Posted: Tue, Jun 3, 2014, 6:32 AM ET (1032 GMT)
Astronomers said Monday that they have discovered an unexpected class of extrasolar planet about the mass of Neptune but rocky like the Earth. The planet, Kepler-10c, has a diameter of 2.3 times that of the Earth but weighs 17 times the Earth, giving it a density consistent with rocks and not gas and ice. That planet is considerably larger than "super-Earth" exoplanets that are also rocky but weigh only a few times that of the Sun, while its mass is more consistent with Neptune-sized worlds of gas and ice. Astronomers detected the planet in data from NASA's Kepler spacecraft and determined its radius; radial velocity observations using a sensitive new instrument at an observatory in the Canary Islands allowed astronomers to determine its mass and thus density. Scientists said current models of planetary formation don't predict the formation of rocky planets as large as Kepler-10c. In addition, the planet is orbiting a very old star that was previously thought not to have enough rocky material to form large planets.
