Located in a region of the Solar System beyond the planets (from 30 AU at Neptune’s orbit to circa 55AU from the Sun), the kuiper Belt, or Edgeworth-Kuiper Belt as many scientists prefer to call it, is similar to an asteroid belt though for greater in size, being 20 times as wide and potentially 200 times as massive. Like asteroid belts, the Kuiper is composed of small celestial bodies, the scattered remnants – the leftover junk if you like from the formation of the Solar System, but unlike asteroid belts whose objects are largely composed of rock and metal, the Kuiper’s objects consist of what are termed ‘frozen volatiles’: methane, ammonia and water in ice form, for example. Another significant differentiating factor between the Kuiper and asteroid belts is the presence in the former of at least four dwarf planets, the most well-known of which is Pluto, which, until 2006, was actually classed as a planet in its own right. The belt has also previously been home to other large objects such as Saturn’s moon Phoebe and Neptune’s Triton, which originated in the region before coming under the influence of the gravitational forces of their respective planets.
Though the existence of a phenomenon like the belt had been hypothesised for some time, its existence wasn’t officially confirmed until 1992, since when over one thousand Kuiper Belt objects (KBOs) have been officially observed. On the basis of what we know about the region, it is speculated that the actual number of KBOs measuring over 100km in diameter is far in excess of the 1000 objects already confirmed and closer to a figure in the region of 70,000. Initially, it was speculated that the Kuiper region was responsible for the appearance of most periodic comets (comets whose orbit lasts less than 200 years), but studies conducted in the mid-90s appear to contradict this assumption and cast the Kuiper region as one which is dynamically stable and, thus, not the repository of periodic comets it was originally assumed to be. Instead, the true source of periodic comets is now thought to be in the father regions of the Solar System outside of the Kuiper Belt in an area referred to as the scattered disc, thought to have been created by the outward motion of Neptune several billion years ago. Scattered-disk objects are KBO-like bodies with orbits which take them as far as 100 AU from the Sun, far beyond the 50 AU boundary of the Kuiper Belt itself.
Pluto is the largest-known Kuiper Belt object and also the tenth most massive body observed directly orbiting the Sun. It was reclassified a Dwarf Planet after it was discovered to be more similar compositionally to KBOs than the major planets, being comprised of rock and ice. Its orbital period is also identical to that of another group of KBOs, referred to as plutinos in its honor. Alongside Pluto, a further three dwarf planets have now been observed within the Kuiper Belt, and these are collectively referred to as plutoids: another planet tribute to the former planet.
The Kuiper’s close proximity to Neptune has a profound effect on its structure due to the consequent orbital resonance (gravitational influence) at work on it. Neptune’s gravity destabilises the orbits of objects lying within certain regions of the Kuiper, doing one of two things: either sending them farther out into the scattered disc region of interstellar space, or into the linear Solar System. For this reason, the Kuiper is characterised by a series of pronounced gaps in its current layout, similar to the gaps present in the asteroid belt on which gravitational forces are also constantly at work.
The classical belt is the region of the kuiper between 42-48 AU. Here the gravitational influence of Neptune is negligible, so KBOs can exist in their original orbits, largely unmolested. This region accounts for about two-thirds of all KBOs observed to date. Within the classical belt are two distinct populations of KBO defined by their differing orbits. The ‘dynamically hot’ population is characterised by more pronounced elliptical orbits. Not only are the populations at odds in their orbits, they are also compositionally distinct. The cold population has red hue not evident in the hot population (the names cold and hot are not in any way indicative of temperature). It is thought that the hot population therefore formed in a different region near Jupiter before being ejected outward by movements among the gas giants, whereas the cold formed roughly where it is still situated, just outside of the orbital range of Neptune.