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

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 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.

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 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.

A plot of the semi-major axes of the KBOs (PS version) (PDF version) 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).

The inclinations of the well observed Plutinos range up to about 20 degrees (see also PS version, PDF version). This is in reasonable agreement with the inclinations expected from the migration hypothesis under plausible assumptions about the motion of Neptune. Some non-resonant KBOs have inclinations much higher than the Plutinos and this is a dynamical surprise, for which no clear explanation currently exists. We expect that resonance trapping should excite the inclinations of Plutinos, but there are no self-evident mechanisms by which the inclinations of Classical KBOs should be pumped.

Dan Green has written a detailed opinion about the perceived status of Pluto in the era of the Kuiper Belt. It's worth a look.

David Jewitt, 2004 Feb.

Kuiper Belt

Last Updated Feb 2004