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.
The Astronomical Research Center A.R.C mentioned that 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 than 4000 getting trapped on Jupiter's orbital path as "Trojan" asteroids.
Now Wesley Fraser at the California Institute of Technology in Pasadena, and colleagues, say that the Trojans have a different size distribution to tilted Kuiper belt objects (Icarus, DOI: 10.1016/j.icarus.2010.08.001). That suggests that the Trojans have a different origin, but if so, the Nice model cannot say where they formed. All it can say is that they could not have formed where they are now, as they would have ended up being shunted elsewhere.
Fraser says he has "lost a lot of sleep" over the puzzle. Alessandro Morbidelli of the Côte d'Azur Observatory in France, who helped develop the Nice model, says he "cannot imagine any scenario that has a chance to explain this result". But he says there is still some disagreement over the sizes of tilted objects in the Kuiper belt, so it may be premature for "modellers to bang their head against the wall".