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Kuiper Belt Objects

Kuiper Belt Objects


Kuiper belt objects (KBOs) are chunks of rock, dust and ice found in the area of the solar system just beyond the orbit of Neptune, starting at about 30 astronomical units (AU) to about 50 AU. (One AU is the average distance between Earth and the sun, about 93 million mi [150 million km].) In 1992, astronomers proposed that there must be at least 70,000 of these objects with diameters larger than 60 mi (100 km).

It is estimated that there are many more such bodies beyond 50 AU, but these are very small and faint bodies. Over 800 KPBs have been discovered, most of them after 1992. Observations do show that the majority of the known KBOs are found within a few degrees of the ecliptic, or the plane of the solar system, just like all the planets except the dwarf planet Pluto.

American astronomer Frederick C. Leonard (1896-1960) suggested the existence of this belt of objects in 1930. Later, in 1943, Irish astronomer Kenneth E. Edgeworth (1889-1972) did the same. In 1951, Dutch-American astronomer Gerard Peter Kuiper (1905-1973) theorized that this belt, or ring, around the solar system could be the source of comets having orbital periods of less than 200 years. Since then, this band of objects has been named the Kuiper belt after Kuiper. Dwarf planet Pluto and its moon Charon are composed of much more ice than the eight major planets, and orbit the sun in a much less circular orbit at a high inclination, or tilt (about 17°). Because of this information, and fact that their composition seems to resemble that of the KBOs, they have been called, by some, the largest known members of this class of objects. In the late 1990s there was even a heated debate among astronomers as to whether Pluto should be reclassified and called a KBO rather than a planet. One argument against this idea, that a minor solar system body (a KBO, an asteroid, or a comet) cannot have a moon had been disproved years before when small asteroids orbiting larger ones had been found. On August 24, 2006, members of the International Astronomical Union (IAU) passed a resolution to officially define a planet as any celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. Consequently, Pluto was disqualified from being a planet due to its highly elliptical orbit that overlapped Neptunes orbit. Instead, Pluto was recognized by the IAU as a dwarf planet a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

Years before Kuiper proposed the existence of these objects, Dutch astronomer Jan Oort (19001992) had noticed that orbital calculations revealed no comets arrived within detection range of Earth from outside the solar system. He also determined that many of them originally came from distances as close as just beyond the orbit of Pluto and as far away as a light-year from the sun.

Oort proposed a huge sphere of icy, rocky objects surrounded the solar system that became known as the Oort cloud. Occasionally, these chunks are pulled by the gravity of one of the planets into a new orbit, which brings it close enough to be observed as a comet from the Earth as its ice is warmed and evaporates, releasing gases and dust to form the well-known cometary tails. Most of the objects from the Oort cloud never come into the solar system at all, and still others probably leave the solar system due to the gravitational pull of nearby stars. The material in the Oort cloud appears to account for most long-period cometsthose with that take more than 200 years to orbit the sun. The Kuiper belt seems to be responsible for the shorter-period comets. Both groups of objects taken together make up the main body of leftover debris from the formation of the solar system. As such, especially when observed at the distances of Neptune and Pluto where they remain in their original frozen state, the study of these objects is very important to understanding the early phases of solar system formation.

Most of the observed KBOs remain far from Neptune, even at perihelion, their closest approach to the sun. These are called classical KBOs (CKBOs) because they follow nearly circular orbits, as do most of the planets. This is what would be expected if they formed with the rest of the solar system. Some KBOs have much larger and more elliptical, tilted orbits that have perihelion distances near 35 AU. These are called scattered kuiper belt objects (SKBOs), the first of which was discovered in 1996. Three more were discovered in wide field scans of the solar system in 1999 and others have been discovered as improved technology allows astronomers to probe larger areas of the sky in ways that allow fainter objects to be seen.

Since the SKBOs reach perihelion distances that are smaller than those of CKBOs, the gravitational pull of Neptune can deflect them into new orbits. This can have the same effect as the outer planets do on Oort cloud objects. It can send them into the inner solar system where they are eventually classified as comets. Other possibilities are that their orbits can change in a way that they remain in the distant reaches near Neptune but in the elliptical, tilted orbits that define them as scattered, or they are ejected into the Oort cloud or out of the solar system into interstellar space. The SKBOs seem to form a fat doughnut that surrounds the classical KBOs in their flatter ring-like region, extending a little closer and also to much larger distances from the sun.

Both the Hubble Space Telescope (HST) and ground-based observatories have detected these populations of comet-like material at the cold fringe of the solar system. Scientists now know, conclusively, that the solar system does not end at Neptune and Pluto. Obviously, the larger objects are easier to find and just as there are more pebbles than boulders on a beach, it is expected that many millions, billions, or even trillions of much smaller objects exist than may ever be found in the Kuiper Belt and the Oort cloud. Detecting even the larger bodies in their distant icy state, at the dim edge of the solar system, pushed Hubble Space Telescope to its limits. One astronomer compared it to trying to see something the size of a mountain, draped in black velvet, located four billion mi (6.4 billion km) away.

The recent discovery of the Kuiper belt and the even newer information about the number and distribution of the objects in it fueled an interest in the possibility of using the New Horizons (also called the Pluto-Kuiper Belt Mission) spacecraft, already scheduled to arrive at Pluto in the summer of 2015 (as of September 2006), to also explore this region of the solar system. Observations of Kuiper Belt Objects


Astronomical unit The average distance between the sun and the Earth. One astronomical unit, symbol AU, isequivalentto92.9 million mi (149.6 million km).

