There is no definitive explanation nor agreed description of the so-called Kuiper Belt. There is even disagreement about who really discovered it, though it is named after Gerard Kuiper (1905 – 1973). In 1949 he proposed an influential theory of the origin of the solar system, suggesting that the planets (the outer planets, at least) had formed by the condensation of a large cloud of gas around the Sun. The Kuiper Belt is the remnant of all this construction work, containing some of the oldest bodies in the solar system—basically builders’ rubble.

Already in 1943 Kenneth Edgeworth suggested that there was a reservoir of comets and larger bodies residing somewhere beyond the planets. The logic was that, since comets are eventually destroyed by the sun, there should be none left by the time we start picking up telescopes. And yet they kept on coming. There must be something resupplying the cometary materials. Although their source was proposed as the Kuiper Belt as early as 1951, its existence remained undetected until the 1990s and the advent of large-scale telescopes.

The Kuiper Belt is thought to be the source of most of the observed short-period comets, particularly those that orbit the Sun in less than 20 years, and for the icy Centaur objects (see below), which have orbits in the region of the giant planets.

The Kuiper Belt is a doughnut-shaped ring of icy objects about 10 billion kilometres thick. Its inner edge is close to the orbit of Neptune. Indeed, Neptune is thought to be the culprit that gravitationally dislodges materials from this region, sending them into the solar system in the form of comets. Their elongated orbits suggested to Kuiper that there must be a vast nesting ground for these comets somewhere beyond the orbit of Neptune. While the proposed Oort Cloud might account for long-period comets, characterised by their random orbital inclinations, those comets with very short periods (of 20 years or less) orbited in the same direction as the planets around the Sun and close to the plane of the solar system. As such they required a nearer, more-flattened source, theorised Gerard Kuiper, and the KB was his answer.

KBOs orbit at a mean distance from the Sun larger than the mean orbital distance of Neptune. The outer edge of the Kuiper belt is more poorly defined but nominally excludes objects that never get closer to the Sun than 7.1 billion kilometres. This is the location of the 2:1 Neptune resonance, where an object makes one orbit for every two of Neptune’s. Thus, most of the members of the Kuiper Belt take about 330 years to orbit the sun— and whose orbits lie close to the plane of the solar system. The Kuiper Belt is inclined at 1.86° to the ecliptic (plane of the sun’s orbit).

It is suggested that some of the planetary moons may have originated from the Kuiper Belt, such as Triton (Neptune) and Phoebe (Saturn). Astronomers seek to study the Kuiper as it may give us clues about the evolution of every planet, including Earth.

The Kuiper Belt is thought to be populated by over a hundred thousand rocky, icy bodies each larger than 100 kilometres across, along with one trillion or more comets. Most of them comprise frozen volatiles such as methane, ammonia and water. But it also has some significantly large objects. For example, it is home to four officially named dwarf planets: Pluto, Eris, Makemake, and Haumea. Pluto, in particular, is compositionally very similar to many KBOs.

Other notable members of the Kuiper community include: Charon, Orcus, Quaoar, Varda, Varuna and Ixion.

Within this belt groups of bodies are thought to form their own “collisional families”, of which Haumea is the main member and prime example. It is the largest body in the solar system to spin so rapidly. Its motion was probably set off by a collision.

KBOs are classified, based on three characteristics: their mean distance from the sun (that is, their semimajor axis); their distance of closest approach to the sun (their perihelion); and, their orbital inclination as measured against the plane of the ecliptic. This gives us three distinct groups: resonant objects; hot classicals; and cold classicals.

Resonant objects

Resonant objects are those that have a mean motion resonance (MMR) with Neptune’s orbit in the ration of 3:2. Pluto is the chief example. This means they complete two orbits around the sun in the time it takes Neptune to complete three. There are an estimated 55 000 such bodies larger than 100 kilometres in diameter. It is estimated that nearly a quarter of all MMR objects are in the 3:2 resonance. And this respect they are sometimes referred to as “Plutinos”.

Hot classicals

These objects have perihelion distances of between 5.2 billion and 6 billion kilometres. There are about 120 000 of such objects with a diameter of 100 kilometres or more. Their orbital inclinations can be as steep as 16°. They are sometimes referred to as outer or “detached” Kuiper Belt objects. The mass of the dynamically hot population is 1% of the mass of Earth.

Cold classicals

Cold classicals have close to flat orbits (inclinations no more than 2.6°), with mean orbital distances of 6.4 – 7.1 billion kilometres. There are some 75 000 such objects with diameters of 100 kilometres or more. There is a sub group called “the kernel objects” with a mean orbital distance from the sun of 6.6 billion kilometres. The dynamically cold population is only 0.03% of the Earth’s mass.

Comets

Comets have finite lifespans. As they near the sun their materials sublime into space and may even disappear. It is now thought that the source of long-period comets is the Oort cloud, a nominal 50 000 AU from the sun (but see entry on the Oort Cloud). Long period comets have orbits lasting thousands of years, but they too will disappear eventually. Short period comets have orbits lasting up to 200 years or less. They are clustered near the plane of the solar system and derive largely from the Kuiper Belt.

The Centaurs

A centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Surveys conducted by the Minor Planet Center and the Jet Propulsion Laboratory’s Deep Ecliptic Survey have catalogued 474 centaurs of an estimated 44 000 such objects (although vague authorities suggest there may be as many as 10 million centaurs of more than one kilometre in diameter). Their most notable members are: Chariklo (diameter 250 kilometres), Chiron (220 kilometres), followed by Hidalgo, Pholus, Nessus, Okyrhoe, and Asbolus. There is a theory that Saturn’s moon Phoebe was once a centaur until it was captured by the planet. Ceres may well be an ex-centaur too. Their general aphelion is a bit outside of Neptune’s orbit.

The Centaurs - main cast of characters
	Discovered	Diameter in kilometres	Distance from the sun in kilometres
Chariklo	1997	250	1.94 billion - 2.77 billion
Chiron	1977	220	1.26 billion - 2.81 billion
Pholus	1992	99	1.30 billion - 4.79 billion
Asbolus	1995	66	1.02 billion - 4.35 billion
Nessus	1993	60	1.77 billion - 5.59 billion
Hidalgo	1920	52	284.2 million - 1.42 billion
Okyrhoe	1998	49	867.66 million - 1.63 billion
*Diameters are from various estimates. Some still need to be confirmed.

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Wednesday, 1st April 2020