Kuiper Gürtel

Plutinos

• How did the 3:2 resonance come to be so full? An exciting idea has been explored by Renu Malhotra. Building on earlier work by Julio Fernandez, she supposes that, as a result of angular momentum exchange with planetesimals in the accretional stage of the solar system, the planets underwent radial migration with respect to the sun. Uranus and Neptune, in particular, ejected many comets towards the Oort Cloud, and as a result the sizes of their orbits changed. As Neptune moved outwards, its mean motion resonances were pushed through the surrounding planetesimal disk. They swept up objects in much the same way that a snow plough sweeps up snow. Malhotra has examined this process numerically, and finds that objects can indeed be trapped in resonances as Neptune moves, and that their eccentricities and inclinations are pumped during the process.

Plutinos

• This scenario has the merit of being a natural consequence of angular momentum exchange with the planetesimals: there is really no doubt that angular momentum exchange took place. However, some researchers are unsure whether Neptune moved out as opposed to in, and question the distance this planet might have moved. They also assert that the inclination of Pluto is larger than typical of the objects in Malhotra's simulations (and notice that the inclination of 1995 QZ9 is still larger than that of Pluto).

Plutinos

• The dynamical situation is presently unclear, but the "moving planets" hypothesis appears as good as any, and better than most.
• A plot of the semi-major axes of the KBOs versus their orbital eccentricities clearly shows a non-random distribution. The Plutinos lie in a band at 39 AU, while most of the other KBOs are further from the sun. Solid blue points in this plot mark KBOs observed on 2 or more years. Their orbits are thought to be reasonably well determined. Unfilled circles mark KBOs observed only in one year. In some cases, these objects were recently discovered and we expect that they will be re-observed next year. In other cases, the KBOs have been lost. The upper diagonal line in the figure separates objects with perihelion inside Neptune's orbit (above the line) from the others. Note that Pluto (marked with an X) falls above the line. The lower diagonal line shows where objects have perihelion at 35 AU (i.e. 5 AU from Neptune's orbit). Note also that 1996 TL66 and the other scattered KBOs are so far off scale that we have not included them in this plot. This plot is updated from a paper describing our 8k CCD observations of the Kuiper Belt (Jewitt, Luu & Trujillo, 1998).