The Kuiper Belt

A short-period comet passes by the Sun every few hundred years or less, losing part of its volatile material each time. Thus, short-period comets cannot be very old because the Sun would have long ago driven away all their ices. This suggests a source that replenishes the supply of short-period comets, like the Oort cloud does for the long-period comets. This source is thought to be a belt of icy bodies in the plane of the Solar System and lying between approximately 30 AU and 100 AU from the Sun that is called the Kuiper belt. The location of the Kuiper belt is illustrated in the figure below.

The Kuiper belt is named in honor of Dutch-American astronomer Gerard Kuiper, who proposed the idea in the early 1950s. Several bodies have been identified that may be members of the Kuiper belt, and it is possible that Charon, Pluto, Triton, and Chiron (discussed later in conjunction with asteroids) are related to the Kuiper belt in some way.

The Oort Cloud

The comets that are most likely to become visible to the naked eye are much rarer than the short-period comets. They are thought to come from a great spherical cloud of cometary material surrounding the Solar System called the Oort cloud. The Oort cloud is named for Dutch astronomer Jan Oort, who first suggested its possible existence in the 1950s.

A Cloud of Icy Bodies
The sphere of the Oort cloud is a light year (50,000 AU) or more in radius, so it is enormous. However, the total mass of cometary material in this cloud is probably less than that of the Earth. The icy bodies in the Oort cloud do not have tails since they are far from the Sun. Their composition probably represents the primordial composition of the solar nebula at great distances from the Sun.

The Oort cloud is shown schematically in the figure below. Note that in this figure the yellow dot representing the Solar System is not drawn to scale. The Solar System would be an almost invisible speck if depicted realistically, since it is 1000 times smaller than the Oort cloud.

Long-Period Comets
Occasionally a comet in the Oort cloud is disturbed gravitationally, for example by a passing star, and started on a long elliptical or parabolic orbit toward the Sun. These long-period comets are primarily responsible for the brighter comets observed historically. The orbits of both the long-period and short-period comets may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself.