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Narrator: NASA's Kepler Mission has discovered

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the first system of multiple planets orbiting

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a pair of stars, or as astronomers call it,

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a circumbinary system.

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Located in the constellation of Cygnus,

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about 4,900 light-years from Earth,

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the discovery of Kepler-47 proves

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that more than one planet can form and persist

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while orbiting two stars.

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One star is similar to the sun in size

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but only 84 percent as bright.

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The second star is only one-third the size of the sun

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and less than one percent as bright.

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The inner planet, Kepler-47b, is the smallest known

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in orbit around two stars.

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The outer planet, Kepler-47c, is a gaseous giant,

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more than four times the size of Earth.

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Astrophysicists believe it might have an atmosphere

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blanketed with thick bright water clouds.

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It orbits its host stars every 303 days,

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placing it in the so-called "habitable zone."

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This is the region in a planetary system

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where liquid water could exist on the surface

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of an orbiting planet.

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While not a world thought to be hospitable for life,

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Kepler-47c is the first known circumbinary planet

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found in the habitable zone of its stars

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and it demonstrates the diversity

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of planetary systems in our galaxy.

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This discovery represents an important step

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for the Kepler mission in the effort to find Earth-size planets

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in the habitable zone of their host stars.