100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64

CHAPTER 32

Kuiper Belt: Dynamics

Alessandro Morbidelli

Observatoire de la C ˆote d’Azur

Nice, France

Harold F. Levison

Southwest Research Institute

Boulder, Colorado

1. Historical Perspective 6. Ecliptic Comets


2. Basic Orbital Dynamics 7. The Primordial Sculpting of the


3. Orbital and Dynamical Structure of the Trans-Neptunian Population
   Trans-Neptunian Population 8. Concluding Remarks


4. Correlations Between Physical and Orbital Properties Bibliography


5. Size Distribution of the Trans-Neptunian
   Population and Total Mass

T

he name Kuiper Belt is generically referred to a popu-
lation of small bodies, the orbits of which have asemi-
major axis—and hence orbital period—larger than those
of Neptune. It can be viewed as a second Asteroid Belt,
but located at the outskirts of the solar system. The Kuiper
Belt objects—having formed at large distances from the
Sun—are rich in water ice and other volatile chemical com-
pounds and have physical properties similar to those of
comets. Indeed, the existence of the Kuiper Belt was first
deduced from observations of the Jupiter-family comets,
a population with short orbital periods and small to mod-
erate orbital inclinations, of which the Kuiper Belt is the
source.
In 14 years since the discovery of the first object, about
1200 Kuiper Belt objects have been detected. Of these,
∼700 objects have been observed for more than 2 years,
a necessary condition to compute their orbital parameters
with significant precision. The results of this detailed ob-
servational exploration of the Kuiper Belt structure have
provided several surprises. Indeed, it was expected that the
Kuiper Belt preserved the pristine conditions of the proto-
planetary disk. But it is now evident that this picture is not
correct: The disk has been affected by a number of pro-
cesses that have altered its original structure.

The Kuiper Belt may thus provide us with a large num-

ber of clues to understand what happened in the outer solar

system during the primordial ages. Potentially, the Kuiper

Belt might teach us more about the formation of the giant

planets than the planets themselves. And, as in a domino

game, a better knowledge of giant planets formation would

inevitably boost our understanding of the subsequent for-

mation of the solar system as a whole. Consequently, Kuiper

Belt research is now considered a top priority of modern

planetary science.

1. Historical Perspective

Since its discovery in 1930, Pluto has traditionally been

viewed as the last vestige of the planetary system—a lonely

outpost at the edge of the solar system, orbiting beyond

Neptune with a 248 year period. Pluto receives very little

light from the Sun (being almost 40 times farther from the

Sun on average than the Earth) and thus it is very cold.

The view was that it was a distant, isolated, and unfriendly

place, with nothing of substance beyond it.

Pluto itself has always appeared to be an oddity among

the planets. Traditionally, the planets are divided into two

Encyclopedia of the Solar System 2e©C2007 by Academic Press. All rights of reproduction in any form reserved. 589