Rocky, overweight planet shakes up theories

BOSTON — Common wisdom in astronomy says that once a planet has collected about 10 Earths’ worth of rock, it becomes a gas giant like Neptune or Saturn. The exoplanet Kepler-10c didn’t get that memo. With 17 times the mass of Earth, the distant planet is the heaviest rocky planet known. And astronomers have no idea how it formed.

With the mass of Neptune squeezed into a ball only about 2.5 times as wide as our planet, gravity on Kepler-10c is three times stronger than Earth’s, astronomer David Latham reported June 2 at the American Astronomical Society meeting. “I wouldn’t want to be a giraffe on this planet,” said Latham, of the Harvard-Smithsonian Center for Astrophysics.

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