In 1950, Dutch astronomer Jan Oort hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is. A year later astronomer Gerard Kuiper suggested that some comet-like debris from the formation of the solar system should also be just beyond Neptune. It was The Kuiper Belt.

The Kuiper belt 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 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger — 20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System’s formation). It is home to at least three dwarf planets – Pluto, Haumea and Makemake. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (dubbed “ices”), such as methane, ammonia and water.

It is now considered to be the source of the short-period comets. Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the giant planets in such a way as to cause the object to cross the orbit of Neptune. It will then very likely have a close encounter with Neptune sending it out of the solar system or into an orbit crossing those of the other giant planets or even into the inner solar system. There are presently nine known objects orbiting between Jupiter and Neptune (including 2060 Chiron (aka 95 P/Chiron) and 5145 Pholus; see the MPC’s list). The IAU has designated this class of objects as Centaurs. These orbits are not stable. These objects are almost certainly “refugees” from the Kuiper Belt. Their future fate is not known. Some of these show some cometary activity (ie, their images are a little fuzzy indicating the presence of a diffuse coma). The largest of these is Chiron which is about 170 km in diameter, 20 times larger than Halley. If it ever is perturbed into an orbit that approaches the Sun it will be a truly spectacular comet. Curiously, it seems that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. Small objects formed near the giant planets would have been ejected from the solar system by gravitational encounters. Those that didn’t escape entirely formed the distant Oort Cloud. Small objects formed farther out had no such interactions and remained as the Kuiper Belt objects. Several Kuiper Belt objects have been discovered recently including 1992 QB1 and 1993 SC (above). They appear to be small icy bodies similar to Pluto and Triton (but mostly smaller). There are more than 800 known trans-Neptunian objects (as of early 2004); see the MPC’s list. Many orbit in 3:2 resonance with Neptune (as does Pluto). Color measurements of some of the brightest have shown that they are unusually red. In late 2002, a Kuiper Belt object over 1000 km in diameter was discovered and provisionally designated 2002 LM60 “Quaoar”. In early 2004 an even larger one, 2004 DW, was found (its size isn’t well known yet, but it’s almost certainly smaller than Pluto). And in late 2005 the discovery of 2003 UB313 (now officially named “Eris”) was announced; it is very likely somewhat larger than Pluto. It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt. A team of astronomers led by Anita Cochran report that the Hubble Space Telescope has detected extremely faint Kuiper Belt objects (left). The objects are very small and faint perhaps only 20 km or so across. There may be as many as 100 million such comets in low-inclination orbits and shining brighter than the HST’s magnitude-28 limit. (A follow-up HST observation failed to confirm this observation, however.) Spectra and photometric data have been obtained for 5145 Pholus. Its albedo is very low (less than 0.1). Its spectra indicates the presence of organic compounds, which are often very dark (e.g. the nucleus of Comet Halley). Some astronomers believe that Triton, Pluto and its moon Charon are merely the largest examples of Kuiper Belt objects (Pluto can still be considered a member of the Kuiper Belt in addition to its classification as a “dwarf planet”).

Since the first discovery in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70 000 KBOs over 100 km in diameter are believed to reside there.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 Kuiper belt is dynamically stable, and that it is the farther scattered disc, a dynamically active region created by the outward motion of Neptune 4.5 billion years ago, that is their true place of origin.Scattered disc objects such as Eris are KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun. The centaurs, comet-like bodies that orbit among the gas giants, are believed to originate there. Neptune’s moon Triton is believed to be a captured KBO. Pluto, a dwarf planet, is the largest known member of the Kuiper belt. Originally considered a planet, it is similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as “Plutinos”.

3 Responses to Kuiper Belt

Pluto is still considered a planet by many scientists. Unlike the other, smaller Kuiper Belt Objects sharing the 3:2 orbital resonance with Neptune, Pluto is spherical, meaning it is large enough to have been pulled into a round shape by its own gravity. This makes it both a Kuiper Belt Object and a planet.