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CHAPTER 33

Kuiper Belt Objects:

Physical Studies

Stephen C. Tegler

Northern Arizona University

Flagstaff, Arizona

1. Discovering Kuiper Belt and
   Centaur Objects
   5. Brightness 10. KBO Binaries
   6. Diameter 11. Mass of Kuiper Belt


2. Naming Objects 7. Albedo 12. New Horizons


3. Databases of Known Objects 8. Brightness Variation 13. Future Work


4. Dynamical Classes 9. Composition Bibliography

O

ur Solar System began as a slowly spinning cloud of
gas and dust about 4.5 billion years ago. As gravity
caused the cloud to shrink in size, conservation of angular
momentum required it to spin faster and evolve into a thin
disk of gas, ice, and dust surrounding the young Sun. In
the outer region of the disk, cold material accreted to first
form boulder-sized objects, then mountain-sized objects,
and then comet nucleus-sized (1–10 km) objects. Eventu-
ally, a small number of objects reached the size of plan-
etary cores. Two cores eventually grew in size to become
Uranus and Neptune. As Uranus and Neptune grew in size,
their gravitational influence stopped the numerous remain-
ing smaller objects from forming an additional large planet.
The first hint of a debris disk of icy material in the outer
Solar System came in 1930 with the discovery of Pluto by
Clyde Tombaugh of Lowell Observatory in Flagstaff, Ari-
zona. It soon became clear that Pluto was much smaller
than any other planet in the Solar System. Pluto’s small size
did not follow the pattern of planetary properties—four
small, rocky terrestrial planets (Mercury, Venus, Earth, and
Mars) close to the Sun followed by four giant, hydrogen-
rich Jupiter-like planets (Jupiter, Saturn, Uranus, and Nep-
tune) farther from the Sun. Why wasn’t Pluto a giant like
the other Jupiter-like planets? In 1978, J. W. Christy and
R. S. Harrington added to the inventory of small bodies

beyond Neptune by discovering Pluto’s satellite, Charon

(pronounced either “Kharon” or “Sharon”), on images taken

at the U.S. Naval Observatory’s Flagstaff station. Figure 1

illustrates the small sizes of Pluto and Charon by comparing

them to the dimensions of the United States.

Perhaps the most important clue to solving the mystery

of Pluto’s small size came in 1988, when Martin Duncan,

Thomas Quinn, and Scott Tremaine presented an extensive

series of numerical simulations of the evolution of comet

orbits due to the gravitational perturbations of the giant

planets. Their simulations provided a dynamical proof that

a belt in the outer Solar System is a far more likely source of

Jupiter-family comets than the Oort cloud. The calculations

set David Jewitt and Jane Luu of the University of Hawaii

looking for the belt. In 1992, they discovered an object much

smaller and fainter than Pluto and Charon orbiting beyond

Neptune. At the present time,∼1000 objects ranging in size

from a large comet nucleus to Pluto are known. It is now

clear that Pluto, Charon, and the numerous smaller objects

are what remain of the ancient disk of icy debris that did not

accrete into a giant, Jupiter-like planet beyond the orbit of

Neptune (Fig. 2). The discovery of an object slightly larger

than Pluto by Michael Brown of the California Institute of

Technology in 2003 triggered the International Astronom-

ical Union (IAU) to downgrade Pluto from its status as a

Encyclopedia of the Solar System 2e©C2007 by Academic Press. All rights of reproduction in any form reserved. 605