606 Encyclopedia of the Solar System
FIGURE 1 The diameter of Pluto (2302 km) and its moon
Charon (1186 km) in comparison to the size of the United States.
The diameters of Pluto and Charon are each smaller than the
diameter of Earth’s Moon, 3476 km. (Courtesy of Dan Boone
and NAU Bilby Research Center)
FIGURE 2 Positions of known bodies in the outer Solar System
on September 24, 2006. The orbits of the outer planets are
shown in light blue. The location of Neptune is marked by a
large blue circle on the outermost orbit. Pluto is marked by a
large white circle. Kuiper Belt objects (KBOs) are marked as red
(classical KBOs), white (Plutinos), and magenta (SDOs) circles.
Centaur objects are marked as orange triangles. Comets are
marked as blue squares. (Courtesy of Minor Planet
Center)
planet in 2006. The icy debris disk beyond Neptune is com-
monly called theKuiper Beltin honor of Dutch-American
astronomer Gerard P. Kuiper, who postulated its existence
in 1951.
There are several dozen icy bodies that make up a class of
objects closely related to Kuiper Belt objects (KBOs). These
Centaur objectsare recent escapees from the Kuiper Belt.
They are on elliptical orbits about the Sun that cross the
near-circular orbits of Saturn, Uranus, and Neptune. Within
a few tens of millions of years after a Centaur object escapes
from the Kuiper Belt, the giant planets scatter it out of the
Solar System, into the Sun or a planet, or cause it to migrate
into the region of the terrestrial planets where it becomes
a Jupiter-family comet. Centaurs are quite important be-
cause they come closer to the Sun and Earth than KBOs.
By virtue of their “close” approach, many of them become
bright enough for certain physical studies that are not pos-
sible on fainter KBOs. However, it is important to remem-
ber that Centaur objects experience a warmer environment
than KBOs, and the warmth may alter their physical and
chemical properties away from their initial properties at
the time of their formation in the Kuiper Belt.
By studying KBOs and Centaurs, we are studying the
preserved building blocks of a planet, and we can therefore
shed some light on the process of planet building in our
Solar System as well as extrasolar planetary systems. After
more than a decade of study, fundamental physical proper-
ties of KBOs and Centaurs—diameter,albedo, period of
rotation, shape, mass, and surface composition—are being
measured with accuracy.1. Discovering Kuiper Belt and
Centaur Objects
Centaur objects and KBOs orbit the Sun every 30 to 330
years. In addition to their own intrinsic motion about the
Sun, the Earth’s motion about the Sun imparts an apparent
(parallactic) motion on these objects as well. These two mo-
tions distinguish KBOs and Centaurs from the multitude of
background stars and galaxies and thereby make it possible
to discover them.
Figure 3 illustrates the motion of the Centaur 1994 TA
against the “fixed” pattern of much more distant stars and
galaxies. Each panel of Figure 3 is a 300-sec exposure of
1994 TA taken with the Keck II 10-m telescope and a charge
coupled device (CCD) camera in October 1998. The image
on the right was taken about an hour after the image on the
left.2. Naming Objects
After a Centaur object or KBO is discovered, it needs a
name. The IAU is responsible for naming celestial objects.