The Kuiper belt is a region between about 30 AU and 50 AU—just outside of the orbit of Neptune (30 AU)—that contains numerous small planetoids composed of ice and rock. The largest and best-known Kuiper Belt objects are Pluto and it's binary companion Charon. While Pluto was discovered in 1930, it is only since the early 1990s that other members of the Kuiper belt have been discovered and studied.
So, why is the Kuiper belt so interesting, if they are nothing more than distant chunks of ice? The three principal reasons are that the Kuiper belt is a remnant of the original solar accretion disk that gave birth to the planets, the current dynamics of the Kuiper belt is a consequence of the evolution of Neptune and Uranus, and the objects within the Kuiper belt are related to a number of objects, such as the Centaur asteroids in the region between Uranus and Neptune, Neptune's moon Triton, and the comets.
The Kuiper Belt objects fall into three classes based on their orbits: classical objects, resonant objects, and scattered objects. Roughly speaking, the classical objects are objects in relatively undisturbed orbits, the resonant objects are in orbits that are tied to Neptune's orbit, and the scattered objects are believed to be objects scattered to a high semimajor axis by Neptune and Uranus early in the history of the Solar System.
The classical Kuiper Belt objects are in orbits that have a uniform distribution of semimajor axes. The range of orbital eccentricities and inclinations is broad, which is not what one expects if the current orbits are the orbits with which the objects were born. The range of observed eccentricities is from 0 to 0.2, with most have an eccentricity less than 0.1, and the range of observed inclinations is from 0° to 35°, with most having an inclination less than 10°. The distribution in inclination is thought to be in part a selection effect, because most of the observations that can find Kuiper Belt objects are confined to the ecliptic. The classical objects account for about 45% of all Kuiper Belt objects.
The theory for the classical Kuiper belt objects is that they began life in orbits with low eccentricities and inclinations, but that with time, collisions among themselves changed their inclinations and eccentricities. Effectively, the collisions “heated” the Kuiper belt objects, so that the belt acquired over time a large velocity dispersion. This can be regarded as an increase in entropy of the Kuiper belt, the belt evolved from a well-ordered state to a more disordered state. This process is similar to the processes now occurring in the rings of Saturn.
A small number of Kuiper Belt objects are classified as scattered objects. These objects are highly eccentric, and they have a broad distribution of semimajor axes, but most have perihelions that are outside of the orbit of Neptune. The eccentricities range from 0.2 to over 0.6, and the semimajor axes range from less than 50 AU to about 120 AU. The scattered objects are estimated to account for 35% of all Kuiper Belt objects.
The theory behind the scattered objects is that early in the history of the Solar System, planetoids with semimajor axes in the range of 20 to 30 AU were scattered by Uranus and Neptune into larger orbits; some of these objects were given extremely large orbits that sent them into the Oort cloud, the hypothetical shell of bodies that give rise to the comets, while others were given orbits that place them in or just outside of the Kuiper Belt. This second group comprises the scattered Kuiper Belt objects.
The orbits of a large number of Kuiper Belt objects are harmonious with Neptune's orbit. The strongest resonance that appears is the 3:2 resonance; a Kuiper Belt object in this orbit completes two revolutions of the Sun in the time it takes Neptune to complete three. The 3:2 resonance corresponds to a semimajor axis of 39.4 AU (this relationship between orbital period and semimajor axis is a consequence of one of Kepler's laws of planetary motion). Objects in this resonance account for 25% of all known Kuiper belt objects, and estimates place the true number of objects at about 15% of all Kuiper belt objects. Other resonances in order of their importance are the 2:1 resonance (47.7 AU), the 4:3 resonance (36.4 AU), and the 5:3 resonance (42.3 AU). Objects in these remaining resonances are estimated to account for 5% of the remaining objects. Pythagoras would have been thrilled: these Kuiper Belt objects dance to the music of the spheres.
The eccentricities of the resonant orbits range from zero to 0.35, which make these orbits much more eccentric than the orbits of the classical objects. Their inclination angles are large, in excess of 25° in some instances, but these are in line with the inclination angles of the classical objects.
Many of the 3:2 objects have perihelions that are inside the orbit of Neptune; most of these objects, however, are in stable orbits that prevent them from ever getting close enough to Neptune to be scattered by Neptune. Among these objects is the Pluto-Charon system.
Theory ties the populating of the resonances with Neptune's drift over the past 4.5 billion years away from the Sun. In the early life of the solar system, it is thought that large numbers of planetoids existed in orbits near the four giant planets. Computer simulations show that over time these planetoids scattered with the giant planets, causing Neptune, Uranus, and Saturn to drift away from the Sun, and causing Jupiter to drift closer to the Sun. This is a variant on the “slingshot effect,” where an interplanetary spacecraft changes the energy of its orbit by passing close to Earth, Venus, or Jupiter. By scattering planetoids, Neptune, Uranus, and Saturn acquire energy and Jupiter loses energy; the planetoids are scattered out of the Solar System or into the Oort cloud. It is estimated that Neptune increased its semimajor orbit from about 22 AU to the current 30 AU. During this process, some planetoids were trapped in the orbital resonances, and they were carried out to larger semimajor axis with Neptune.
The pinball-like interaction of Kuiper objects with each other and with the giant planets led to a gradual erosion of the Kuiper belt and to the populating of other classes of object. The Kuiper belt is estimated to have about 0.1 times the mass of Earth, but theoretical studies suggest that early in its life, its mass was 100 times this value. The inner edge was depleted through scattering with the giant planets. Farther out, collisions among the Kuiper belt objects pulverized these objects to dust that then drifted inward towards the Sun. Finally, observations strongly suggest that the belt is truncated at about 50 AU; one theory for this is that a close encounter with a star early in the Sun's life disrupted the disk outside this radius.
1 This article is based on nice review of Kuiper Belt Objects can be found in the 2002 issue of Annual Reviews of Astronomy and Astrophysics. Luu, Jane X., and Jewitt, David C., “Kuiper Belt Objects: Relics from the Accretion Disk of the Sun.” In Annual Reviews of Astronomy and Astrophysics, edited by Geoffrey Burbidge, Allan Sandage, and Frank H. Shu, vol. 40. Palo Alto, California: Annual Reviews, 2002.