Kuiper Belt: Dynamics 603
FIGURE 10 The distribution of semimajor axes and eccentricities in the Kuiper Belt. Left panel:
Result of a simulation based on the recent model on the origin of the LHB. Right panel: The observed
distribution. The model reproduces fairly well the outer edge of the Kuiper Belt at the 1:2 resonance
with Neptune, the characteristic shape of the (a, e) distribution of the classical belt, the scattered and
the extended scattered disks, and the resonant populations. The vertical solid lines mark the main
resonance with Neptune. The dotted curve denotes perihelion distance equal to 30 AU, and the
dashed curve delimits the region above which only high inclination objects or resonant objects can be
stable over the age of the solar system. The overabundance of objects above this curve in the
simulation is therefore an artificial consequence of the fact that the final orbits of the giant planets are
not exactly the same as the real ones.of the Kuiper Belt remarkably well. It is also consistent with
its low mass because in the simulations the probability of
capture in the Kuiper Belt is roughly of 1/1000 (which pre-
dicts a final mass of 0.03M⊕). Moreover, the Kuiper Belt
objects with final low inclinations and those with final large
inclinations are found to come predominantly from differ-
ent portions of the original planetesimal disk (respectively
outside and inside 29 AU), which can explain, at least at a
qualitative level, the correlations with physical properties.
We finally come to the issue of the origin of the extended
scattered disk. Simulations show that, in the same process
described earlier, bodies are also delivered to orbits with
moderate semimajor axis and perihelion distance, like that
of the extended scattered disk object 1995 TL 8 and its com-
panions. The origin of Sedna is probably different. The key
issue is that bodies with comparably large perihelion dis-
tances (∼80 AU) but smaller semimajor axis (a<500 AU)
have never been discovered despite the more favorable ob-
servational conditions. Therefore, they probably do not ex-
ist. If this is true, and the population of extended scattered
disk bodies with large perihelion distance starts only beyond
several hundreds of AUs, then an “external” perturbation
is required. The best candidate is a stellar passage at about
1000 AU from the Sun, lifting the perihelion distance of the
distant members of the primordial, massive scattered disk.
Such a stellar encounter is very unlikely in the framework
of the current galactic environment of the Sun, but it would
have been probable if the Sun formed in a moderately dense
cluster, as mentioned earlier.8. Concluding Remarks
At the time of the first edition of this encyclopedia,
60 objects had been discovered in the Kuiper Belt. Now,
we know 20 times more objects, and our view of the Kuiper
Belt has become much more precise. It is now clear that
the trans-Neptunian population has been sculpted in the
primordial phases of the history of the solar system, by pro-
cesses that are no longer at work.