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Narrator: NASA's Kepler Mission has discovered the

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first Earth-size planet orbiting in the habitable

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zone of a star outside of our solar system.

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The newly discovered planet is called Kepler-186f

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and is about 10 percent larger than Earth.

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Elisa Quintana: Kepler-186f is the first validated

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Earth-size planet in the habitable zone of its

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star.  It's the outmost of five planets to orbit a

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star that is smaller and cooler than the sun.

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This planet orbits its star every 130 days and so

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this places it in the habitable zone, where it's

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in a region where it could have liquid water

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on its surface.

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Narrator: Kepler-186f resides in the Kepler-186

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system, about 500 light-years from Earth in the

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constellation Cygnus.

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Thomas Barclay: This planet, Kepler-186f, orbits

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a star that is cooler and dimmer than the sun.

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So while we may have found a planet that is

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the same size as Earth and receives a similar

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amount of energy as to what Earth receives,

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it orbits a very different star.

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So, perhaps instead of an Earth twin, we've

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discovered an Earth cousin.

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Narrator: On the surface of Kepler-186f, the

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brightness of its star at high noon is only as

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bright as our sun appears to us about an hour

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before sunset.

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Scientists believe Kepler-186f is likely to be a

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rocky world, but are unable to confirm its mass

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and density.

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Thomas Barclay: This is one of the big milestones

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that we've been looking for in our attempts to

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find out if there are places just like home and if

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there's life out there.  One of the big steps is

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to say "Is there somewhere that looks, to all

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intents and purposes, like Earth?"  Well, we don't

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know just yet, but we know that there are at least

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places that look similar.

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Narrator: Managed by NASA's Ames Research Center,

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the Kepler mission collected this data using a

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space-based telescope to search one part of the

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galaxy for potentially habitable planets.