572 PART 4^ |^ THE SOLAR SYSTEM
comets from the Oort cloud and had their orbits altered by a
close encounter with Jupiter. However, that process can’t explain
all of the short-period comets: Some follow orbits that could not
have been produced by Oort cloud objects interacting with
Jupiter or the other planets. Th ere must be another source of icy
bodies in our solar system. To fi nd the answer, you need only
look beyond the orbit of Neptune and study the icy Kuiper belt
objects.Comets from the Kuiper Belt
You fi rst met the Kuiper belt in Chapter 19 when you studied the
origin of the solar system. In Chapter 24, you learned about the
largest Kuiper belt objects as examples of dwarf planets. In this
chapter, it is important to study the smaller bodies of the Kuiper
belt because they are one of the sources of comets.
In 1951, astronomer Gerard Kuiper proposed that the for-
mation of the solar system should have left behind a belt of small,
icy planetesimals beyond the Jovian planets and in the plane of
the solar system. Such objects were fi rst discovered in 1992 and
are now known as Kuiper belt objects (KBOs).*
Th e Kuiper belt objects are small, icy bodies (■ Figure 25-18)
that orbit in the plane of the solar system extending from the
orbit of Neptune out to about 50 AU from the sun. Some objects
are known to loop out as far as 1000 AU, but those may have
been scattered into those orbits by gravitational interactions with
passing stars. Th e entire Kuiper belt, containing as many as
100,000 objects 100 km or larger in diameter and hundreds of
millions of smaller bodies, would be hidden behind the yellow
dot representing the solar system in Figure 25-17. Some Kuiper
belt objects are as large, or larger than, Pluto, but most are quite
small.
Can this belt of ancient, icy worlds generate short-period
comets? Because Kuiper belt objects orbit in the same direction
as the planets and in the plane of the solar system, it is possible
for an object perturbed inward by the infl uence of the giant
planets to move into an orbit resembling those of the short-
period comets. Rare collisions and interactions among the KBOs
could also add to a continuous supply of small, icy bodies from
the Kuiper belt sent into the inner solar system.
Comets vary in brightness and orbit. Nevertheless, there are
two basic types of comets in our solar system. Some originate in
the Oort cloud far from the sun. Others come from the Kuiper
belt just beyond Neptune. Th ey all share one characteristic—
they are ancient icy bodies that were born when the solar system
was young.■ Figure 25-1 7
Long-period comets appear to originate in the spherical Oort cloud. Objects
that fall into the inner solar system from that cloud arrive from all directions.
Solar
system*In 1943 and 1949, astronomer Kenneth Edgeworth published papers that
included a paragraph speculating about objects beyond Pluto. Consequently, you
may occasionally see the Kuiper belt referred to as the Edgeworth–Kuiper belt,
but most astronomers refer to this part of the solar system as the Kuiper belt.that the star Gliese 710 will pass within 1 light-year (about
63,000 AU) of the sun, crossing through the Oort cloud, in
about a million years. Th e result may be a shower of Oort cloud
objects perturbed into the inner solar system, where, warmed by
the sun, they will become comets.
Saying that comets come from the Oort cloud only pushes
the mystery back one step. How did those icy bodies get there?
In preceding chapters, you have studied the origin and evolution
of our solar system so carefully that the answer may leap out at
you. Th ose Oort cloud comets are some of the icy planetesimals
that formed in the outer solar nebula. Th e bodies in the Oort
cloud, however, could not have formed at their present location
because the solar nebula would not have been dense enough so
far from the sun. And, if they had formed from the solar nebula,
they would be distributed in a disk and not in a sphere.
Astronomers think the Oort cloud planetesimals formed in the
outer solar system near the present orbits of the giant planets. As
those planets grew, they swept up many of these planetesimals,
but they also would have ejected some out of the solar system.
Most of those ejected objects vanished into space, but perhaps
10 percent had their orbits modifi ed by the gravity of stars pass-
ing nearby and became part of the Oort cloud. Some of those
later became the long-period comets.
Long-period comets originate in the Oort cloud, and some
of the short-period comets do also. A few short-period comets,
including Comet Halley, appear to have begun as long-period