Astronomers have determined that the planet Kepler-10c, discovered by NASA’s Kepler Space Telescope back in 2011 and thought to be a gas giant, is actually solid, making it an unprecedented "mega-Earth" find.

The planet orbits around the star Kepler-10, in the constellation Draco, which is roughly 560 light years away from Earth. Kepler-10c is so big, more than twice the size of the Earth, that researchers initially described it as a smaller Neptune, which is the densest gaseous planet in our solar system. But, upon further observations, scientists learned that Kepler-10c is a different type of planet, similar to our own. New Scientist explains

Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues have used the HARPS-N telescope in the Canary Islands to pin down Kepler-10c's mass. They found it is actually 17 times as heavy as Earth: more or less the same mass as Neptune. But because Kepler-10c is much smaller than Neptune, it must be an incredibly dense, rocky world, the like of which has never been seen before.

Dumusque presented his findings at a meeting of the American Astronomical Society in Boston this week. "All of the major existing planetary formation models were not predicting this type of planet, and it is why we could not believe our result at the beginning," he said. According to earlier theories, such a dense planet would collapse on itself before reaching the size attained by Kepler 10-c. 

According to Dumusque, the planet is likely made from the same materials that make up Earth, but in different proportions. Fellow researcher Dimitar Sasselov added that "Kepler-10c has positive implications for life." This is, in part, because of the implications the find has for our understanding of planet formation, per the press release announcing the discovery

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. When those stars exploded, they scattered 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. However, Kepler-10c shows that the universe was able to form such huge rocks even during the time when heavy elements were scarce.

Sasselov adds, "finding Kepler 10-c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life." 

The planet itself, however, is likely too hot to be habitable. Sydney University's Dennis Stello told the Sydney Morning Herald that "in theory it could hold life if it wasn't as hot as it is,’’ adding that this finding would prompt researchers to reevaluate previous assumptions. "We probably need to look at more data to see if this sort of planet is common or whether its a special event. If it turns out to be common that we can have these very huge solid planets then certainly our theories need to be revised," he said.