Astronomers have discovered the "Godzilla" of all Earths - a new type of rocky planet that weighs 17 times as much as our planet and is more than twice as large in size, a surprising find that could change scientists' understanding of the origins of the universe.
The newly found mega-Earth, Kepler-10c, circles a Sun-like star once every 45 days. It is located about 560 light-years from Earth in the constellation Draco.
Theorists believed such a world could not form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant.
This planet, though, is all solids and much bigger than previously discovered "super-Earths," making it a "mega-Earth."
"We were very surprised when we realised what we had found," said astronomer Xavier Dumusque of the Harvard-Smithsonian Centre for Astrophysics (CfA), who led the data analysis and made the discovery.
"This is the Godzilla of Earths!" added CfA researcher Dimitar Sasselov.
"But unlike the movie monster, Kepler-10c has positive implications for life," said Sasselov.
The system also hosts a 3-Earth-mass "lava world," Kepler-10b, in a remarkably fast, 20-hour orbit.
Kepler-10c was originally spotted by NASA's Kepler spacecraft.
Kepler-10c was known to have a diameter of about 18,000 miles, 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.
The team used the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to measure the mass of Kepler-10c.
They found that it weighed 17 times as much as Earth - far more than expected. This showed that Kepler-10c must have a dense composition of rocks and other solids.
"Kepler-10c did not lose its atmosphere over time. It's massive enough to have held onto one if it ever had it. It must have formed the way we see it now," said Dumusque.
Planet formation theories have a difficult time explaining how such a large, rocky world could develop. However, a new observational study suggests that it is not alone.
The CfA astronomer Lars A Buchhave found a correlation between the period of a planet (how long it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous.
This suggests that more mega-Earths will be found as planet hunters extend their data to longer-period orbits.
The discovery that Kepler-10c is a mega-Earth also has profound implications