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    The First Earth-Sized Rocky Exoplanet Has Been Found

    Hello, Kepler-78b.

    This is Kepler-78b. It's a planet that orbits a star 400 light years from Earth.

    David A. Aguilar (CfA)

    It's 80% more massive than Earth with a radius 20% bigger. So its density is pretty much the same as our planet's, meaning it probably has a similar composition. The astronomers who found Kepler-78b say it's made mostly of rock and iron.

    It's the first rocky, Earth-sized exoplanet astronomers have found.

    David A. Aguilar (CfA)

    A team of astronomers announced they'd discovered the planet in August this year. Now they've been able to measure its size and density too, confirming it as the most Earth-like exoplanet yet.

    A year on Kepler-78b lasts just 8.5 hours because it's so close to its star. So it's really quite hot on the surface – between 1500 and 3000C.

    David A. Aguilar (CfA)

    In that respect it's not much like Earth at all, and there is no way we'd be able to live there.

    The weird thing about Kepler-78b is that earlier on in its life, the star would have been big enough to engulf the planet where it is now. But if it had formed further out and migrated in, it should have fallen into the star.

    "This planet is an enigma," says Dimitar Sasselov from Harvard-Smithsonian Center for Astrophysics.

    The result is announced in two papers published in the journal Nature today.

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