Pluto is now considered to be a large Kuiper Belt object and is classed as a dwarf planet.
The Kuiper Belt objects may be leftover material from the formation of the outer planets and are arranged in a large flat disc.
Photo: Artist's impression of the Kuiper Belt object Quaoar (NASA)
Icy objects beyond Neptune may be a source of comets.
The distant icy objects may help explain Uranus and Neptune's formation.
Scientists studying how the Solar System formed are interested in the Kuiper Belt, a region of icy objects beyond Neptune. It is thought that the Kuiper Belt objects may explain the formation of the outer planets Uranus and Neptune.
Enjoy Astronomer Marek Kukula's guide to the Solar System.
If Jupiter were much larger it would be a star in its own right! Enjoy Astronomer Marek Kukula's eloquent guide to the Sun, the planets and the outer reaches of the Solar System.
The early Solar System was a shooting gallery.
Professor Brian Cox explains how the orbiting gas giants may have caused an enormous asteroid and comet bombardment in the inner Solar System 3.6 billion years ago. Earth and the other planets were peppered by asteroids and comets.
A flyby of Phoebe yields clues about the moon's origins.
The Cassini-Huygens probe, launched in 1997, made a close flyby of Saturn's moon Phoebe and answered a fundamental question about this unusual moon's origins. It turned out that Phoebe offers scientists a chance to learn more about the distant, icy outer Solar System.
The Kuiper belt /ˈkaɪpər/, sometimes called the Edgeworth–Kuiper belt (after the astronomers Kenneth Edgeworth and Gerard Kuiper), is a region of the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although some 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. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. 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.[nb 1]
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. 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 the second largest known TNO, the largest being Eris in the scattered disk.[nb 1] Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a "dwarf planet" in 2006. 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", that share the same 2:3 resonance with Neptune.