The Kuiper belt (to rhyme with "viper"), 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.
Since the first was discovered 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".
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs).
In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth hypothesised that, in the region beyond Neptune, the material within the primordial solar nebula was too widely spaced to condense into planets, and so rather condensed into a myriad of smaller bodies. From this he concluded that “the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system,” becoming what we call a comet.
In 1951, in an article for the journal Astrophysics, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, however, he did not believe that such a belt still existed today. Kuiper was operating on the assumption common in his time, that Pluto was the size of the Earth, and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. By Kuiper's formulation, there would not be a Kuiper belt where we now see it.
The hypothesis took many other forms in the following decades: in 1962, physicist Al G.W. Cameron postulated the existence of “a tremendous mass of small material on the outskirts of the solar system,” while in 1964, Fred Whipple, who popularised the famous "dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of Uranus that had sparked the search for Planet X, or at the very least, to affect the orbits of known comets. Observation, however, ruled out this hypothesis.
In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a blink comparator; the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before. In 1992, another object 5145 Pholus, was discovered in a similar orbit. Today, an entire population of comet-like bodies, the centaurs, is known to exist in the region between Jupiter and Neptune. The centaurs' orbits are unstable over periods longer than roughly 100 million years, a relatively short span when compared to the age of the Solar System. From the time of Chiron's discovery, astronomers speculated that they therefore must be frequently replenished by some outer reservoir.
Further evidence for the belt's existence later emerged from the study of comets. That comets have finite lifespans has been known for some time. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space, eating them gradually away. In order to still be visible over the age of the Solar System, they must be frequently replenished. One such area of replenishment is the Oort cloud; the spherical swarm of comets extending beyond 50 000 AU from the Sun first hypothesised by astronomer Jan Oort in 1950. It is believed to be the point of origin for long period comets, those, like Hale-Bopp, with orbits lasting thousands of years.
There is however another comet population, known as short period or periodic comets; those with orbits lasting less than 200 years. By the 1970s, the rate at which short-period comets were being discovered was becoming increasingly inconsistent with them having emerged solely from the Oort cloud. For an Oort cloud object to become a short-period comet, it would first have to be captured by the giant planets. In 1980, in the monthly notice of the Royal Astronomical Society, Julio Fernandez stated that for every short period comet to be sent into the inner solar system from the Oort cloud, 600 would have to be ejected into interstellar space. He speculated that a comet belt from between 35 and 50 AU would be required to account for the observed number of comets. Following up on Fernandez's work, in 1988 the Canadian team of Martin Duncan, Tom Quinn and Scott Tremaine ran a number of computer simulations to determine if all observed comets could have arrived from the Oort cloud. They found that the Oort cloud could not account for short-period comets, particularly as short-period comets are clustered near the plane of the Solar System, whereas Oort cloud comets tend to arrive from any point in the sky. With a belt as Fernandez described it added to the formulations, the simulations matched observations. Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named this region the "Kuiper belt.
In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System." He encouraged then-graduate student Jane Luu to aid him in his endeavour to locate another object beyond Pluto's orbit, because, as he told her, "If we don't, nobody will." Using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator. Initially, examination of each pair of plates took about eight hours, but the process was sped up with the arrival of electronic Charge-coupled devices or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90 percent of the light that hit them, rather than the ten percent achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for most astronomical detectors. In 1988, Jewitt moved to the Institute of Astronomy at the University of Hawaii. He was later joined by Jane Luu to work at the University of Hawaii’s 2.24 m telescope at Mauna Kea. Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly. Finally, after five years of searching, on August 30, 1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" ; Six months later, they discovered a second object in the region, 1993 FW.
Studies since the trans-Neptunian region was first charted have shown that in fact, the region now called the Kuiper belt is not the point of origin for short-period comets, but that they instead derive from a separate but linked population called the scattered disc. The scattered disc was created when Neptune migrated outward into the proto-Kuiper belt, which at the time was much closer to the Sun, and left in its wake a population of dynamically stable objects which could never be affected by its orbit (the Kuiper belt proper), and a separate population whose perihelia are close enough that Neptune can still disturb them as it travels around the Sun (the scattered disc). Because the scattered disc is dynamically active and the Kuiper belt relatively dynamically stable, the scattered disc is now seen as the most likely point of origin for periodic comets.
The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of the Pan-STARRS survey telescope, which should reveal many currently unknown KBOs, to determine more about this.
The Kuiper belt is believed to consist of planetesimals; fragments from the original protoplanetary disc around the Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than 3000 km in diameter.
Modern computer simulations show the Kuiper belt to have been strongly influenced by Jupiter and Neptune, and also suggest that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are believed to have formed closer to Jupiter, but to have been flung outwards during the course of the Solar System's early evolution. Work in 1984 by Fernandez and Ip suggests that exchange of angular momentum with the scattered objects can cause the planets to drift. Eventually, the orbits shifted to the point where Jupiter and Saturn existed in an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational pull from such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing them to switch places and for Neptune to travel outward into the proto-Kuiper belt, sending it into temporary chaos. As Neptune traveled outward, it excited and scattered many TNOs into higher and more eccentric orbits.
However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles, the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".
