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Before they started finding exoplanets back in the 1990s, astronomers thought alien worlds would be pretty much like the planets you see in our own solar system. But from the moment the discoveries began rolling in, it was clear how wrong they were. The first of the new planets, 51 Pegasi b, discovered in 1995, was a massive, gaseous world like Jupiter — but unlike the real Jupiter, it circles its star so closely that it’s searing hot. More recently, astronomers have been finding “mini-Neptunes” — planets not much bigger than Earth, with a rocky core, but with a much higher proportion of water than we have.

Now comes yet another surprise. Speaking at a press conference in Boston, Harvard astronomers have announced the discovery of a world they call a “mega-Earth.” Known as Kepler 10c, it’s about 18,000 miles (29,000 km) across, compared with about 8,000 (13,000 km) for our planet, and weighs about 17 times as much as Earth does. It is, says Harvard astrophysicist Dimitar Sasselov, “the Godzilla of Earths.”

It’s a bit astonishing that the planet exists at all, says Xavier Dumusque, who made the discovery. “Our first thought,” he says, “was that we couldn’t really believe it.” The reason is that when a chunk of mostly rock and iron reaches about 10 times the mass of Earth, its powerful gravity begins vacuuming up all of the gases in the neighborhood — and at the time planets are forming, you’d expect there to be lots of gas to vacuum. Close to home, Jupiter, Saturn, Uranus and Neptune all have rocky centers bigger than Earth, but most of their mass comes from the hydrogen, helium, H20 and other gases they sucked in pretty much as soon as they were born.

Somehow, Kepler 10c failed to manage this trick, and it isn’t at all clear why. “We really don’t have any good ideas,” says Sasselov. Jack Lissauer, a planet-formation theorist at the NASA Ames Research Center, who wasn’t involved with the new study, suggests that maybe all the gas was somehow gone by the time 10c reached its Godzilla-esque size, or maybe it formed from the collision of two smaller rocky planets, neither of which was big enough to hold on to its gas — but he doesn’t think either theory is especially compelling.

The mystery gets even thicker when you factor in the parent star’s age, which is about 11 billion years. That means the star — Kepler 10, about 560 light-years from Earth, in the constellation Draco — formed only 3 billion or so years after the Big Bang. At that time, iron and silicon, the latter a major component of rock, were in far lower abundance than they were just 4.6 billion years ago, when the sun was born. Yet Kepler 10c is full of the stuff.

The planet’s general Earthiness does not mean it could sustain life. For one thing, with an orbit of just 45 days, it’s too close to its star for water to remain liquid on its surface. It’s also not quite the same as Earth in composition: about 1% of our planet’s mass is made up of volatile substances like oxygen and water. Kepler 10c has more like 10%. “It’s definitely not an Earth twin,” says Lissauer, of the Ames Research Center, home base of the Kepler planet-hunting spacecraft.

Under the crushing pressure of gravity generated by all of Kepler 10c’s mass, most of whatever water it has may be bound up in minerals, or squeezed into solid form despite the high temperatures. “I call it a solid planet,” says Sasselov, “rather than a rocky planet.

Kepler 10c is not alone out there; it has a little sibling named, appropriately enough, Kepler 10b. This one, discovered in 2011, is also predominantly rocky, but it’s only three times Earth’s mass. It’s also much closer to the star, however, and therefore even hotter. If not for the searing temperatures that give it the nickname “lava planet,” a world like 10b might be a plausible place to look for living organisms. And the fact that this star system has two solid planets means it wouldn’t be crazy to think there could be a third planet that’s a true Mirror Earth — although there’s no evidence yet that such a world actually exists.

“Five years ago,” says Sasselov, “I was arguing that we shouldn’t bother looking for solid planets around such old stars.” After this week, he says, “I’ve changed my mind.”

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