Astronomers using the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands have found that the extrasolar planet Kepler-10c, once thought to be a gas giant, is actually a ‘mega-Earth.’

Kepler-10c was originally spotted by NASA’s Kepler Space Telescope in May 2011.

The planet lies in the constellation Draco, about 564 light-years away, and circles its host star, Kepler-10, once every 45 days. The system also hosts another previously known exoplanet, Kepler-10b.

Kepler-10c was known to have a diameter of about 29,000 km, about 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes.

But Dr Xavier Dumusque from the Harvard-Smithsonian Center for Astrophysics and his collagues have found that Kepler-10c has a mass about 17 times that of Earth – far more than expected. This shows that the exoplanet must have a dense composition of rocks and other solids.

“We were very surprised when we realized what we had found,” said Dr Dumusque, who is the lead author of a paper accepted for publication in the Astrophysical Journal (arXiv.org version).

Co-author Dr Dimitar Sasselov from the Harvard-Smithsonian Center for Astrophysics added: “this is the Godzilla of Earths! But unlike the movie monster, Kepler-10c has positive implications for life.”

“Kepler-10c didn’t lose its atmosphere over time. It’s massive enough to have held onto one if it ever had it. It must have formed the way we see it now,” Dr Dumusque said.

The discovery that Kepler-10c is a mega-Earth also has profound implications for the history of the Universe and the possibility of life.

The Kepler-10 system is about 11 billion years old, which means it formed less than 3 billion years after the Big Bang.

“Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life,” Dr Sasselov concluded.

______

Xavier Dumusque et al. 2014. The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet. ApJ, accepted for publication; arXiv: 1405.7881