Despite being currently offline, the Kepler space telescope is still turning up surprises. One of them is an Earth-like planet that’s so large that astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) call it a “mega-Earth.” Revealed in a press conference at a meeting of the American Astronomical Society (AAS), planet Kepler-10c is 17 times heavier than the Earth, and may require scientists to rethink their ideas on planet formation and the likelihood of life in our galaxy.

Kepler-10c lies 560 light-years from Earth in the constellation Draco, where it orbits Kepler-10 with a year of 45 days. This makes it so close to its star that it’s too hot to sustain life. It shares the star system with Kepler-10b, which is a “lava world” three times the size of Earth and has a year only 20 hours long.

When first discovered by the Kepler space probe, Kepler-10c’s diameter was measured at 2.3 times that of Earth (about 18,000 mi / 29,000 km), which led scientist to think that it was a “mini-Neptune” with a structure similar to that of the gas giants of the Solar System. But the true "mega" nature of the planet has since been uncovered by a CfA team led by astronomer Xavier Dumusque. Using the Kepler data as a starting point, the team looked at Kepler-10c with the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands. While Kepler was only able to determine the size of Kepler-10c, the telescopic analysis by CfA combined with the Kepler data allowed the team to deduce the mass as well.

Astronomers now believe that Kepler-10c is 17 times heavier than the Earth, which, with its diameter, is significant. Despite their great size, gas giants are actually not very dense because of they’re made up of vast atmospheres of hydrogen and helium around a small rocky core. The planet Saturn, for example, is so light that it would float in water – if you could find a large enough bathtub. Kepler-10c, on the other hand, is solid, which is unheard of for a planet that size.

According to current theory, such a planet shouldn't exist. The CfA team says that any planet that size would have enough gravity to collect a massive atmosphere around it, turning it into a gas giant the size of Jupiter or larger. This one appears to be a giant ball of rock. If this so, then it raises questions about how planets form.

"Kepler-10c didn't lose its atmosphere over time," says Dumusque. "It's massive enough to have held onto one if it ever had it. It must have formed the way we see it now."

In addition, the Kepler-10 star system is 11 billion years old. This means it formed only 3 billion years after the Big Bang. The presence Kepler-10c with its rocky structure indicates that the heavy elements needed to make such a planet were available earlier than thought. If this is the case, then the CfA team says that there may be habitable planets that may be very old, which increases the chances of life being found elsewhere in the universe.

Source: NASA, CfA