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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.”
