Astronomers have spotted the "Godzilla" of all Earths, a huge rocky planet orbiting a star 560 light years away that is changing scientists' understanding of the origins of the universe.

This new mega-Earth weighs 17 times as much as our planet, and was found by NASA's Kepler mission, experts said at a meeting in Boston of the American Astronomical Society.

The new discovery, named Kepler 10c, is 18,000 miles (29,000 kilometers) wide or 2.3 times the size of the Earth.

"We were very surprised when we realized what we had found," said astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics.

Until now, astronomers did not think a rocky planet could get so large, because its growing girth would make it collect more hydrogen gas and turn it into a gas-giant, like Jupiter.

"This is the Godzilla of Earths!" said researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative.

"But unlike the movie monster, Kepler-10c has positive implications for life."

The US space agency's planet-hunting Kepler mission can only spot planets and classify them based on the number of times they pass in front of their star. It cannot reveal much about whether they are rocky or gassy.

What lifted Kepler-10c out of the category of super-Earths and mini-Neptunes -- planets outside our solar system that are larger than Earth and smaller than dense, gassy Neptune -- was a special telescope in the Canary Islands which measured its mass.

"Kepler-10c didn't lose its atmosphere over time. It's massive enough to have held onto one if it ever had it," said Dumusque.

"It must have formed the way we see it now."

Since Kepler-10c is far denser than expected, that puts it in a new class, which more planets are likely to join once discovered, experts said.

Kepler-10c, circles its Sun-like star once every 45 days, suggesting it is likely too hot for life to survive.

It is located in a planetary system that is also home to Kepler 10b, a lava-world that spins around in a fast 20-hour orbit.

The Kepler-10 system is some 11 billion years old, and formed less than three billion years after the Big Bang, scientists said.

That means that somehow, the universe was able to form massive rocks even when heavy elements like silicon and iron were scarce. The early universe was made of only hydrogen and helium.

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