Also known as the Edgeworth-Kuiper Belt, this is a region of space that falls in an area outside of the orbits of the major planets but still within the gravitational influence of the Sun, thus forming part of the Solar System proper. However, the area is delineated with a clearly defined “edge” at 30 Astronomical Units (AU), which is beyond the orbit of Neptune, and a “drop-off” point at around 50 AU, much like the drop-off of the continental shelves around the continents. The sharply defined edges of the Kuiper Belt imparts to the structure more the appearance of a torus or gigantic doughnut than the shape of a belt, with the area containing mostly small icy objects composed of frozen gases, such as methane, ammonia and water, although it is also home to the dwarf planets Pluto, Haumea and Makemake.
History of the Kuiper Belt
The discovery of Pluto in 1930 caused many observers to speculate that the erstwhile planet might be only one of several that inhabit the same region of space, and several hypotheses were formed. However, the sheer number of theories, hypotheses, and scientific debates around the possibility (or otherwise) of a large population of trans-Neptunian objects existing has caused much uncertainty on the subject of who should be credited with first proposing the possibility, and for the purposes of this article we will look at the leading contenders.
Early Hypotheses
Soon after the discovery of Pluto by Clyde Tombaugh in 1930, Frederick C. Leonard proposed the possibility that it might be “…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“. Soon after, the astronomer Armin Otto Leuschner offered the suggestion that “Pluto may be one of many long-period planetary objects yet to be discovered.”
During 1943, Kenneth Edgeworth speculated in the Journal of the British Astronomical Association that the material that constituted the primordial solar nebula was too dispersed in the region beyond Neptune for it to have condensed into planets, and that instead, the material condensed in to a “myriad” of less massive, and thus smaller objects. This view led him to the opinion 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 occasionally “one of their number wanders from its own sphere and appears as an occasional visitor to the inner solar system” as a comet.
Gerard Kuiper on the other hand, waited until 1951 to enter the debate, which he did in an article published in the scientific journal Astrophysics in which he stated that although a structure as proposed by other astronomers might very well have formed during the early evolutionary history of the solar system, he was of the opinion that it could no longer exist. Kuiper, as did many astronomers in his day, believed that Pluto was of the same size as the Earth, and that such a massive body would have either ejected smaller bodies from the solar system, or accelerated them into the Oort Cloud.
Subsequent Hypotheses
The basic hypothesis that a large number of icy objects inhabited the region beyond Neptune took several twist and turns over the subsequent decades. Al G.W. Cameron, a physicist, posited in 1962 that “a tremendous mass of small material on the outskirts of the solar system” existed. Fred Whipple, who coined the phrase “dirty snowball” to describe cometary structure, on the other hand, was of the opinion that a “belt of comets” could be sufficiently massive to cause the supposed orbital discrepancies of the planet Uranus. The purported orbital discrepancies of Uranus sparked a frenzied search for a so-called missing planet dubbed Planet X, but not finding a Planet X, it was thought that the “comet belt” might be massive enough to cause perturbations in the orbits of the known cometary bodies. However, close observation of the orbits of known comets have not revealed such anomalies.
On August 30, 1992, David Hewitt, of MIT, and graduate student Jane Luu identified the first trans-Neptunian object found after Pluto and Charon, (15760) 1992 QB1, which they discovered in the region that has since become known as the Kuiper Belt. A second discovery of an object in the same region a few months later, dubbed (181708) 1993 FW, was generally taken as proof of the existence of the Kuiper belt, and several thousand objects have since been found and confirmed.
Origin and structure of the Kuiper Belt
Despite extensive research, the exact origin of the Kuiper belt is still unclear, although it is believed to be relatively densely populated by several billion small, icy objects that have somehow failed to clump together to form proper planets. The largest known object, Pluto, is less than 2,326 kilometers in diameter and until the data from several wide-field survey research programs such as Pan-STARRS and LSST have been fully analyzed, the exact parameters with regard to population, population density, and fine structural details of the Kuiper Belt remain unknown.
However, what is known is that the entire region, including its outlying parts, ranges from about 30 AU from the Sun, out to around 55 AU, with the main body of the structure lying between the 2:3 resonance at 39.5 AU, and the 1:2 resonance point at approximately 48 AU.
Although the Kuiper belt is relatively thick, the main concentration of mass extends fully ten degrees both above and below the ecliptic, with the mean inclination to the ecliptic being 1.860. However, the total thickness of the structure extends several tens of degrees above and below the ecliptic, with the furthest extent in either direction being progressively less densely populated as the distance from the ecliptic increases.
Neptune exerts a profound influence on the general structure of the Kuiper Belt due to the effect of orbital resonance. Over solar system compatible timescales, Neptune’s tidal effects disrupts the orbits of all and any objects that fall within some regions, which can either eject such an object from the Solar System entirely, but more likely into the scattered part of the Kuiper belt, or into interstellar space. These disruptions have created pronounced “gaps” or “avenues” in the main body of the structure that closely resemble those in the rings of Saturn, or more locally, the Kirkwood-gaps in the asteroid belt. One such region falls between 40 and 42 AU, in which no object can maintain a stable orbit due to the influence of Neptune’s gravity for extended periods, and the objects that are currently there must have migrated there from other parts of the structure in comparatively recent times.