The Kuiper belt is an area of the solar system extending from the orbit of Neptune (at 30 AU) to 50 AU from the Sun. The Kuiper belt is similar to the asteroid belt, although it is vastly larger. It consists mainly of small bodies; remenants from the Solar System's formation, though a fair portion of its material has condensed into dwarf planets. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt is composed largely of ices, such as methane, ammonia, and water.
Long suspected to exist, the Kuiper belt was only discovered in 1992, by the team of David Jewitt and Jane Luu at the University of Hawaii's main telescope at Mauna Kea. Since its discovery, the number of Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70,000 KBOs over 1 km in diameter are believed to reside there.
The Kuiper belt is believed to be the repository for periodic comets, those, like Halley's Comet, with orbits lasting less than 200 years. The centaurs, comet-like bodies that orbit among the gas giants, are also believed to originate there, as are the scattered disc objects such as Eris; KBO-like bodies with massive orbits that take them as far as 100 AU from the Sun. Neptune's moon Triton is believed to be a captured KBO.
Pluto, a dwarf planet, is the largest known member of the Kuiper belt. Originally considered a planet, it has many physical properties in common with the objects of the Kuiper belt, and has been known since the early 1990s to share its orbit with a number of similarly-sized KBOs, now called Plutinos. This led many astronomers to conclude that, much like Ceres before it, which was considered a planet before the discovery of its fellow asteroids, Pluto should be reclassified as a Kuiper belt object. The issue was brought to a head by the discovery of Eris, an object in the scattered disc far beyond the Kuiper belt, that is now known to be 27 percent more massive than Pluto.
In response, the International Astronomical Union (IAU), was forced to define a planet for the first time, and in so doing included in their definition that a planet must have "cleared the neighbourhood around its orbit." As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.
The objects within the Kuiper belt, together with the members of the scattered disc extending beyond and any hypothetical Hills cloud and Oort cloud objects, are collectively referred to as trans-Neptunian.
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant.
Since the discovery of Pluto, many have speculated that it might not be alone. The first astronomer to suggest the existence of a trans-Neptunian population was Frederick C. Leonard in 1930, soon after Pluto's discovery, who pondered whether it was "not likely that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected". In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth hypothesized that, in the region beyond Neptune, the material within the primordial solar nebula was too widely spaced to consense into planets, and so rather condensed into myriad of smaller bodies. From this he concluded that “the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies" and that, from time to time, one of their number 'wanders from its own sphere and appears as an occasional visitor to the inner solar system,' becoming what we call a comet.
In 1951, in an article for the journal Astrophysics, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, however, he did not believe that such a belt still existed today. Kuiper was operating on the assumption common in his time, that Pluto was the size of the Earth, and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. By Kuiper's formulation, there wouldn't be a Kuiper belt where we now see it.
The hypothesis took many other forms in the following decades: in 1962, physicist Alistair Cameron speculated on a "tremendous mass of small material on the outskirts of the solar system," while in 1964, Fred Whipple, who popularised the famous "dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepencies in the orbit of Uranus that had sparked the search for Planet X, or at the very least, to affect the orbits of known comets. Observation, however, ruled out this hypothesis.
In 1977, Charles Kowal, using a blink comparator, the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before, discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus.
In 1992, another object 5145 Pholus, was discovered in a similar orbit. Today, an entire population, the centaurs, is known to exist in that region. The centaurs' orbits are unstable over periods longer than roughly 100 million years, a relatively short span when compared to the age of the Solar System. From the time of Chiron's discovery, astronomers speculated that it therefore must be frequently replenished by some outer reservoir.
By the 1970s, the rate at which short-period comets (those comets with orbits with less than 200 years) were being discovered was becoming increasingly inconsistent with them having emerged solely from the Oort cloud, the spherical cloud of comets extending beyond 50,000 AU from the Sun from which, in 1950, astronomer Jan Oort had cited as the point of origin for all long-period comets.
For an Oort cloud object to become a short-period comet, it would first have to be captured by the giant planets. In 1980, in the monthly notice of the Royal Astronomical Society, Julio Fernandez stated that for every short period comet to be sent into the inner solar system from the Oort cloud, 300 would have to be ejected into interstellar space. He speculated that a comet belt from between 35 and 50 AU would be required to account for the observed number of comets.
Following up on Fernandez's work, in 1988 the Canadian team of Martin Duncan, Tom Quinn and Scott Tremaine ran a number of computer simulations to determine if all observed comets could have arrived from the Oort cloud. They found that the Oort cloud could not account for short-period comets, particularly as short-period comets are clustered near the plane of the Solar System, wheras Oort cloud comets tend to arrive from any point in the sky. With a belt as Fernandez described it added to the formulations, the simulations matched observations. Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentance of Fernandez's paper, Tremaine named this region the "Kuiper belt."
In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System." He encouraged then-graduate student Jane Luu to aid him in his endeavour to locate another object beyond Pluto's orbit, because, as he said to her, "If we don't, nobody will." Using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro-Toledo Inter American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator.
Initially, examination of each pair of plates took about eight hours, but the process was speeded up with the arrival of electronic Charge-coupled devices or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90 percent of the light that hit them, rather than the ten percent achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for all astronomical detectors.
In 1988, Jewitt moved to the Institute of Astronomy at the University of Hawaii. He was later joined by Jane Luu to work at the University of Hawaii’s 2.24 m telescope at Mauna Kea. Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly. Finally, after five years of searching, on August 30, 1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" (15760) 1992 QB1. Six months later, they discovered a second object in the region, 1993 FW.
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