Salem Press

Solar System
Kuiper Belt

Category: Small Bodies

The observation of object 1992 QB₁ led to the discovery of a vast, previously unknown region of the solar system, the Kuiper belt, which is thought to be the source of most short-period comets.

Overview
Two lines of evidence led scientists to suppose that the Kuiper belt exists. First, comets that come close enough to the Sun to grow spectacular tails are melting, and so they cannot last forever. Furthermore, if they approach too close to the Sun (or the gas giant Jupiter), gravity may pull them apart, as it did Comet Shoemaker-Levy 9, whose life ended when it impacted Jupiter in 1994. A comet can survive only a limited number of close approaches to the Sun, so why, scientists wondered, were there still so many comets? Where did they come from?

The basic answer is the Oort cloud, a hollow, spherical cloud of comets with the Sun at its center, extending nearly halfway to neighboring stars. It is generally accepted that long-period comets (those with periods greater than two hundred years) come from the Oort cloud, and that, if they come close enough to Jupiter to be affected by Jupiter's gravity, some can become short-period comets (those with periods less than two hundred years).

Another feature of comets is that long-period comets come toward the Sun from all directions; that is, their orbital planes may make large angles with the ecliptic plane, the plane of the Earth's orbit around the Sun. It is called the ecliptic because when the Moon is also in this plane, an eclipse can occur. The orbits of long-period comets can be explained if they come from the Oort cloud, and if the Oort cloud is spherical. The orbits of short-period comets, however, are more nearly in the ecliptic plane, so their source ought therefore to lie more nearly in the ecliptic plane.

A second line of evidence for the Kuiper belt comes from modeling the formation of the solar system. There should have been 40 or 50 Earth masses of icy-rocky material beyond Neptune. What happened to that material? In 1949, Kenneth Edgeworth speculated that a reservoir of comets existed beyond the orbit of Neptune. In 1951 Gerard Kuiper reasoned that there should have been large amounts of icy material beyond Pluto and that Pluto's gravity should stir up the icy bodies that formed there, flinging some of them sunward as short-period comets and flinging others into interstellar space. It was thought then that Pluto's mass was far larger than it actually is.

In 1980 Julio Fernández published a paper that used the words "Kuiper" and "comet belt" in the first sentence. Subsequent researchers combined the words, and the term "Kuiper belt" was born. According to Fernández, a belt of potential comets lies beyond the orbit of Neptune in the ecliptic plane. This belt provides the short-period comets and repopulates the Oort cloud. Fernández estimated that the Oort cloud lost three hundred comets per year as they were flung inward toward the Sun or outward to interstellar space. Therefore, the Oort cloud could not endure for the life of the solar system unless it was being repopulated.

Pluto's strange properties offer another clue that there ought to be a Kuiper belt. In 1977 James Christy discovered the large satellite of Pluto subsequently named Charon. Using Charon's orbital period and its separation from Pluto, scientists could calculate the masses of both Pluto and Charon. Pluto's mass is 0.0021 Earth mass, only one-sixth the mass of Earth's Moon. Charon's mass is 0.0003 Earth mass. Pluto's orbit does not lie in or near the ecliptic plane, as do the orbits of the other planets. Charon was most likely formed in a collision between Pluto and another object perhaps one-fifth the size of Pluto. For this to have any likelihood, there must have been many such objects present, far more than seem to be around today.

The discovery of the Kuiper belt object (KBO) 2003 UB₃₁₃, later named Eris, caused astronomers to reevaluate the definition of a planet, since Eris is about 25 percent larger than Pluto. A parallel situation occurred when the first asteroids were discovered and included in the list of planets until it was decided that they belonged to a different class of objects. Likewise, Pluto is not a planet, but the first of the KBOs to be discovered. Pluto and Eris are the largest of the known KBOs and are now classed as dwarf planets, along with the largest asteroid, Ceres.

Also in 1977, Charles Kowal discovered an object that was eventually named Chiron, for the mythological centaur of that name. Chiron is estimated to be 170 kilometers (106 miles) in diameter and orbits between Saturn and Uranus. Apparently Chiron is a comet, because on various occasions it has produced a coma (the large vapor cloud typically identified as the "tail" of a comet). About a hundred such objects have been found with orbits that cross the orbit of at least one of the giant planets. All such objects are referred to as centaurs, and at least three of them have produced comas. Because of gravitational tugs from the giant planets, centaur orbits are unstable over periods of millions of years, so the centaurs now seen must have migrated to their present locations during the past 10 million years. The centaurs will eventually either crash into one of the giant planets or migrate to stabler orbits.

Knowledge Gained
The centaurs and KBOs can be grouped by color: Some are red, and others are blue-gray. The red color can arise as cosmic rays and solar ultraviolet rays strike carbon compounds on an object's icy surface to form reddish-brown compounds. The blue-gray color may arise as smaller objects strike and crater a KBO, covering its surface with new layers of ices. Based on analyses of comets, KBOs must be largely water ice along with carbon dioxide ice, carbon monoxide ice, methane ice, methanol ice, ammonia ice, amorphous (noncrystalline) carbon, silicates and other stony materials, sodium, carbonates, simple hydrocarbons, and clays. The Deep Impact mission found that Comet 9P/Temple was covered by a dust layer tens of meters deep and the nucleus of the comet was 75 percent empty space, which made it structurally weak. The minor constituents in particular may differ from one comet to another, implying that comets formed at various distances from the Sun.

