The Godzilla of Earths: A planet that shouldn't exist

Space · Published 21 October 2026 Swipe more like this →

In short: Astronomers expected a gas giant but found something far denser. Kepler-10c defies current planetary formation theories, existing as a massive rocky world that challenges our understanding of how planets grow and retain their composition in the cosmos.

For years, astronomers believed there was a physical limit to how large a rocky planet could become. The prevailing theory suggested that any celestial body hefty enough to reach a certain mass would inevitably grab hydrogen gas as it grew, transforming into a Jupiter-like gas giant. However, the discovery of Kepler-10c—a 'mega-Earth' located about 560 light-years away in the constellation Draco—upended this logic.

While NASA's Kepler spacecraft initially identified the planet via the transit method, it could only determine its diameter: approximately 18,000 miles (2.3 times the size of Earth). Based on its size alone, scientists assumed it was a 'mini-Neptune' with a thick gaseous envelope. It wasn't until researchers used the HARPS-North instrument in the Canary Islands to measure its mass that the surprise emerged: Kepler-10c weighs 17 times as much as Earth.

This extreme density confirms the planet is composed of rocks and solids rather than gas. Because it is so massive, it was capable of holding onto an atmosphere if it ever had one, yet it remains a solid world. This discovery forces a re-evaluation of planetary evolution, as researchers now look for correlations between orbital periods and the specific size thresholds where planets transition from rocky to gaseous.

The system hosting this mega-Earth also contains Kepler-10b, a 3-Earth-mass 'lava world' with a rapid 20-hour orbit. While these worlds are common in our galaxy, they remain unique because we have no analogs for them in our own solar system.

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