Explaining the Kuiper Belt "Kernel"

As defined by Wikipedia, the Kuiper Belt is:

"a region of the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation"

But while we may know to some extent what it is, the more fundamental problem - yet to be resolved- has been how to explain it, i.e. its origin in terms of solar system formation models. Now, that impasse may well have come to an end,

A recent study by David Nesvorny, of the Southwest Research Institute - as reported in a recent Newsletter of the Dynamical Division of Astronomy, may not only account for how Neptune arrived at its present orbit, but also how its "migration" may have created the Kernel.

Let's first note that the orbits of objects composing the Kuiper Belt all have semimajor axes of 44 A.U. (where 1 A.U. = 149,000,000 km). These orbits also display low eccentricities (e) and low inclinations. The latter angles (i) denote the angle made by the object's orbit with respect to the plane of the solar system. The eccentricity of orbit defines how much the orbit diverges from circularity - with e = 0 defining a circular orbit, and e = 0.4 a highly elliptic one.

Nesvorny has proposed that the Kernel actually resulted from Neptune's outward migration through the solar system. In the model, Neptune began at a distance of 24 A.U. and migrated rapidly further out to 28 A.U.  As it traveled it swept up bodies in the outer disk in a 2:1 resonance that migrated along with Neptune - from an original distance of about 40 A.U. out to 44 A.U.  (The 2:1 resonance meant Neptune made two orbits for every one of the Kuiper objects.)  Such periodicities are of great interest in astronomy, and especially in explaining phenomena such as the "Kirkwood gaps" of the asteroid belt.

At the point of the shift of the Kuiper objects to 44 A.U. a close encounter with an unnamed fifth gas giant planet (since expelled from the solar system) caused Neptune's orbit to suddenly jump by 0. A.U.  Unable to keep up the 2:1 population were released from resonance and they remain today at the orbital distance of 44 A.U. , effectively becoming the Belt Kernel.

Nesvorny did previous simulations of the outer planets and these suggested that after the Neptune encounter, the fifth gas giant was ejected from the solar system by Jupiter.

We will have to follow this line of inquiry further to see if Nesvorny's findings are confirmed.