has found 3,538 candidate planets, Jason Rowe, a research scientist at the SETI Institute, said at the news conference Monday. Of those, 647 are Earth-size, 104 are in the star’s habitable zone and 10 meet both criteria.
William Borucki, the principal investigator for Kepler, noted that some of the larger planets in the habitable zone might have moons amenable to life. “All of those 104, I think, are very interesting planets,” Borucki said.
If the new estimate is correct, there should be about 25 billion Earth-size planets in habitable zones in our galaxy, by Borucki’s calculation.
“Those numbers are pretty soft,” he said. “We have a lot of corrections to make before we have really definitive numbers.”
Monday’s news can be viewed as progress in filling in numbers in the famous Drake Equation, developed in 1961 by astronomer Frank Drake as a way of estimating the abundance of communicative civilizations.
“It’s encouraging,” Drake, a senior astronomer at the SETI Institute, said Monday. “It gives us a number, and it’s a number we can stick into the equation.”
He said other factors in the Drake Equation remain highly uncertain — in particular, the evolutionary probability of intelligence.