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Facts about the Kuiper Belt for Kids


While most asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed “ices”), such as methane, ammonia and water. The classical belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake.

  • Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, are also believed to have originated in the region.
  • Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist.
  • However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets’ true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.
  • The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs).
  • Pluto is the largest known member of the Kuiper belt, and one of the two largest known TNOs, together with scattered disc object Eris.
  • It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs known as “plutinos” which share the same 2:3 resonance with Neptune.
  • In 1930, soon after Pluto’s discovery by Clyde Tombaugh, Leonard pondered whether it was “not 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”.
  • From this he concluded 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, from time to time, one of their number “wanders from its own sphere and appears as an occasional visitor to the inner solar system”, becoming a comet.
  • One such area of replenishment is the Oort cloud, the spherical swarm of comets extending beyond 50 000 AU from the Sun first hypothesised by astronomer Jan Oort in 1950.
  • Astronomers sometimes use the alternative name Edgeworth–Kuiper belt to credit Edgeworth, and KBOs are occasionally referred to as EKOs.
  • The model predicts a higher average eccentricity in classical KBO orbits than is observed.
  • Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice, a composition they share with comets.
  • Despite its vast extent, the collective mass of the Kuiper belt is relatively low.
  • Conversely, models of the Solar System’s formation predict a collective mass for the Kuiper belt of 30 Earth masses.
  • However, in some scientific circles the term “Kuiper belt object” has become synonymous with any icy minor planet native to the outer Solar Systembelieved to have been part of that initial class, even if its orbit during the bulk of Solar System history has been beyond the Kuiper belt.
  • This is its moon Triton, which is the only large moon in the Solar System to have a retrograde orbit; it orbits in the opposite direction to Neptune’s rotation.
  • Beyond this, 15–20% of solar-type stars have an observed infrared excess which is believed to indicate massive Kuiper belt-like structures.