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.
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
David Jewitt
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Kuiper Belt |
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