Astronomers have discovered the heaviest planet yet that's predominantly rocky, a hefty body 17 times more massive than Earth. Called Kepler-10c, the planet orbits a star that is similar to the sun, though nearly twice as old, and located about 560 light-years away in the constellation Draco.
The exoplanet, which has been dubbed a "mega-Earth," could be the first of a new class of massive rocky planets found at more distant orbits from their stars, said the astronomers who announced their discovery this week at the American Astronomical Society meeting in Boston.
Kepler-10c weighs as much as Neptune. But while Neptune has a radius about 3.9 times wider than Earth's, Kepler-10c has a radius only 2.3 times bigger. For a planet to be so compact and heavy, it must be primarily made of rock, the scientists reason.
Astronomers assume that rocky planets are necessary for habitability, since any life would likely need to have evolved near a solid surface. The discovery of a massive rocky planet like Kepler-10c "increases the number of planets out there which could be potentially habitable," explained one member of the research team, Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Almost All Rock
Finding a rocky planet of such a massive size was not the biggest surprise, though, said Xavier Dumusque of Harvard-Smithsonian, who led the research. "The surprise is that there is no gas around it."
Planets are born from the disk of gas and dust that surrounds an embryonic star. A body as massive as Kepler-10c has so much gravity that it should have collected enough hydrogen and helium to turn into a giant gas planet like Jupiter.
"It's very difficult to put together a large solid planet like this without accreting even a small amount of hydrogen and helium, which is there in the disk," Sasselov explained.
The Kepler Space Telescope detected the planet in 2011, along with its companion, Kepler-10b, which was the first confirmed rocky planet found outside the solar system. Astronomers could use data from Kepler to measure the planets' radii, but they could only get a rough estimate of the planets' masses.
To better determine how heavy the planets are, Dumusque, Sasselov, and their colleagues used the Galileo National Telescope in the Canary Islands to measure how fast the planets were orbiting their star. From the speeds, the researchers deduced what the masses were and that the planets must be made of rock.
The Diversity of Planets
Kepler-10c is certainly interesting and appears to be an outlier for now, but it may not be that bizarre.
"Something on the order of the mass of Neptune and [that] is rocky with metallic material and perhaps a thin veneer of a hydrogen and helium atmosphere—that doesn't seem outside the realm of reasonable possibility," said Gregory Laughlin of the University of California, Santa Cruz, who was not involved in the finding.
There are a number of conceivable ways to create a planet like Kepler-10c, said Jack Lissauer of the NASA Ames Research Center in Moffett Field, California, who also wasn't a part of the team. The planet could have resulted from the collision of two smaller bodies that didn't have much gas to begin with. Or all of the gas in the embryonic star's disk may have somehow dissipated quickly, before the planet had time to accumulate any.
But calling Kepler-10c "rocky"—or even a "mega-Earth"—is a little misleading, since it's nothing like Earth, Lissauer noted. "I would call it a rock-rich world," he said. Although the planet is made nearly entirely of rock, there may be enough surrounding gas to create extreme pressures at the planet's surface, he explained.
"This is an important discovery," Lissauer said, because it "shows how diverse planets can be."