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KEY WORDS: Neptune, Pluto, planet formation, outer solar system, short-period comets
ABSTRACT
The region of the solar system immediately beyond Neptune's orbit is densely populated with small bodies. This region, known as the Kuiper Belt, consists of objects that may predate Neptune, the orbits of which provide a fossil record of processes operative in the young solar system. The Kuiper Belt contains some of the Solar System's most primitive, least thermally processed matter. It is probably the source of the short-period comets and Centaurs, and may also supply collisionally generated interplanetary dust. I discuss the properties of the Kuiper Belt and provide an overview of the outstanding scientific issues.
HISTORY OF THE KUIPER BELT
Edgeworth (1943) was the first to speculate on the existence of planetary material beyond the orbit of Pluto. Referring to the solar nebula, he wrote, "It is not to be supposed that the cloud of scattered material which ultimately condensed to form the solar system was bounded by the present orbit of the planet Pluto; it is evident that it must have extended to much greater distances." He also suggested that the trans-Plutonian region was the repository of the comets: "From time to time a member of this swarm of potential comets wanders from its own sphere and appears as an occasional visitor to the inner regions of the solar system." These qualitative ideas were later repeated by Edgeworth (1949) and echoed by Kuiper (1951), who gave no indication that he was aware of Edgeworth's papers. Rather, his motivation was in part to correct Oort's (1950) assertion that the comets formed near, and were ejected by, Jupiter and had a composition like that of the main-belt asteroids. Edgeworth's and Kuiper's remarks were purely speculative rather than predictions in the accepted, scientific sense. Perhaps for this reason, the notion of a transNeptunian belt had a less immediate impact on the subsequent development of cometary science than did the contemporaneous but more quantitative work by Oort (1950) and Whipple (1951). Nevertheless, the possibility of a transPlutonian ring was clearly recognized before the middle of the 20th century. This concept was sustained by Whipple (1964) and others, while much later Fernandez (1980) explicitly re-proposed a trans-Neptunian ring as a source of...