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Scientists have been hunting for an Earth-like planet beyond our solar system. Now they've found one that's remarkably similar to ours. It's almost the same size as Earth and it orbits in the so-called Goldilocks zone. That's where temperatures are not too hot and not too cold, so it could be friendly enough for life.

Still, as NPR's Nell Greenfieldboyce reports, a lot about this planet remains a mystery.

NELL GREENFIELDBOYCE, BYLINE: Researchers detected this planet while poring over data collected by NASA's Kepler Space Telescope. The telescope spent years staring at 150,000 stars, watching for telltale dips in brightness, that might mean a planet was circling around a star. One small star in the constellation Cygnus showed signs of five planets. Four of them about the size of Earth, but tucked in close to the star. So they're probably too hot for life. Then there is the fifth planet.

ELISA QUINTANA: So this planet orbits its star every 130 days.

GREENFIELDBOYCE: Elisa Quintana is a scientist with the SETI Institute and NASA's Ames Research Center. She says the planet is called Kepler-186f. It's just 10 percent bigger than Earth. And at least in our solar system, Earth-sized planets are made of rock and iron and gas.

QUINTANA: So we can guess that Kepler-186f, being so close in size to Earth, it has a high probability of being rocky also, and composed of those sorts of materials.

GREENFIELDBOYCE: She says conditions on the surface would depend on what kind of atmosphere it had, if any. If it was like Earth, temperatures wouldn't be balmy.

QUINTANA: Being on this planet would probably be like being in San Francisco on a cool day. It would be a much colder place to live.

GREENFIELDBOYCE: It would be warm enough for one thing that's thought to be essential for life.

QUINTANA: If this planet had the right atmospheric conditions, and if there were water on the surface, it would be likely in liquid form.

GREENFIELDBOYCE: But if it has oceans they would look different.

TOM BARCLAY: It's not going to have a deep rich blue ocean, such as we have, because there's less blue light coming from the star. So the ocean would probably be a duller, grayer blue.

GREENFIELDBOYCE: Tom Barclay, of NASA's Ames Research Center, is also on the team that that describes the planet in the journal Science. He says because it orbits a dim, red dwarf star, midday on this planet wouldn't be bright. It would look like an hour before sunset on Earth.

BARCLAY: You know, it's very romantic to imagine there'd be places out there that look like Earth. And that's what we're trying to find, places that remind us of Earth.

GREENFIELDBOYCE: He says this is the first time anyone has found an Earth-sized planet in the habitable zone of a distant star.

BARCLAY: This is a really profound discovery. It's a major milestone.

DAVID CHARBONNEAU: This planet really is the same size as the Earth and the same temperature. Up until this point, planets satisfied one of those two, but we really didn't have one that was both those things together.

GREENFIELDBOYCE: David Charbonneau of Harvard University says both those things are key to life on Earth. But we'll probably never know if this new planet has life.

CHARBONNEAU: And the reason is that this star system is just too far away from us.

GREENFIELDBOYCE: Its 500 light-years away, too distant for follow-up work with other telescopes. Still, if scientists could find another planet like this around a nearby star...

CHARBONNEAU: We could really study the atmosphere and really figure out something about whether it truly is Earth-like, and maybe whether it actually has life on the surface.

GREENFIELDBOYCE: That's why Charbonneau and other scientists will keep searching for other Earth-like planets closer to home.

Nell Greenfieldboyce, NPR News.

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