Comet An object usually seen in the inner solar system that results when a dusty, rocky chunk of ice left over from the formation of the solar system moves close enough to the sun that its ices evaporate. The resulting release of gases and dust surrounds the original object with a cloud called the coma and a tail that can extend for 100 million miles (161 million km) across space. This creates the sometimes spectacular objects observed from the Earth known as comets.

Ecliptic In the sky, the ecliptic is the apparent path of the sun against the star background, due to the Earth orbiting the sun. The term ecliptic plane is used to describe the average location in space of the orbits of the planets of the solar system, except dwarf planet Pluto that orbits the sun at a 17° angle to the others.

Inclination The orbital tilt of a planet or other object in the solar system. The eight planets with very low inclinations to the eclipitic plane. Dwarf planet Plutos orbit has a 17° tilt or inclination.

Infrared Spectral Mapper A device used to detect heat (frequencies lower than visible light) and map the intensity of the radiation received in order to determine chemical processes at work in the object being observed.

Light year A unit of measure used between stars and galaxies. A light year is the distance that light travels (at about 186,272 mi or 300,000 km per second) in one year. One light year is equal to about six trillion miles (9.6 trillion km).

Perihelion The closest approach of an object to the sun in its orbit.

Ultraviolet spectrometer A device that receives, and breaks into its component frequencies, electromagnetic radiation in the region above (of higher frequency) than visible light. The spectrometer splits up the received energy allowing analysis of chemical elements and processes that caused the radiation.

Voyager spacecraft A pair of unmanned robot spacecraft that left the Earth in 1977 to fly by all the gas giant planets (Jupiter, Saturn, Uranus, and Neptune).

past Pluto should occur approximately between 2016 and 2020. The main scientific reason for attempting KBO flybys is the opportunity to explore a whole new region of the planetary system. The mounting evidence that the Kuiper belt is a region where planet-building processes ended is also an intriguing aspect of solar system evolution to study. The opportunity to study comet nuclei that have been undisturbed by the warming influence of the sun is an additional important goal for the mission. Such study may reveal many secrets about the formation of the sun and planets.

The New Horizons spacecraft is well suited for flybys of KBOs because, since the composition of Pluto is similar and the planet is also out on the dim, cold edge of the solar system, the scientific instrument packages already installed can adequately observe them. The high-resolution instruments aboard New Horizons should be able to provide detailed information about many aspects of these as yet mysterious objects. Maps obtained even from many tens of thousands of miles away would have a feature resolution of a few miles across. This would provide geological and color information about of surface features. Not only will it be the first spacecraft from Earth to observe Pluto at close range, but the only one to travel to this distant region of the solar system since Voyagers 1 and 2 crossed the distance of Plutos orbit in the early 1990s.

In late 2005, an astronomy team used the Hubble Space Telescope to discover two tiny moons orbiting Pluto. They were temporarily designated S/2005 P1 and S/2005 P2 but have since then been approved by the International Astronomical Union to be called Nix and Hydra, respectively. In the tradition of naming bodies in the far reaches of the solar system after underworld characters, the team originally chose the figures Nyx, the Greek goddess of the night (and mother of Charon), and Hydra, a nine-headed monster that guarded the entrance to the underworld. (The team altered one name after finding that Nyx was already a name for a small asteroid.)

Nix and Hydra are estimated to be between 30 and 125 mi (48 and 200 km) in diameter and about 27,000 mi (44,000 km) away from Plutoabout two to three times further away than Charon. Hydra is the further out of the two satellites. The two new moons appear to move counterclockwise around Pluto, as does Charon. When viewed with cameras onboard Hubble, the moons are about 5,000 times fainter than Pluto. However, scientists conclude that these numbers are currently only rough estimates. With the confirmation of these newly discovered bodies, astronomers are more confident in being able to learn more about the nature of Pluto and its origin. Based on this discovery, and knowing that Pluto resides within the heart of the Kuiper Belt, astronomers are already speculating that many larger bodies within the Kuiper Belt may have several moons orbiting them like Pluto.

Since 2000, many KBOs have been discovered, some larger and some smaller than Pluto and Charon. In 2002, 5000 Quaoar was discoveredand found to be half the size of Pluto. In July 2005, 136108-2003EL61 and 136472-2005FY9 were found to be larger than 5000 Quaoar. When Pluto was downgraded to a dwarf planet in 2006, the larger of the KBOs were re-classified as dwarf planets, since they have similar sizes to Pluto.



Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Natures Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Brandt, John C. Introduction to Comets. Cambridge, UK, and New York: Cambridge University Press, 2004.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Davies, John Keith. Beyond Pluto: Exploring the Outer Limits of the Solar System. Cambridge, UK, and New York: Cambridge University Press, 2001.

Fernandez, Julio A. Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance. Dordrecht, Netherlands: Springer, 2005.

Wickramasinghe, Chandra. Cosmic Dragons: Life and Death on Our Planet. London, UK: Souvenir, 2001.


Jewitt, David, Institute for Astronomy, University of Hawaii. Kuiper Belt Page. <> (accessed October 14, 2006).

Clint Hatchett

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