The Kuiper Belt

The Kuiper Belt (pronounced kyper belt and sometimes referred to as the Edgeworth-Kuiper belt) is a region of our Solar System, beyond the orbit of Neptune, between around 30 to 50 astronomical units from the Sun. The Kuiper Belt is home to possibly trillions of small Solar-System bodies (SSSBs), ranging from tiny icy planetesimals to dwarf planets.

The dwarf planet Pluto, discovered in 1933 by Clyde Tombaugh, was the first trans-Neptunian object to be found in the region now known as the Kuiper belt. At the time of its discovery, Pluto was considered to be a ninth planet.

In 1943, Irish astronomer Kenneth Edgeworth, proposed the existence of a disk of small planets or comets, which may have formed beyond the orbit of Neptune. In 1953, the Dutch-American astronomer Gerard Kuiper made a similar proposal; however, he believed that such objects would have been swept clear by planetary gravitational perturbations, so that few would remain there today. Canadian astronomer Scott Tremaine is credited with giving the Kuiper Belt its name, in the late 1980s, although it is sometimes referred to as the Edgeworth-Kuiper belt, since Kenneth Edgeworth had made his prediction of the belt’s existence 10 years prior to Kuiper’s.

1992 QB1

On 30 August, 1992, an object, provisionally named 1992 QB1 (since it was the 27th object reported to the International Astronomic Union’s Minor Planet Centre in the second half of August, 1992 – see Asteroid Names and Numbers) was discovered by David Jewitt and Jane Luu. The pair had been hunting for objects beyond the orbit of Pluto for about five years before they made their discovery of 1992 QB1. This object is over 100 km in size, and was officially named Albion in 2018, after a character created by the poet and painter William Blake. (Jewitt and Luu originally suggested the name Smiley, after the American astronomer Charles Hugh Smiley, although this name had already been used for an asteroid). However, QB1 could be considered a fortuitous name for the first Kuiper belt object discovered since Pluto and its moon Charon.

This marked the beginning of a flurry of further discoveries, and it is estimated that there could be over 100,000 Kuiper belt objects (KBOs) of around the size of 1992 QB1 or larger.

Plutinos, Cubewanos and Neptune Trojans

The two main types of Kuiper Belt object are often referred to as cubewanos, (from QB1-o), or classical Kuiper belt objects, and plutinos (after the dwarf planet Pluto).

The plutinos have more eccentric orbits than the cubewanos and are mostly found in a 2:3 orbital resonance with Neptune. Whereas the cubewanos have much more circular orbits and are mainly found between the 2:3 orbital resonance and the 1:2 resonance with Neptune.

Several other populations of trans-Neptunian objects also exist, such as those with a 1:2 resonance, also known as the twotinos, or those with a 2:5 or 1:3 resonance.

A few objects have also been discovered with a 1:1 orbital resonance with semi-major axes similar to that of Neptune near to the Sun-Neptune Lagrangian points. These are known as the Neptune trojans by analogy to the Trojan asteroids of Jupiter.

The Scattered Disc

The scattered disk is a more sparsely populated region, which overlaps the Kuiper belt and extends out to about 100 astronomical units from the Sun. Scattered disc objects are in highly eccentric, unstable orbits and most short-period comets are believed to have originated there.

Objects from the scattered disc are believed to have formed along with Kuiper belt objects in the Sun’s primordial protoplanetary disc. Since they were so far out from the Sun, they wouldn’t have had time to coalesce into a large planet. Neptune and Uranus are theorised to have been formed closer in to the Sun than their current orbits and were sent outwards by gravitational interactions with Jupiter and Saturn. Neptune’s gravitational influence then caused around 99 per cent of the original Kuiper belt objects to be scattered out of the belt.

The remaining objects in the Kuiper belt finally settled into their current stable orbits, although further out than where they were originally formed. Many of the scattered objects were either sent outwards into highly elliptical orbits or inwards where they either ending up in stable orbits, within the orbit of Neptune, were ejected out of the Solar System by Jupiter, or became short period comets.


The current population of objects found between the orbits of Jupiter and Neptune, known collectively as “centaurs”, are believed to have originated in the original Kuiper belt and were scattered inwards by Neptune.

It is believed that comets, centaurs and scattered disc objects, therefore, have similar compositions to the icy objects of the Kuiper belt.

Neptune’s largest moon, Triton, is also believed to be a captured Kuiper belt object.

Top 10 Largest Known Trans-Neptunian Objects:

As of 2022, the top 10 largest known trans-Neptunian objects, by diameter, are:

  • Pluto – Discovered: 23 January 1930,
    Location: the Kuiper belt
    Mean Diameter: 2,376.6 ± 1.6 km
  • Eris – Discovered: 21 October 2003,
    Location: the Scattered disc
    Mean Diameter: 2,326 ± 12 km
  • Haumea – Discovered: 7 March 2003
    Location: the Kuiper belt
    Approx Diameter: 1,595 km
  • Makemake – Discovered: 31 March 2005,
    Location: the Kuiper belt
    Approx Diameter: 1,430 km
  • Gonggong – Discovered: 17 July 2007
    Location: the Scattered disc
    Approx Diameter: 1,230 km
  • Quaoar – Discovered: 4 June 2002
    Location: the Kuiper belt
    Approx Diameter: 1,070 km
  • Sedna – Discovered: 14 November 2003
    Location: Detached (origin uncertain)
    Approx Diameter: 995 km
  • Orcus – Discovered: 17 February 2004
    Location: the Kuiper belt
    Approx Diameter: 910 km
  • Salacia – Discovered: 22 September 2004
    Location: the Kuiper belt
    Approx Diameter: 866 km
  • 2002 MS4 – Discovered: 18 June 2002
    Location: the Kuiper belt
    Approx Diameter: 800 km

Source: Wikipedia: List of trans-Neptunian objects

See also Dwarf Planets, for more information on most of these objects.

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Astronomy, Cosmology, Space and Astrophysics