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'Mega-Earth' planet discovered orbiting distant Sun-like star

Artist’s conception of Kepler-10c (foreground) and Kepler-10b.
Artist’s conception of Kepler-10c (foreground) and Kepler-10b.
Harvard-Smithsonian Center for Astrophysics/David Aguilar

Astronomers on Monday, June 2 made a “big” announcement about exoplanets, and it is big – literally. Another new world has been discovered, which is quite routine now these days, but this one is different, and unexpected; a planet which is more than twice as large as Earth and about 17 times heavier, a sort of “mega-Earth” as some have referred to it. Nothing else like it has been seen before, until now. The new discovery was announced at a press conference during a meeting of the American Astronomical Society (AAS).

Why is this so significant?

The planet, Kepler-10c, orbits a sun-like star about 560 light-years away in the constellation Draco. It presents a conundrum for astronomers because it is a type of planet that was thought to be impossible to exist – any planets of that size and mass were presumed to always become a gas or ice giant like Uranus or Neptune, or even larger like Jupiter or Saturn. This world, however, appears to defy that expectation, and has remained a massive rocky planet, kind of like an Earth on steroids.

As astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA) notes, “We were very surprised when we realized what we had found.” Dumusque led the analysis which used data from the Kepler space telescope to make the discovery.

The new findings were made using the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands. It was previously known that Kepler-10c had a diameter about 2.3 times that of Earth, which wasn’t too unusual in itself, but it’s mass was still a question. Now that question has been answered, but the result is still more questions as to how this planet formed.

As for habitability, Kepler-10c is not a likely candidate, since it orbits very close to its star, in only 45 days. Much too hot, at least for life as we know it. But it’s existence also means that other ones like it are probably out there and that may be good news for finding life elsewhere:

“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,” said Dimitar Sasselov, another CfA researcher and director of the Harvard Origins of Life Initiative.

“This is the Godzilla of Earths!” he added. “But unlike the movie monster, Kepler-10c has positive implications for life.”

There is also at least one other planet known to orbit this star, a smaller rocky world about 3 times Earth’s mass called Kepler-10b, which is even closer to the star, completing an orbit in only 20 hours. This one is so hot it’s referred to as a “lava world.”

Rocky planets similar to Earth in terms of size and mass are being found more frequently now, but they have been within expectations of just how large and massive planets like that could be. Kepler-10c has thrown a wrench into those expectations, which is exciting because it means discovering something new that was thought to be unlikely, if not impossible. One type of planet being discovered, but not found in our solar system, is the “super-Earth” which is a rocky world a bit larger than Earth, but smaller than Uranus or Neptune. Those worlds are thought to be usually rocky, with some having potentially global oceans on their surfaces. Kepler-10c, however, is like a step up from a super-Earth, becoming a “mega-Earth.”

Natalie Batalha, Kepler mission scientist at NASA’s Ames Research Center in Moffett Field, California sums it up nicely:

“Just when you think you’ve got it all figured out, nature gives you a huge surprise – in this case, literally.” She added, “Isn’t science marvellous?”

Yes, yes it is.

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