Why don't exoplanets match astronomers' expectations?

The most exhilarating science conference I've ever been to took place in San Antonio 15 years ago this week, when planet hunters Geoff Marcy and Paul Butler announced they had found two planets orbiting sunlike stars beyond our solar system. Coming a couple of months after another team, led by Michel Mayor and Didier Queloz, made the first such discovery, Marcy and Butler's presentation was a veritable Woodstock of astronomy, such was the crush of scientists trying to get into the room. I'll never forget the chill that went up my spine when they said one of the planets orbited within the habitable zone of its star, the range of distances where liquid water could be stable.

The excitement has scarcely dissipated since then. This week Marcy, now at the University of California, Berkeley, spoke again at the American Astronomical Society's winter meeting here to take stock. He proceeded to demolish the prevailing theoretical models for how planets form. Observers in any field of science take a peculiar pleasure in seeing their theorist colleagues collapse into sobbing heaps, but it happens with unnerving regularity with exoplanets. Modelers have consistently failed to predict the diversity of planetary systems out there. And they are the first to admit it. "These models are crap," says Hal Levison of the Southwest Research Institute in Boulder, Colo. "They may be the best we can do, but they are still crap."