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By now, it’s not surprising that NASA’s Kepler space telescope is turning up extrasolar planets by the bushel. Last week, at the first Kepler science conference at NASA’s Ames Research Center in Moffett Field, California, mission scientists announced that the space telescope has identified 2,326 candidate planets, nearly doubling its haul since February.

But what has puzzled observers and theorists so far is the high proportion of planets — roughly one-third to one-half — that are bigger than Earth but smaller than Neptune. These ‘super-Earths’ are emerging as a new category of planet — and they could be the most numerous of all (see ‘Super-Earths rising’). Their very existence upsets conventional models of planetary formation and, furthermore, most of them are in tight orbits around their host star, precisely where the modellers say they shouldn’t be.

“It poses a challenge,” says Douglas Lin, a planet-formation modeller and director of the Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing, China. “You can’t just tweak the parameters. You need to think about the physics.”

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