The Kuiper belt ( /ˈkaɪpər/, rhyming with "piper" and "viper"), sometimes called the Edgeworth–Kuiper belt, 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.[1] It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive.[2][3] Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While most asteroids are composed primarily of rock and metal, Kuiper belt objects[nb 2] are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The classical (low-eccentricity) 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.[4][5]
Since the belt was discovered in 1992,[6] the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70,000 KBOs over 100 km (62 mi) in diameter are believed to exist.[7] 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;[8] 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).[9]
Pluto is the largest known member of the Kuiper belt, and the second largest known TNO, after the scattered-disc object Eris.[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" which share the same 2:3 resonance with Neptune. In Pluto's honor, the four currently accepted dwarf planets beyond Neptune's orbit are called "plutoids".