KUIPER BELT

The Kuiper Belt is a disk-shaped region past the orbit of Neptune extending roughly from 30 to 50 AU from the Sun containing many small icy bodies. 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").

Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."^[76] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.^[77]

Kuiper Belt Facts:

Kuiper belt is the most recently observed section of the Solar System.

Kuiper belt extends from Neptune's orbit to three billion kilometers beyond it.

Contains lumps of icy material with organic compounds; makes them like comets. Called "Kuiper belt objects" (KBO's) or minor planets.

Kuiper belt may have formed when the gravity of the young Jupiter sent the Kuiper belt objects out to where they are now.

Scientists consider Pluto to be one of the largest Kuiper belt objects. It is 2390 km across.

After Pluto, next largest known Kuiper belt objects are Orcus, which is about 1600 km (1,000 miles) across, 2003 EL61 at 70% the size of Pluto and 2005 FY9 at 50% to 70% of Pluto's size.

Recently, scientists found an object, 2003 UB313, that is bigger than Pluto. The scientists don't know its exact size, but the estimate it is about 20% larger than Pluto. At the time it was discovered, it was almost 100 times further away from the Sun than Earth is. It can come about as close to the Sun as Pluto is. Its orbit is tilted almost 45 degress compared to Earth's orbit. Pluto's orbit is tilted by only 17 degrees. Some people are calling it the tenth planet. Others don't think that 2003 UB313 is a planet and don't think that Pluto should be considered one either.

Other large Kuiper belt objects about or over 1000 km across are Pluto's moon Charon, Quaoar, Varuna, Ixion, 1996 TL66, 2002 TX300, 2002 TC302, 2002 UX25 and 2002 AW197. Ceres, the largest asteroid, is about 950 km across.

By 2004, over 800 Kuiper belt objects had been found. Scientists think that there are many more yet to be discovered, and some might be as big as Earth (though much colder).

After the first object in the belt was spotted from the Mauna Kea Observatory in Hawaii, the belt was named after the astronomer Gerard Kuiper, who, in 1951, wrote that he thought it had existed very long ago.

Astronmomers including Frederick Leonard, Kenneth Edgeworth, and Julio Fernandez thought that the belt still existed; and some astronomers call it the Edgeworth-Kuiper belt.

When an object is discovered in space, it is given a temporary name called a "provisional designation". This temporary name begins with the year the object was discovered, followed by some letters and numbers that tell in what month and in what order it was discovered. Later on, important objects are given other names, often from mythology.

Orcus, Charon, and Varuna were named after mythological gods of the underworld. Ixion was named after a mythological person in the underworld. Quaoar was named after a creation god of the Native American Tongva people.

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