It's been a little more than a decade since astronomers confirmed the existence of the Kuiper Belt — the ancient disk of planetesimals circling the Sun outside the orbit of Neptune. In that time astronomers have found more than 700 icy, asteroid-size objects in the classical Kuiper Belt and have learned that it has a sharp outer edge around 50 astronomical units from the Sun. There seem to be no objects larger than 200 kilometers in diameter beyond that boundary.

But it remains unclear how the Kuiper Belt came to be. A new study published in the November 27th Nature by Harold F. Levison (Southwest Research Institute, Boulder) and Alessandro Morbidelli (Observatoire de la Côte d'Azur) sheds some light on the mystery. Their work suggests that Kuiper Belt objects formed inside the present orbit of Neptune and that the planet itself gradually pushed them outward.

The biggest issue surrounding Kuiper Belt formation is the "missing mass problem." For the objects to form in their current locations like normal planets do, they would have needed an environment 100 times denser than exists there today — totaling about 10 Earth masses of material instead of the 0.1 Earth mass observed. Scientists have modeled various solutions to get rid of 99 percent of the original disk, but each fix requires that Neptune winds up well outside its current location.

Levison and Morbidelli instead propose that the primordial solar system stretched from the Sun to about 30 a.u. and that Neptune originally formed at 20 a.u. Over time, as objects came close to the gas giant, they were ejected. Some were thrown outward into wider orbits and others were tossed inward toward the Sun to encounter Uranus, Saturn, or Jupiter.

In each interaction Neptune exchanged orbital energy with the ejected body. Sometimes it lost energy, which drifted the planet in toward the Sun; sometimes it stole energy and widened its orbit a skosh as a result. But on average Neptune gained more energy than it lost, so eventually the planet crept through the planetesimals, dust, and gas between it and the edge of the disk. It stopped at 30 a.u. when there weren't any more planetesimals to steal energy from. It has remained there ever since.

Thus the Kuiper Belt we see beyond Neptune today represents the last fragmentary evidence of Neptune's journey. Objects thrown inward eventually collided with Uranus, Saturn, or Jupiter or were flung out of the solar system by them. Objects sent far outward were lost to space forever. Only bodies lucky enough to land in or migrate between one of the many stable orbital resonances with Neptune survived. Indeed, Levison notes, the current edge of the Kuiper Belt "appears to be at exactly the same location as Neptune's 1:2 resonance."