The Plutinos

a: semimajor axis; e: eccentricity; i: inclination.
q: perihelion distance; Q: aphelion distance.
Object a [AU] e i [deg] q [AU] Q [AU]
1996 TP66 39.71 0.34 5.7 26.38 53.05
1993 SZ4 39.82 0.26 4.7 29.57 50.07
1996 RR20 40.05 0.19 5.3 32.55 47.55
1993 SB 39.55 0.32 1.9 26.91 52.18
1993 SC 39.88 0.19 5.2 32.24 47.52
1993 RO 39.61 0.20 3.7 31.48 47.73
1993 RP 39.33 0.11 2.8 35.00 43.66
1994 JR1 39.43 0.12 3.8 34.76 44.11
1994 TB 39.84 0.32 12.1 27.05 52.63
1995 HM5 39.37 0.25 4.8 29.48 49.26
1997 QJ4 39.65 0.22 16.5 30.83 48.47
1995 KK1 39.48 0.19 9.3 38.67 46.98
1995 QZ9 39.77 0.15 19.5 33.70 45.85
1995 YY3 39.39 0.22 0.4 30.70 48.08
1996 TQ66 39.65 0.13 14.6 34.59 44.71
Pluto 39.61 0.25 17.17 29.58 49.30

The Plutinos are a specific type of Resonant KBO. They sit in the 3:2 mean-motion resonance with Neptune as does Pluto. (Orbital properties of a few Plutinos are listed in the Table to the left).

The name is simply a humorous attempt to indelibly link these objects together as a group pdf reference. I think we succeeded.

Approximately 1/4 of the known trans-Neptunian objects are Plutinos. Others are residents of different resonances (e.g. 1995 DA2 is probably in the 4:3). Objects in the 2:1 MMR resonance are sometimes dubbed Two-tinos. 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.

How did the 3:2 resonance come to be so full? The accepted 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 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. The issues concern how rapidly the migration occurred, and how jumpy it was (jumpy migration occurs because the masses of scattered objects are discrete; each one imparts a "kick" to the planet, making it jump).

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).

The inclinations of the well observed Plutinos range up to about 40 degrees. This is in reasonable agreement with the inclinations expected from the migration hypothesis under plausible assumptions about the motion of Neptune (although, to be fair, migration struggles to fit the most highly inclined objects). Some non-resonant KBOs have inclinations much higher than the Plutinos and this is a dynamical surprise, for which no single, clear explanation currently exists.

David Jewitt

Kuiper Belt