Experimental evidence of the Kuiper belt came with the 1992 discovery by Jane Luu and David Jewitt of object 1992 QB₁. It was 41 AU from the Sun, which is about the orbit of Pluto. They estimated its diameter at 250 kilometers (155 miles). Six months later they found 1993 FW, a second candidate for the Kuiper belt. By early 2008, more than one thousand KBOs had been discovered. The main belt is shaped like a doughnut centered on the Sun. The doughnut itself goes from 30 AU from the Sun to perhaps 100 AU or even 1,000 AU or more, although there is a sharp falloff in numbers at 50 AU. Objects in the belt are called CKBOs, for classical Kuiper belt objects, also called cubewanos, named for the pronunciation of the first CKBO discovered, 1992 QB₁ (Q-B-1-0-s). In order not to be influenced by Neptune at 30 AU from the Sun, these objects must have an average distance from the Sun of 40 and 50 AU.

A large fraction of known KBOs are plutinos. Like Pluto, plutinos make two trips around the Sun for every three trips made by Neptune. This guarantees that if they are not now close to Neptune, they will not be close in the future, and therefore their orbits are stable. Since they never get close to Neptune, they may approach to within 30 AU of the Sun with impunity. Pluto never comes closer to Neptune than 17 AU, even though Pluto's orbit crosses Neptune's orbit.

The final group of Kuiper belt objects are scattered disk objects, or SDOs. Their orbits tend to be more elliptical than the classical Kuiper belt objects' orbits, and they also travel considerably above and below the ecliptic plane. Scattered disk objects and centaurs seem to form a continuous distribution. If a KBO is ejected to the inner solar system, it becomes a comet or a centaur. It is estimated that there may be 70,000 KBOs between 30 and 50 AU with diameters greater than 100 kilometers (60 miles), and perhaps 1,000,000 with diameters of 1 kilometer (0.6 mile) or larger. There may be as many as 30,000 SDOs that are 100 kilometers (62 miles) in diameter.

Context
It is believed that stars like our Sun form from a more or less spherical cloud of gas and dust within a larger cloud. When conditions are right for one star to form, they are often right for many stars to form; hence, stars, like puppies, tend to be born in litters. The close approach of one or more neighboring stars may have sheared off the Sun's Kuiper belt at about 50 AU from the Sun. Solid material left over from the formation of the Sun would have developed into a rotating disk with the newborn Sun at its center. Concentrations in the disk formed into asteroid-sized bodies that in turn combined to form the planets. Icy asteroids would have formed among the giant planets and beyond. Those beyond Neptune became the Kuiper belt. In the inner belt, from about 30 AU to perhaps 50 AU, collisions occasionally occurred with the result that large objects grew larger (such as Pluto and Eris) and small objects were eventually ground to dust. Beyond 50 AU, objects (if they exist) should be pristine samples of the solar nebula, since they would be so far apart that collisions would be unlikely.

As the newly formed planets interacted with icy asteroids around them, some icy bodies would have been flung inward toward the Sun, and some of these may have hit the Earth and formed the oceans; the Earth was probably born with far less water than it now has. Others were flung outward to form the Oort cloud and perhaps scattered disk objects. Interactions between KBOs today should occasionally propel them out into the Oort cloud or inward as comets in the inner solar system or as centaurs in the outer solar system.

Some questions about Pluto and other KBOs may be answered by the New Horizons mission, sponsored by the National Aeronautics and Space Administration (NASA). Launched in 1995, it should reach Pluto by 2015. After taking images in the visible, infrared, and ultraviolet bands of Pluto and its satellite Charon, the spacecraft will go on to one or more KBOs. The infrared images should reveal the surface composition of these objects.

Charles W. Rogers

Further Reading
Davies, John. Beyond Pluto: Exploring the Outer Limits of the Solar System. New York: Cambridge University Press, 2001. Excellent and easy-to-read discussion of the ideas and observational evidence leading up to the discovery of the Kuiper belt. Describes the properties of the various classes of objects found.

Kaufmann, William J., III. Universe. 8th ed. New York: W. H. Freeman, 2007. College-level introductory text covering the field of astronomy. Contains descriptions of astrophysical questions and their relationships.

Lin, Douglas N. C. "The Chaotic Genesis of Planets." Scientific American 278, no. 5 (May, 2008): 50-59. Discusses the formation of the solar system, including the formation of the Kuiper belt and the Oort cloud.

Luu, Jane X., and David C. Jewitt. "The Kuiper Belt." Scientific American 274, no. 5 (May, 1996): 46-52. This landmark article is accessible to layperson. Luu and Jewitt discuss how they found KBOs and helped to establish the existence of the Kuiper belt.

Malhotra, Renu. "Migrating Planets." Scientific American 281, no. 3 (September, 1999): 56-63. During the formation of the solar system, Jupiter migrated inward, while Saturn, Uranus, and Neptune migrated outward by slinging icy bodies to the inner solar system and to the Kuiper belt and Oort cloud.

Stern, S. Alan. "Into the Outer Limits." Astronomy 28, no. 9 (September, 2000): 52-55. Discusses history of the Kuiper belt and how Pluto fits as a KBO.

See Also
Comets; Dwarf Planets; Oort Cloud; Planetary Classifications.