First mega-Earth found

The giant exoplanet, Kepler-10c, doesn’t play by the rules. It has as much mass as Neptune — yet it’s made of rock, just like Earth. Astronomers are now calling it a “mega-Earth.” Our solar system’s massive planets, such as Jupiter and Saturn, are made from gas. And scientists used to think any planet that massive must also be made primarily of helium and hydrogen. But Kepler-10c is now forcing experts to throw such assumptions out the window.

Kepler-10c is one of two planets orbiting a sunlike star 564 light-years away. It sits in the constellation Draco. Although as massive as Neptune, this heavyweight is only 2.5 times as wide as Earth. (Neptune’s diameter, for comparison, is 22.4 times that of Earth’s.) So Kepler-10c is dense and its gravity exceptionally strong — about three times stronger than Earth’s, explains David Latham. He’s an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Latham shared his team’s findings June 2 at an astronomy meeting in Boston.

All that mass would give the exoplanet extreme gravity, the researchers note. Kepler-10c also orbits close to its star — so close that it completes an orbit in only 45 days. This proximity to its sun means the planet’s climate also should be brutally hot. (A companion planet, called Kepler-10b, orbits even closer and faster. Earth, by comparison, takes a little more than 365 days to complete one orbit of the sun.)

Kepler-10c was one of the first exoplanets found by the Kepler space telescope. The robotic space mission has been searching for planets beyond our solar system since 2009. Astronomers measured the size of Kepler-10c three years ago. But until now, they didn’t realize its heft.

To measure the mass of an exoplanet, astronomers focus on its sun. A star’s gravity keeps a planet moving in orbit, like Earth moves around our sun. But a planet’s gravity also tugs on the star — and causes the star to wobble. The more mass a planet has, and the closer it is to the star, the more it makes that star wobble. By measuring this stellar wobble, scientists can estimate a planet’s mass. That’s what Latham and his colleagues did with Kepler-10c.

Once they had measured its mass and size, the scientists determined the planet’s density. Density is calculated by dividing mass by volume. Rocky planets are dense: They pack a lot of stuff into a small space. Gas giants aren’t: They are large and fluffy. Latham and his team have now determined that Kepler-10c has the density of rock.

The planet weighs 17 times as much as Earth. Astronomers used to think that planets with at least 10 times the mass of Earth had to be gas giants. As Kepler-10c now shows, that’s not always true. Astronomers have no idea how the rule-breaking planet formed.

Even though Kepler-10c is the only mega-Earth known, it’s probably not the only one out there.

“When one type of planet is found, that’s usually the tip of the iceberg,” says Sara Seager. She’s an exoplanet hunter at the Massachusetts Institute of Technology, in Cambridge, who did not work on the new study. “There are probably many, many more of them,” she says of mega-Earths.