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 than 4000 getting trapped on Jupiter鈥檚 orbital path as 鈥淭rojan鈥� asteroids.
Now 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, ). 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.
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Fraser says he has 鈥渓ost a lot of sleep鈥� over the puzzle. of the C么te d鈥橝zur Observatory in France, who helped develop the Nice model, says he 鈥渃annot imagine any scenario that has a chance to explain this result鈥�. But he says there is still some , so it may be premature for 鈥渕odellers to bang their head against the wall鈥�.
