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Sly Stallone has nothing on this rocky heavyweight. Twice the size of Earth and with 17 times our planet's mass, Kepler-10c is so unusual that it has been placed in a brand new class of exoplanet.

Kepler-10c was discovered in 2011 by NASA's Kepler space telescope. The planet orbits a star that is about 560 light years away from us. It has a radius slightly more than double that of Earth's – a size that led astronomers to assume it was a shrunken version of gassy planet Neptune, which is four times larger than Earth.

Now Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues have used the HARPS-N telescope in the Canary Islands to pin down Kepler-10c's mass. They found it is actually 17 times as heavy as Earth: more or less the same mass as Neptune. But because Kepler-10c is much smaller then Neptune, it must be an incredibly dense, rocky world, the like of which has never been seen before.

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