Beyond the furthest planet of our system, Neptune, lies a dense and large torus-shaped region composed of rocky and icy bodies, comet-like objects, and dwarf planets, amongst which are the infamous Pluto and the more recently explored Arrokoth. Starting at around 30 Astronomical Units (AU) from our Sun and stretching out until nearly 1,000 AU from it, this region known as the Kuiper Belt holds the fingerprints of the early history of our solar system and the remnants of the eight planets’ past.
Like other asteroid belts, the Kuiper Belt is a source of comets and, more importantly, a boundary between the known and the unknown, a place we are beginning to understand, where we are constantly pushing out understanding.
The discovery of the Kuiper Belt
The Kuiper Belt was named after Dutch astronomer Gerard Kuiper who had conducted studies on objects, he speculated, existed beyond Pluto. However, he made no formal mention of such a structure, and it was only years later, after a large-scale collaboration, that the Kuiper Belt region was formally recognized.
Indeed, between the discovery of Pluto and the discovery of the objects that make up the Kuiper Belt (Kuiper Belt Objects or KBOs), amongst which Pluto does figure, passed more than 60 years. Therefore, Gerard Kuiper is mainly considered the catalyst for more thorough research in that deep region of the Solar System and not its discoverer. Hence the preference of some astronomers to call it the Trans-Neptunian Region.
The formation of the Kuiper Belt
Only little certitude surrounds our knowledge of the exact steps of the Kuiper Belt’s formation; however, we do know, from modern computer simulations, that the formation and disruptions caused by Jupiter, Neptune, and Uranus, as well as their gradual relative movements, were strongly influential towards the Kuiper Belt’s structure. Indeed, studies show that Saturn, Neptune, and Uranus would have formed much closer to Jupiter than the position we currently know them in.
When the planets were forming, small icy objects would scatter away from Jupiter, disrupting the other giant planets’ orbits, which would then move into orbits further away from the Sun. After gradual shifting, Jupiter and Saturn finally reached an exact 1:2 ratio of orbital resonance, meaning Jupiter orbited the Sun twice while Saturn would do so only once. This change ultimately had repercussions on the orbits of Neptune and Uranus, and so forth went the process: building our Solar System in perfect harmony.
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During this period of orbital fine-tuning, as planets reached their optimal resonance, scattered planetesimals migrated outwards and were captured in the resonance of the planets and formed an orbital belt. Indeed, this phenomenon may also explain Jupiter’s Trojans or the composition of Saturn’s rings.
The composition of the Kuiper Belt
Spectroscopic analysis of the Kuiper Belt indicates that its objects are various rocky bodies or ice forms such as water, ammonia, and methane. Besides, due to the Kuiper Belt region’s low temperature – around 50 Kelvin – most compounds that would typically be found in the gaseous state around the Sun are here found in the solid state. The typical density of a Kuiper Belt object ranges from 0.4 to 2.6 grams per centimeter, the denser objects being rocky ones surrounded with a thin ice coat, whereas the least dense ones being porous ice.
The Kuiper Belt objects mainly fall into two categories of properties. Either the objects resemble a greyish color and have low albedos (absorbs most of the light that hits it), or they hold a reddish color and have high albedos (reflect most of the light reaching their surface). Thus, the Kuiper Belt’s peculiar composition and its large distance from the Sun explain its value as a Solar System fingerprint nowadays. Indeed, at such distances, its objects and bodies have remained unaffected by the later developments and upheavals of the Solar System and therefore provide valuable insight into materials found at the initial states of our Solar System.
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Pluto is amongst the largest KBOs and, in recent years, has been comparatively similar to other large KBOs, one of the reasons it was demoted from the status of a planet. Indeed, they are of similar composition, all holding amounts of methane and carbon monoxide in them. Therefore, the Kuiper Belt is a source of changing understanding for our planetary classification. As our understanding of it progresses, so does the way we understand why a certain body of our solar system is located where it is. Indeed, comparing Pluto’s behavior to that of gradually discovered KBOs allowed us to firmly confirm its place beyond the planets, but the detailed reasons are yet to be firmly confirmed.