A Potentially Habitable 'Godzilla of Earths' Is an Entirely New Class of Planet

By all accounts, the latest exoplanet discovery shouldn't exist. But, as astronomers are increasingly learning, space is a weird place, with something new lurking wherever we point our most advanced instruments. They've just discovered the "mega-Earth," an entirely new type of potentially habitable planet.

Kepler-10c has a diameter roughly 2.3 times that of our own planet. The existence of the planet itself is not a new finding—the discovery of the planet, made with the help of the Kepler Space Telescope, was first announced back in 2011. It's also part of a system that contains the first confirmed rocky exoplanet.

When it was first discovered, researchers didn't know what to make of it. However, a new analysis made with the help of a telescope located on Spain's Canary Islands suggests that it's an entirely new class of planet.

The super-heavy planet has a mass of about 17 times that of Earth, leading Harvard University's Dimitar Sasselov, one of the researchers who helped discover it, to call it the "Godzilla of Earths."

"Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods," the team wrote in a paper on the arXiv pre-print server, which means it has not yet been submitted for official publication. "Understanding the transition from rocky planets to planets defined by a hydrogen-dominated envelope [gas giants] is critical to planet formation theory. Kepler-10c stands out … in direct challenge to theory."

Sasselov and his colleague Xavier Dumusque announced the discovery today in a press conference at the American Astronomical Society.

Normally, a planet of Kepler-10c's size would be expected to collect and absorb hydrogen as it formed, creating a gas giant similar to Jupiter or Neptune. But Sasselov and Dumusque say that the planet's huge mass means it's almost entirely made of rock, heavy elements, and a "significant amount of water," according to the team's report.

The planet is located about 560 light-years from Earth, and has years that are about 45 days long. Despite being so heavy, the team says that the planet likely still has its atmosphere and is located in its star's habitable zone, leading them to suggest that it has "positive implications for life."

Its mass isn't the only unusual thing about Kepler-10c: The planet's star system (which contains a lava-world similar to several previously discovered) is extremely old, having formed less than 3 billion years after the Big Bang.

When the universe formed, it contained just hydrogen and helium, with the heavier elements necessary to make rocky planets coming later. Kepler-10c would thus be one of the oldest rocky planets ever discovered, and suggests that rocky planets could have formed earlier in the universe's history than previously expected.