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The
Kuiper belt (
/ˈkaɪpər/, rhyming with "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.<sup id="cite_ref-0" class="reference">
[1]</sup> It is similar to the
asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive.<sup id="cite_ref-beyond_1-0" class="reference">
[2]</sup><sup id="cite_ref-2" class="reference">
[3]</sup> Like the asteroid belt, it consists mainly of
small bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of
rock, ices, and metal, the Kuiper objects 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.<sup id="cite_ref-3" class="reference">
[4]</sup><sup id="cite_ref-4" class="reference">
[5]</sup>
Since the belt was discovered in 1992,<sup id="cite_ref-qbee_5-0" class="reference">
[6]</sup> 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.<sup id="cite_ref-6" class="reference">
[7]</sup> The Kuiper belt was initially believed 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 region created by the outward motion of Neptune 4.5 billion years ago;<sup id="cite_ref-book_7-0" class="reference">
[8]</sup> scattered disc objects such as
Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.<sup id="cite_ref-KB_9-0" class="reference">
[nb 1]</sup>
Pluto is the largest known member of the Kuiper belt, if the
scattered disc is excluded. Originally considered a planet, Pluto's position as part of the Kuiper belt has caused it to be reclassified as a "
dwarf planet". It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "
plutinos". In Pluto's honour, the four currently accepted dwarf planets beyond Neptune's orbit are called "
plutoids".
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).<sup id="cite_ref-10" class="reference">
[10]</sup>
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