Astronomers digging through data from NASA's planet-hunting Kepler space telescope say they have discovered the closest Earth-like "twin" circling another star ever found.
The planet, called Kepler-452b, is about 60% larger in diameter than Earth and circles a sun-like star that sits about 1,400 light-years away in the constellation Cygnus. It orbits at roughly the same distance as Earth – its year is 385 days, just 20 days longer than our own – which means it probably gets a similar amount of sunshine.
"Today we're announcing the closest twin, so to speak, to Earth 2.0, that we've found so far in the data set," John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington, said in a briefing Thursday.
This planet is not the only Earth-sized planet to have been found circling its planet's habitable zone, but the previous best candidates, such as Kepler-186f (which was closer in size to Earth), were typically spotted around much dimmer stars than our own.
This particular planet's combination of factors – including the type of star, the planet's distance from it and its size – make it the closest analogue to Earth ever found, the scientists said.
Rather than an exact twin, this distant planet is a little more like a bigger, older cousin – Kepler-452b's star system is about 6 billion years old, making it senior to our solar system's mere 4.6 billion years.
"That's considerable time and opportunity for life to arise somewhere on its surface or in its ocean, should all the necessary ingredients and conditions for life exist on this planet," said Jon Jenkins, the Kepler data analysis lead at NASA's Ames Research Center in Northern California who led the discovery of the new planet.
Whether those conditions exist or not is anyone's guess. Very little is known about this distant planet; researchers can't say precisely how much mass it holds. Other than the fact that the planet has a good chance of being rocky rather than gassy, they can't say exactly what it's made of and they don't know what's in its atmosphere.
But they do think it would have a much thicker atmosphere than Earth's and it would probably still have very active volcanoes, Jenkins said.
This planet would probably have five times the mass as Earth, and humans walking on this planet would experience twice the gravity they feel here, the researchers said.
"Gravity really sucks," Grunsfeld, a former astronaut, said, recalling returning to Earth after extended missions in space. "You just feel incredibly heavy."
Luckily, the human body is adaptable, the scientists added. Jenkins suggested that future generations of humans living on such a planet would become stockier over time.
Grunsfeld said change could be even more immediate.
"Even in real time, if we were there we'd get stronger, our bones would get stronger; it would be like a workout every day," he said.
Kepler-452b was announced along with the release of a fresh catalogue of 521 exoplanet candidates, culled from the data taken with the spacecraft between 2009 and 2013, before the spacecraft's second reaction wheel malfunctioned, ending its primary mission. (The spacecraft has since found new life as the K2 mission.) The new catalogue brings the total number of candidate planets detected by Kepler to 4,696, including a dozen candidates in their respective stars' habitable zones that have diameters that are between one and two times that of Earth's.
Kepler-452b is a confirmed planet, but this fresh catalogue of planets will require additional observations to be confirmed.
Of course, humans won't be able to visit Kepler-452b or any of the candidate planets (assuming they're confirmed) anytime soon, the scientists added.
"You and I probably won't be traveling to any of these planets without some unexpected breakthrough, but our children's children's children may," said Jeffrey Coughlin, a Kepler research scientist at the SETI Institute in Mountain View who led the work on the new catalogue. "So I think this really gives you something to aim for. Kepler is the first step. We're finding out if planets like Earth are common – and the answer seems, so far, to be yes."
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