An artist's rendition shows Kepler-69c, an Earth-size planet in… (NASA )
There's no place like home, but scientists now say that Earth-size planets orbiting sun-like stars in a so-called habitable zone are so common that there could be as many as 11 billion in the Milky Way alone.
Using a clever method to detect Earth-size exoplanets they may have missed, astronomers calculated that 1 in 5 stars like the one at the center of the solar system hosts a planet capable of holding liquid water on its surface and — if it had the right chemical ingredients — supporting life.
The finding, published Monday in the Proceedings of the National Academy of Sciences, "represents one great leap toward the possibility of life, including intelligent life, in the universe," said UC Berkeley astronomer Geoffrey Marcy, who worked on the new study.
The results suggest that the Milky Way is home to 11 billion Earth-like planets. If lukewarm planets orbiting cooler red dwarf stars are included, the total rises to 40 billion, Marcy said. The nearest could be just 12 light-years away.
The new planetary census was based on data gathered by the Kepler space telescope, NASA's primary planet-hunting observatory. The analysis also identified 603 potential exoplanets, including 10 Earth-size candidates orbiting in a habitable zone.
Since its launch in 2009, Kepler has helped uncover a "bewildering variety" of alien worlds throughout the Milky Way, said William Borucki, lead scientist for the Kepler mission at NASA's Ames Research Center in Mountain View, Calif.
Many in this strange menagerie — which includes mini-Neptunes and super-Jupiters — are not like any of the planets in our solar system. Some distant planets may be composed entirely of water; others have densities greater than iron or lower than Styrofoam.
Of the thousands of candidate exoplanets spied by Kepler, UC Berkeley astronomer Erik Petigura had one driving question: How many were like Earth?
"How common is this ball of rock that we're sitting on right now?" said Petigura, lead author of the new report.
It's not easy to find planets orbiting other stars, let alone smaller ones like Earth, Petigura said. A transit-watching telescope like Kepler looks for dips in a star's brightness as a planet travels in front of it and blocks a tiny fraction of its light. But that works only if the telescope is properly aligned to catch a planet in the act of transiting. There's another problem: Some stars are too active for this to work well; they flicker so much that it's hard to find the tiny 0.01% dip in light caused by an Earth-size planet.
Clearly, some exoplanets had evaded detection. And while scientists couldn't do much about the planets that didn't transit, perhaps they could find out how many planets were already hidden in the data.
"We're doing a census of extrasolar planets, but not everyone's answering the door," Petigura said.
So he, Marcy and Andrew Howard of the University of Hawaii-Manoa decided to see whether they could figure out how many planets they were missing. They added fake planets into the starlight data and then ran the software that searched for planetary signals. Not all of the planets were found. Because they knew how many faux planets they'd started with, it was a simple matter to figure out how many they were missing.
The researchers figured that they were probably missing just as many in their real planet search.
"Understanding how many of the fake planets we find, we understand how many of the real ones we're finding," Petigura said.
The researchers surmised that about 22% of stars like the sun have planets in their habitable zone that are about the size of Earth.
"If we ever get star travel, we'd probably see a lot of traffic jams," Borucki said.
Of course, there's no guarantee that any of these planets are habitable — that depends on what they're made of, whether they have protective atmospheres and whether they contain ingredients like water and other chemical building blocks necessary to support life as we know it.
That sort of planet won't be found any time soon. It will require missions that "are likely to be done by our children or grandchildren," Borucki said at the Kepler Science Conference II, being held this week at the Ames Research Center.
But scientists are making progress. A separate analysis by Borucki's Kepler team announced 833 new planetary candidates, bringing the total to 3,538.
Given that Kepler gets better at finding smaller planets as it observes more transits and gathers more data, "the trend of finding Earth-like planets is likely to continue," said Jason Rowe, a research scientist at the SETI Institute in Mountain View who led the work.
Kepler suffered a critical malfunction with its reaction wheels last spring and is no longer able to gather transiting data with the precise pointing power it once wielded. But the data from its fourth and final year of stargazing have yet to be fully analyzed.
"We still have a lot more to discover," Borucki said.