A FAMILY of asteroids that travels in lockstep with Jupiter appears to be different in one important respect from their purported kin in the outer solar system. The mismatch could spell trouble for the leading theory of how our solar system evolved.

This theory, called the Nice model, suggests that as Jupiter and Saturn moved to their current orbits, they wreaked gravitational havoc in the early solar system, scattering lumps of rock in their vicinity. Some of these ended up on tilted orbits in the distant Kuiper belt, beyond the orbit of Neptune. Others were hurled inwards, with more …