The planets, described at a NASA press conference, orbit a sun that's cooler than ours but is at the right distance to allow water to remain liquid, which is considered essential for a planet to support life. (Read about a related discovery in 2011: "NASA's Kepler Finds Two Earth-Size Planets Around Sunlike Star.")
And because of their sizes and orbits, the newfound planets are likely either rocky—like Earth—or watery, NASA scientists said. The two planets are located 1,200 light-years away in a five-planet system orbiting a star dubbed Kepler-62.
Called Kepler-62e and -62f, the planets "are by far the best candidates for habitability of any found so far," said William Borucki of NASA's Ames Research Center, the science principal investigator for the agency's Kepler Space Telescope.
"If you were on Kepler-62f and looking at the sun, it would be a little less yellow than ours," said Borucki, whose announcement coincided with the release of a study on the topic in the journal Science.
"And at sunset the sky would be more red. But otherwise it would basically look and feel the same," he said.
"I would call this a breakthrough discovery."
The newly found extrasolar planets, or exoplanets, are part of a vast menagerie of celestial bodies discovered in recent years. Our Milky Way galaxy alone is now known to have hundreds of billions of planets circling its stars.
Exoplanets that are Earth-size, are rocky or watery, or that exist in habitable zones in relation to their suns have all been found before. But the two new planets are the first that appear to meet all three criteria, Borucki said.
Although these planets are now believed to be habitable, Borucki cautioned there's no evidence that they actually support life.
The planets were found by the Kepler Space Telescope, an orbiting observatory that studies 170,000 stars and the planets that circle them in a small, distant patch of the Milky Way. It works on the understanding that if an exoplanet crosses in front of its sun, the light from the star will slightly dim; this shows researchers there's a planet in orbit around that star.
Those early discoveries overturned a lot of assumptions about how solar systems could be organized, since the prevailing understanding had been that other solar systems resembled our own. That was why finding a Jupiter-like planet in the place where Mercury or Venus would be in our solar system was unexpected.
But as Kepler enters its fourth year of observing its selected stars, it is able to detect smaller planets with orbiting periods much longer than those found earlier in the mission.
To be confident they've identified a small planet, Kepler researchers need to see the faint sign of a transit across its sun a number of times. (Related: "Smallest Exoplanets Found—Each Tinier Than Earth.")
So far Kepler scientists have identified 350 Earth-size exoplanets. The two planets introduced today—called "super Earths"—are 1.4 and 1.6 times larger than Earth, and orbit around their sun in 122 and 267 days, respectively.
In Search of Super Earths
There's some debate in the exoplanet community about what constitutes an Earth-size planet. But Kepler has also found planets that are Mars-size—about half the diameter of Earth—and even moon-size.
"There's been a steady progression in the field from easier-to-find planets to harder-to-find planets," said Kepler team member Geoff Marcy of the University of California, Berkeley.
"Our goal is to find a extrasolar planet that is within 5 percent [the size of] Earth."
The question of whether or not the planets are solid, liquid, or consist largely of gas is more complex and not easily teased out using the Kepler's method of tracking planetary transits.
Borucki said the team's conclusion that Kepler-62e and -62f are not gas planets but rather potentially habitable rocky or water planets is based on their size and place in the Kepler-62 planetary system. No planet the size of Kepler-62f has ever found to be gaseous, he said.
The density of exoplanets is most effectively determined using a different form of measurement, one that captures the "wobble" of a star caused by the gravitational pull of an exoplanet. (See "'Backward' Planet Has Density of Foam Coffee Cups.")
So far that method has not been effective for planets as small and as distant as those around Kepler-62. But Marcy said his team, based at the W. M. Keck Observatory in Hawaii, will be working to better understand the density of the newfound planets.
New Sun Discovered
Scientists also described another recent discovery: Kepler-69. It's "the sun most like our own that we've found that has a planet orbiting in the habitable zone," said study leader Thomas Barclay, a Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California.
He said planets orbiting hot stars like our own are more difficult to detect than those circling cooler stars, so the discovery of Kepler-69 is a sign of major technical progress.
"We're finding planets now that were unimaginable not very long ago—small ones pretty far from their stars, and now with very bright suns," Barclay said.
Marcy, a co-author of the Kepler-62 study, called the new exoplanet discoveries "profound."
That's in part because it dovetails with another recent finding that most of the 200 billion stars in the Milky Way have an Earth-size planet orbiting them.
He said he didn't think the importance of the Milky Way discovery has really sunk in with the public.
"It's not just that most stars have Earth-sized planets, but some of them—many of them actually—are at a distance from their suns that allow for lukewarm temperatures," Marcy said. "That means they just might support life."
He added, "It's a very moving moment in humanity's efforts to understand our home planet and the possibility of other habitable planets in the universe."