602 Encyclopedia of the Solar System
As Neptune moved through the disk on a quasi-circular
orbit, it scattered the planetesimals with which it had
close encounters. Through multiple encounters, some plan-
etesimals were transported outward on eccentric, inclined
orbits. A small fraction of these objects still exist today and
constitute the scattered disk. Occasionally, some scattered
disk objects entered a resonance with Neptune. Resonances
can modify the eccentricity of the orbits. If decreased,
the perihelion distance is lifted away from the planet;
the sequence of encounters stops, and the body becomes
“decoupled” from Neptune like a Kuiper Belt object.
If Neptune had not been migrating, the eccentricity
would have eventually increased back to Neptune-crossing
values—the dynamics being reversible—and the sequence
of encounters would have restarted again. Neptune’s mi-
gration broke the reversibility so that some of the decou-
pled bodies managed to escape from the resonances and
remained permanently trapped in the Kuiper Belt. These
bodies preserved the large inclinations acquired during
the Neptune-encountering phase, and they can now be
identified with the “hot” component of the Kuiper Belt
population.
At the same time, while Neptune was migrating through
the disk, its 1:2 and 2:3 resonances swept through the disk,
capturing a fraction of the disk planetesimals as explained
earlier. When the 1:2 resonance passed beyond the edge
of the disk, it kept carrying its load of objects. Because
the migration of Neptune was presumably not a perfectly
smooth process, the resonance was gradually dropping ob-
jects during its outward motion. Therefore, like a farmer
seeding as he advances through a field, the resonance dis-
seminated its previously trapped bodies all along its way up
to its final position at about 48 AU. This explains the current
location of the outer edge of the Kuiper Belt. Because the
1:2 resonance does not significantly enhance the orbital
inclinations, the bodies transported by the resonance pre-
served their initially small inclination and can now be iden-
tified with the cold component of the Kuiper Belt.
This scenario, reproduced in numerical simulations,
explains qualitatively the orbital properties of the trans-
Neptunian population, but it has difficulties explaining why
the hot and the cold classical populations have different
physical properties. Indeed, the members of these two pop-
ulations should have formed more or less in the same region
of the disk, although they followed two different dynamical
evolutions toward the Kuiper Belt.
An alternative possibility is that the hot population
formed as explained earlier, but the cold population formed
in situ, where it is now observed. Thus, the formation places
being well separated, the corresponding physical proper-
ties could be different. However, this model has difficulties
explaining how the cold population lost most of its primor-
dial mass. It has been proposed that the objects grind down
to dust in a collisional cascade process, but the latter has
not been shown to be really effective, and seems inconsis-
tent with a number of constraints, such as the existence of
binary objects with large separations, or the total number of
comet-sized bodies in the scattered disk. Moreover, if the
cold population is local, one is faced again with the problem
of explaining why the outer edge of the population is exactly
at the location of the 1:2 resonance with Neptune.
A further possibility may be offered by a recent model,
on the evolution of the outer solar system, that has been de-
veloped in order to explain the origin of the so-called Late
Heavy Bombardment (LHB) of the terrestrial planets. The
latter is a cataclysmic period characterized by huge impact
rates on all planets that occurred between 4.0 billion and
3.8 billion years ago, namely about 600 million years after
planet formation. In this model, the giant planets are as-
sumed to be initially on quasi-circular and coplanar orbits,
with orbital separations significantly smaller than the cur-
rent ones. In particular, Saturn is assumed to be closer to
Jupiter than their mutual 1:2 resonance (they are now close
to the 2:5 resonance). The planetesimal disk is assumed to
exist only from about 1.5 AU beyond the location of the out-
ermost planet, up to∼35 AU, with a total mass of∼ 35 M⊕.
With this setting, the planetesimals at the inner edge of
the disk acquire Neptune-scattered orbits on a timescale of
a few million years. Consequently, the migration of the gi-
ant planets proceeds at very slow rate, governed by the slow
escape rate of planetesimals from the disk. This slow migra-
tion continues for hundreds of millions of years, until Jupiter
and Saturn cross their mutual 1:2 resonance. This resonance
crossing excites their eccentricities, which destabilizes the
planetary system as a whole. The planetary orbits become
chaotic and start to approach each other. Both Uranus and
Neptune are scattered outward, onto large eccentricity or-
bits (e∼ 0. 3 − 0 .4) that penetrate deeply into the disk. This
destabilizes the full planetesimal disk and triggers the LHB.
The interactions with the planetesimals damp the planetary
eccentricities, stabilizing the planetary system once again,
and forcing a residual short radial migration of the planets,
which eventually reach final orbits when most of the disk
has been eliminated. Simulations show that this model is
consistent with the current orbital architecture of the giant
planets of the solar system.
In this model, objects can be implanted into the current
Kuiper Belt during the large eccentricity phase of Neptune.
In fact, the full Kuiper Belt is unstable at that time, so that
it can be visited by objects that leave the original planetes-
imal disk when the latter is destabilized. When Neptune’s
eccentricity is damped, the Kuiper belt becomes stable so
that the objects which, by chance, are in the Kuiper Belt re-
gion at that time, become trapped forever. Because the large
eccentricity phase of Neptune is short, the inclinations of
these objects remain predominantly small, consistent with
the cold population of the current Kuiper Belt. The objects
with the largest inclinations, conversely, are captured later,
during the final bit of Neptune’s migration, as explained be-
fore. As Fig. 10 shows, this model reproduces the structure