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Surprised scientists find a monster rocky world

Paul Sutherland, Feature writer
Jun 2, 2014, 18:59 UTC

Sen—Planet hunters using the Kepler space telescope have discovered a monster rocky world that came as a big surprise because they didn’t think it could exist.

The exoplanet, dubbed a “mega-Earth”, is around 17 times more massive than our own world and lies 560 light-years away. Its discovery was announced today at a meeting of the American Astronomical Society (AAS).

Though the Galaxy has been found to be swarming with gaseous planets known as “hot Jupiters”, it was thought that no rocky body could exist at such a size because it would instead collect hydrogen gas and become like a gasball itself.

No “super-Earth” previously discovered comes anywhere near the size of the new find, which is labelled Kepler-10c, after the NASA space telescope that revealed its existence in the constellation of Draco, the Dragon.

It orbits its home star once every 45 days in a solar system that also includes a “lava world”, Kepler-10b, that is three times the Earth’s mass and zips around the star in just 20 hours.

Incidentally, this planetary system’s location may come as a surprise because it is often said that the Kepler telescope was aimed at a field of sky covering the constellations Cygnus and Lyra. In fact a small region of Draco also falls within the field of view.


An artist’s impression of Kepler operating in space, and indicating the field of view that it was watching for its primary mission. Image credit: NASA/JPL

After Kepler-10c was spotted by Kepler as it passed in transit in front of its star, astronomers were able to use the data to get an idea of its size. But they could not tell at that stage what it was made of.

It appeared to have a diameter of about 29,000 km (18,000 miles) making it 2.3 times as large as Earth, and suggesting that it would fall into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.

Instead, when they examined the exoplanet more closely, using the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands, they found it “weighed” 17 times as much as Earth, which is much more than expected. This told the scentists that Kepler-10c must be densely composed of rocks and other solids.

Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the data analysis and made the discovery, said: “We were very surprised when we realized what we had found.”

He added: “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.”

Current theories of planet formation theories make it difficult to explain how such a large, rocky world could develop. But CfA astronomer Lars A. Buchhave told the AAS meeting he had found a correlation between how long it takes to orbit its star and the size at which a planet switches from being rocky to gaseous. This suggests that more mega-Earths will be found as planet hunters begin to study exoplanets with longer-period orbits.

The discovery team say that Kepler-10c’s status as 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.

The early universe contained only hydrogen and helium. Heavier elements needed to make rocky planets, like silicon and iron, had to be created in the first generations of stars. The discovery of Kepler-10c shows that exploding stars spread these planet-forming materials much earlier than had been imagined.

CfA researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative, said of the new planet: “This is the Godzilla of Earths! But unlike the movie monster, Kepler-10c has positive implications for life. 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.”

This research suggests that astronomers should not rule out old stars when they search for Earth-like planets.

Kepler’s primary mission ended a year ago after two of the wheels failed that allow it to point accurately in space, but a new technique is being used to allow it to continue to observe the sky.