Worlds such as this were not thought possible to exist. The enormous gravitational force of such a massive body would accrete a gas envelope during formation, ballooning the planet to a gas giant the size of Neptune or even Jupiter. However, this planet is thought to be solid, composed primarily of rock.

A newly discovered type of planet called mega-Earth, announced this week, challenges the notion that planets so large can't support life.

Located about 560 light-years from Earth, the rocky planet of Kepler-10c is more than double the size of Earth and 17 times heavier.

"Here you have a planet which should belong to the group of the giants, like Neptune and Uranus, but instead it is a solid planet," said Dimitar Sasselov, director of Harvard Origins of Life Initiative. Sasselov was part of the team with Harvard-Smithsonian Center for Astrophysics that determined Kepler-10c was a solid mass.

Sasselov said it's unlikely Kepler-10c, first spotted by NASA's Kepler telescope, is alone. The Harvard team is now searching for other mega-Earths.

Scientists have already known of a category of planets called super-Earths, which are typically 50% larger than Earth and up to 10 times more dense.

Mega-Earth raises questions of how planets are formed. Planets of this size were previously thought to be so big that the gravitational force would "accrete a gas envelope" during the planet's formation, instead of becoming a solid, according to NASA.

The discovery of this planet also suggests that scientists should not rule out Earth-like planets around older stars. Kepler-10c's star is twice as old as our solar system, Sasselov said.

Kepler-10c is composed of the same type of materials that Earth is made up of, but much more tightly packed together. The planet also has about 10 times more water than Earth has, Sasselov said.

Despite its similarities to Earth, Kepler-10c has an orbit of only 45 days — too close to its sun and therefore too hot to be habitable for humans.

"We're still searching for the perfect Earth twin," Sasselov said.

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