A surprising result of the new observational work is that many of the distant objects are in or near the 3:2 mean motion resonance with Neptune. This means that they complete 2 orbits around the sun in the time it takes Neptune to complete 3 orbits. The same resonance is also occupied by Pluto. To mark the dynamical similarity with Pluto, we have christened these objects as "Plutinos" (little Plutos).

Probably, the 3:2 resonance acts to stabilize the Plutinos against gravitational perturbations by Neptune. Resonant objects in elliptical orbits can approach the orbit of Neptune without ever coming close to the planet itself, because their perihelia (smallest distance from the sun) preferentially avoid Neptune. In fact, it is well known that Pluto's orbit crosses inside that of Neptune, but close encounters are always avoided. This property is also shared by a number of the known Plutinos (e.g. 1993 SB, 1994 TB, 1995 QY9), further enhancing the dynamical similarity with Pluto.

Approximately 40% (12/32) of the trans-Neptunian objects are Plutinos. A few more are suspected residents of other resonances (e.g. 1995 DA2 is probably in the 4:3). By extrapolating from the limited area of the sky so far examined, we have estimated that the number of Plutinos larger than 100 km diameter is of order 10,000. Pluto is distinguished from the Plutinos by its size: it is the largest object identified to date in the 3:2 resonance.

How did the 3:2 resonance come to be so full? An exciting idea has been explored by Renu Malhotra. 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 a great 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.

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).

The dynamical situation is presently unclear, but the "moving planets" hypothesis appears as good as any, and better than most.

References to Malhotra's papers may be found on the Kuiper Belt Bibliography included on the Kuiper Belt Home Page.

David Jewitt, 1996 March 10.