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Two new exoplanets are more Earth-like than any we’ve found before

This artist's conception depicts an Earth-like planet orbiting an evolved star that has formed a stunning "planetary nebula." Earlier in its life, this planet may have been like one of the eight newly discovered worlds orbiting in the habitable zones of their stars. (David A. Aguilar)
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NASA's Kepler Space Telescope has found a lot of planets outside our solar system, but now it may have tracked down a pair that are truly Earth-like.

Scientists recently verified Kepler's 1,000th planetary discovery, and there are still more to sort through. At a meeting of the American Astronomical Society on Tuesday, researchers announced 554 new candidates found by the telescope, bringing its total to 4,175.

Of those possible planets, eight are in the so-called "Goldilocks zone," or habitable zone, where the host star is at just the right distance to keep water liquid. Too close, and water heats up and boils off. Too far, and the planet is covered in permanent, solid ice.

These findings nearly double the number of known planets in the habitable zone, but researchers are especially excited about two of the new exoplanets: Their size, location, and star type means they could be rocky planets like Earth -- which means they could have evolved life as we recognize it.

One of the planets, Kepler-438b, is only 12 percent bigger than Earth in diameter. That means it's quite likely a rocky planet. Scientists have given it a 70 percent chance. Kepler-442b is a bit bigger at around 33 percent larger than Earth, but still has a 60 percent chance of being rocky.

But while 438b hits the sweet spot in size, 442b has it beat when it comes to distance from the sun. Both planets orbit a small red dwarf star, cooler than Earth's Sun, but they also orbit more closely. 438b gets 40 percent more light than Earth, which means it has around a 70 percent chance of being able to hold liquid water. But with 66 percent as much light as our own planet, 442b has a 97 percent chance of being in the habitable zone.

So neither planet is a sure shot -- and both still need to be studied further -- but these are exciting finds. And they're more promising than anything else Kepler has ever found.

'We are now closer than we’ve ever been to finding a twin for the Earth around another star," NASA scientist Fergal Mullally said while introducing his portion of the research at the conference  Tuesday. He and his colleagues hope to verify these planets for further study, then use their similarities to -- and differences from -- Earth to learn something about the formation of the planet.