Kuiper Gürtel

Plutinos

• 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).

Plutinos

• 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.

Plutinos

• Approximately 1/4 of the known 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 1400, to within a factor of a few, corresponding to a few % of the total. The number is uncertain for several reasons. First, the Plutinos are observationally over-assessed due to their being closer (brighter), on average, than the Classical KBOs giving rise to an observational bias in favor of the Plutinos. The intrinsic fraction is smaller than the actual fraction. Second, the initial orbits published by the IAU are little more than guesses, only weakly constrained by the limited orbital arcs. Pluto is distinguished from the Plutinos by its size: it is the largest object identified to date in the 3:2 resonance.