Pluto is one member of a flat ring of icy small bodies that revolve around the Sun beyond the orbit of the planet Neptune. Named for the Dutch American astronomer Gerard P. Kuiper, it comprises hundreds of millions of objects—presumed leftovers from the formation of the outer planets—whose orbits lie close to the plane of the solar system. It is thought to be the source of most of the observed short-period comets, particularly those that orbit the Sun in less than 20 years, and for the icy Centaur objects, which have orbits in the region of the giant planets. (Some of the Centaurs may represent the transition from Kuiper Belt objects to short-period comets.)
Although its existence had been assumed for decades, the Kuiper Belt remained undetected until the 1990s, when the prerequisite large telescopes and sensitive light detectors became available. The Irish astronomer Kenneth E. Edgeworth speculated in 1943 that the distribution of the solar system's small bodies was not bounded by the present distance of Pluto. Kuiper developed a stronger case in 1951. Working from an analysis of the mass distribution of bodies needed to accrete into planets during the formation of the solar system, he demonstrated that a large residual amount of small icy bodies—inactive comet nuclei—must lie beyond Neptune.
A year earlier the Dutch astronomer Jan Oort had proposed the existence of a much more distant, spherical reservoir of icy bodies, now called the Oort cloud, from which comets are continually replenished. This distant source adequately accounted for the origin of long-period comets—those having periods greater than 200 years. Kuiper noted, however, that comets with very short periods (20 years or less), which all orbit in the same direction as all the planets around the Sun and close to the plane of the solar system, require a nearer, more flattened source. This explanation, clearly restated in 1988 by the American astronomer Martin Duncan and coworkers, became the best argument for the existence of the Kuiper Belt until its direct detection.
The first Kuiper Belt object (KBO) was discovered in 1992 by the American astronomer David Jewitt and graduate student Jane Luu. Designated 1992 QB1, the body is about 200-250 km (125-155 miles) in diameter, as estimated from its brightness. It moves in a nearly circular orbit in the plane of the planetary system at a distance from the Sun of about 44 AU. This is outside the orbit of Pluto, which has a mean radius of 39.5 AU. The discovery of 1992 QB1 alerted astronomers to the feasibility of detecting other KBOs, and within 12 years more than 800 had been discovered. Most of them lie between 30 and 50 AU from the Sun, and all must be larger than 100 km (60 miles) in diameter to be seen. On the basis of brightness estimates, the sizes of the larger known KBOs approach or exceed that of Pluto's largest moon, Charon, which has a diameter of 1,250 km (780 miles). One KBO, given the name Eris, appears to be twice that diameter—i.e., larger than Pluto itself. Because of their location outside Neptune's orbit (mean radius 30.1 AU), they are also called trans-Neptunian objects.
Two broad groups of KBOs can be distinguished: those in nearly circular stable orbits and those in highly eccentric (elongated) orbits that are much inclined to the plane of the solar system. Most of the latter objects survive only because they are in a stabilizing 3:2 resonance with Neptune—i.e., they complete two orbits around the Sun in the time it takes Neptune to complete three. Pluto is likewise in an eccentric, inclined orbit having a 3:2 resonance with Neptune. Because of its orbital characteristics, composition of ice and rock, and diminutive size compared with the planets (being smaller than Earth's Moon), Pluto is generally considered a very large KBO. In recognition of this kinship, the other KBOs with 3:2 resonances have been dubbed Plutinos ("little Plutos"). A small percentage of KBOs form a third group: those in elongated, inclined orbits that are not in resonance with Neptune. Because of their unstable orbits, they have the potential to be perturbed onto paths either far from the Sun or into the inner solar system, where they may become short-period comets.
An extrapolation of KBO discovery statistics suggests that 100,000 objects with diameters larger than 100 km—and probably many millions more smaller objects down to 1 km (0.6 miles)—exist between 30 and 50 AU. The population may extend farther outward, but at what distance it ends and whether it reaches the innermost boundary of the Oort cloud is not known. Astronomers generally consider the Kuiper Belt to be the outer remnant of the massive protoplanetary disk that existed some 4.5 billion years ago but failed to form large planets beyond the present orbit of Neptune.
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