Kepler-10c "The Mega Earth"

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Astrophysicists announced at the American Astronomical Society (AAS) meeting that they have found a new type of planet — a rocky world weighing 17 times as much as Earth. Theorists thought such a world couldn’t form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant. This planet, however, is all solids and much larger than before found “super-Earths,” making it a “mega-Earth.”

The fresh found mega-Earth, Kepler-10c, circles a Sun-like star once every 45 days. It is situated about 560 light-years from Earth in the constellation Draco. The system also hosts a 3-Earth-mass “lava world,” Kepler-10b, in a extraordinarily fast 20-hour orbit.


“We were very astonished when we understood what we had discovered,” said astrophysicist Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts. “This is the Godzilla of Earths! But unlike the movie monster, Kepler-10c has positive effects for life.” said Dimitar Sasselov from CfA.


NASA’s Kepler spacecraft initially discovered Kepler-10c. Kepler hunts planets using the transit method, looking for a star that dims when a planet passes in front of it. By calculating the amount of dimming, astrophysicists can measure the planet’s physical size or diameter. Though, Kepler can’t tell whether a planet is rocky or gassy.

Kepler-10c was recognised to have a diameter of about 18,000 miles (29,000 kilometers), 2.3 times as large as Earth. This proposed it fell into a category of planets known as mini-Neptunes, which have thick gaseous wrappers. The team used the HARPS-North tool on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to calculate the mass of Kepler-10c. They discovered that it weighed 17 times as much as Earth — far more than predicted. This presented that Kepler-10c must have a dense composition of rocks and other solids.

“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.” said Dumusque.

Planet formation theories have a hard time explaining how such a large rocky world could develop. Though, a new observational study proposes that it is not alone. Also presenting at AAS, Lars A. Buchhave from CfA discovered a correlation between the period of a planet — how long it takes to orbit its star — and the size at which a planet changes from rocky to gaseous. This proposes that more mega-Earths will be discovered as planet hunters extend their data to longer-period orbits.

The finding that Kepler-10c is a mega-Earth also has profound implications for the history of the universe and the likelihood 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 had only hydrogen and helium. Heavier elements required to make rocky planets, like silicon and iron, had to be formed in the first generations of stars. When those stars exploded, they dispersed these crucial ingredients through space, which then could be incorporated into later generations of stars and planets.

This process should have taken billions of years. Though, Kepler-10c shows that the cosmos was able to form such huge rocks even during the time when heavy elements were scarce.

“Discovering Kepler-10c tells us that rocky planets could form much earlier than we believed. And if you can make rocks, you can make life,” said Sasselov.

This study suggests that astrophysicists shouldn't rule out old stars when they search for Earth-like planets. And if old stars can host rocky Earths too, then we have a better chance of finding potentially habitable worlds in our cosmic neighborhood.

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