Kuiper Belt

Comets are some of the most spectacular objects in the night sky. About once per decade, a truly bright comet comes along and can be viewed by the unaided eye. Where do these comets come from? Where do they spend most of their lives?

Some comets orbit the Sun once every thousand years or so and can be easily viewed only when they are in the inner solar system. These are known as long period comets. Nonperiodic comets appear in the inner solar system only once. Some comets, however, enter the inner solar system repeatedly and predictably. These are the short period comets.

All of these comets are in orbit around the Sun but, unlike the planets, which all revolve around the Sun in the same direction and are confined to approximately the same plane as Earth's orbit (the plane of the ecliptic ), cometary orbits show no preferred orientations. The shortest period comets (orbital periods of less than twenty years) are an exception. Comets in this group, called the Jupiter family comets (JFCs), revolve around the Sun near the plane of the ecliptic in the same direction as Earth's orbit.

Noting the different nature of the JFC orbits, astronomers sought explanations. It had been believed that all comets originated in the Oort cloud, a halo of comets at extremely large distances from the Sun. But in 1988, Martin J. Duncan, Thomas R. Quinn, and Scott Tremaine showed that it was impossible to have the random orientations of Oort cloud comets converted to the planar orientations of JFCs. They proposed that, in addition to the Oort cloud as a reservoir for comets, there must be a disk-like reservoir of comets with its inner edge near Neptune. They called this disk the Kuiper belt after Dutch-born American astronomer Gerard P. Kuiper, who postulated in 1951 that the solar system could not end at Neptune because that would imply a sharp edge to the disk out of which planets formed.^*

The objects in the Kuiper belt represent remnants from the formation of our solar system. When the planets formed 4.6 billion years ago, they formed from an agglomeration of many planetesimals, or small solid celestial bodies. Beyond Neptune, the density of planetesimals was too low and the time for them to collide and accumulate was too long for another planet to form. Thus, the planetesimals remained in the outer solar system, past Neptune's orbit. They are called Kuiper belt objects (KBOs).

The existence of the Kuiper belt was confirmed in the 1990s. In 1992 David Jewitt and Jane Luu found the first KBO, designated 1992 QB₁. By 2000 many surveys had been performed and a total of 345 KBOs had been found.

In 2001, some one trillion planetesimals still existed in KBO orbits from Neptune outwards. Most remained in the Kuiper belt, interacting with one another or Neptune. Some of these were just barely visible from Earth through the most sensitive telescopes; others were too faint to see. But they do exist. And, as time passes, some will be perturbed into the inner solar system, where they will become Jupiter-family comets and appear periodically.

see also Comets (volume 2); Kuiper, Gerard Peter (volume 2); Oort Cloud (volume 2); Orbits (volume 2); Planetesimals (volume 2); Small Bodies (volume 2).

Anita L. Cochran

Bibliography

Duncan, Martin J., Thomas R. Quinn, and Scott Tremaine. "The Origin of Short-Period Comets." Astrophysical Journal (Letters) 328 (1988):L69-L73.

Edgeworth, Kenneth. E. "The Origin and Evolution of the Solar System." Monthly Notices of the Royal Astronomical Society 109 (1949):600.

Jewitt, David, and Jane Luu. "Discovery of the Candidate Kuiper Belt Objects 1992 QB₁." Nature 362 (1993):730-732.

Kuiper, Gerard P. "On the Origin of the Solar System." In Astrophysics: A Topical Symposium, pp. 357-424, ed. J. A. Hynek. New York: McGraw-Hill, 1951.

Malhotra, Renu, Martin J. Duncan, and Harold Levison "Dynamics of the Kuiper Belt." In Protostars and Planets IV, eds. Vince Manning, Alan P. Boss, and Sara S. Russell. Tucson: University of Arizona Press, 2000.

^*In 1949 Irish astronomer Kenneth Edgeworth published an analysis similar to Kuiper's, but Kuiper's work was better known; in recognition of Edgeworth's contribution, some people call the comet reservoir the Edgeworth-Kuiper belt.

Kuiper Belt A region of the outer Solar System containing an estimated 10⁷ − 10⁹ icy planetesimals, or comet nuclei. The Kuiper Belt is an inner, flattened extension of the Oort Cloud. It lies more or less in the same plane as the planets and extends outwards from around 30 AU (the orbit of Neptune) to perhaps 1000 AU. Members of the Kuiper Belt are also known as trans-Neptunian objects. Such a vast reservoir of comets beyond Neptune was proposed in 1951 by G. P.Kuiper. The Irish engineer and astronomer Kenneth Essex Edgeworth (1880–1972) wrote papers about objects beyond Pluto in 1943 and 1949, so it is also known as the Edgeworth–Kuiper belt. In 1992, the British-born American astronomer David Clifford Jewitt (1958– ) and the Vietnamese-born American astronomer Jane Luu (1963– ) discovered the first Kuiper Belt object, 1992 QB₁, now numbered (15760). This has a diameter of about 200 km, semimajor axis 43.8 AU, orbital period about 290 years, perihelion 40.9 AU, aphelion 46.7 AU, and inclination 2 °.2. Since then, hundreds more have been found. The Kuiper Belt is thought to be the source of most periodic comets. Members of the Kuiper Belt that pass close to Neptune can be diverted inwards by gravitational perturbations to become Centaurs, or outwards to become scattered‐disk objects. See also cubewano; plutino.

Kuiper belt A region of bodies, made mainly from ice, that lies just beyond the orbit of Pluto and supplies the solar system with short-period comets, e.g. Halley. At its outer margins the Kuiper belt extends into the Oort cloud. The existence of the Kuiper belt was proposed by Gerard P. Kuiper, in 1988 its role as the source of short-period comets was suggested, and in the early 1990s the first objects belonging to the Kuiper belt were discovered.

Edgeworth–Kuiper Belt An alternative name for the Kuiper Belt.

Kuiper belt see comet ; Kuiper, Gerard Peter .