The presence of Neptune has a profound effect on the Kuiper belt's structure due to orbital resonances. Over a timescale comparable to the age of the Solar System, Neptune's gravity destabilises the orbits of any objects which happen to lie in certain regions, and either sends them into the inner Solar System or out into the Scattered disc or interstellar space. This causes the Kuiper belt to possess pronounced gaps in its current layout, similar to the Kirkwood gaps in the Asteroid belt. In the region between 40 and 42 AU, for instance, no objects can retain a stable orbit over such times, and any observed in that region must have migrated there relatively recently.
The classical Kuiper belt appears to be a composite of two separate populations. The first, known as "dynamically cold" population, has orbits much like the planets; nearly circular, with an orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). The second, the "dynamically hot" population, has orbits much more inclined to the ecliptic, by up to 30°. The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up. The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, is believed to have formed more or less in its current position although it may also have been later swept outwards by Neptune during its migration.
When an object's orbital period is an exact ratio of Neptune's (a situation called a mean motion resonance), then it can become locked in a synchronised motion with Neptune and avoid being perturbed away if their relative alignments are appropriate. If, for instance, an object is in just the right kind of orbit so that it orbits the Sun two times for every three Neptune orbits, then whenever it returns to its original position, Neptune will always be half an orbit away from it, since it will have completed 1½ orbits in the same time. This is known as the 2:3 (or 3:2) resonance, and it corresponds to a characteristic semi-major axis of about 39.4 AU. This 2:3 resonance is populated by about 200 known objects, including Pluto together with its moons. In recognition of this, the other members of this family are known as Plutinos. Many Plutinos, including Pluto, often have orbits which cross that of Neptune, though their resonance means they can never collide. Many others, such as 90482 Orcus and 28978 Ixion, are large enough to likely qualify as plutoids when more is known about them. Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune. The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7AU, and is sparsely populated. Its residents are sometimes referred to as twotinos. Minor resonances also exist at 3:4, 3:5, 4:7 and 2:5. Neptune possesses a number of trojan objects, which occupy its L4 and L5 points; gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are often described as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits and are unlikely to have been captured by Neptune, but rather to have formed alongside it.
Additionally, there is a relative absence of objects with semi-major axes below 39 AU which cannot apparently be explained by the present resonances. The currently accepted hypothesis for the cause of this is that as Neptune migrated outward, unstable orbital resonances moved gradually through this region, and thus any objects within it were swept up, or gravitationally ejected from it.
Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU; so this sudden drastic falloff, known as the "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown. Bernstein and Trilling et al. have found evidence that the rapid decline in objects of 100 km or more in radius beyond 50 AU is real, and not due to observational bias. Possible explanations include that material at that distance is too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those which did form. Patryk Lykawka of Kobe University has claimed that the gravitational attraction of an unseen large planetary object, perhaps the size of Earth or Mars, might be responsible.
Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as methane), ammonia, and water ice, a composition they share with comets. The temperature of the belt is only about 50K, so many compounds that would remain gaseous closer to the Sun are solid. Due to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine. The principal method by which astronomers determine the composition of a celestial object is spectroscopy. When an object's light is broken into its component colours, an image akin to a rainbow is formed. This image is called a spectrum. Different substances absorb light at different wavelengths, and when the spectrum for a specific object is unravelled, dark lines (called absorption lines) appear where the substances within it have absorbed that particular wavelength of light. Every element or compound has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine what it is made of.
Initially, such detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their colour. These first data showed a broad range of colours among KBOs, ranging from neutral grey to deep red. This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons. This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of their volatile ices to the effects of cosmic rays. Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing. However, Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in colour was too extreme to be easily explained by random impacts.
Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition. In 1996, Robert H. Brown et al obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of Pluto, as well as Neptune's moon Triton, possessing large amounts of methane ice.
Water ice has been detected in several KBOs, including 1996 TO66, 2000 EB173 and 2000 WR106. In 2004, Mike Brown et al determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs, 50000 Quaoar. Both of these substances would have been destroyed over the age of the solar system, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.
Despite its vast extent, the collective mass of the Kuiper belt is relatively low. The upper limit to the total mass is estimated at roughly a tenth the mass of the Earth, with some estimates placing it at a thirtieth an Earth mass. Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses. This missing >99% of the mass can hardly be dismissed, as it is required for the accretion of any KBOs larger than 100 km in diameter. At the current low density, these objects simply should not exist. Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent," resulting in destruction rather than accretion. It appears that either the current residents of the Kuiper belt have been created closer to the Sun or some mechanism dispersed the original mass. Neptune’s influence is too weak to explain such a massive "vacuuming". While the question remains open, the conjectures vary from a passing star scenario to grinding of smaller objects, via collisions, into dust small enough to be affected by solar radiation.
Bright objects are rare compared with the dominant dim population, as expected from accretion models of origin, given that only some objects of a given size would have grown further. This relationship N(D), the population expressed as a function of the diameter, referred to as brightness slope, has been confirmed by observations. The slope is inversely proportional to some power of the diameter D.
Less formally, there are for instance 8 (=2³) times more objects in 100–200 km range than objects in 200–400 km range. In other words, for every object with the diameter of 1000 km there should be around 1000 (=10³) objects with diameter of 100 km.
The law is expressed in this differential form rather than as a cumulative cubic relationship, because only the middle part of the slope can be measured; the law must break at smaller sizes, beyond the current measure.
Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects)
rect 646 1714 2142 1994 The Earth
circle 1786 614 142 (136472) Makemake
circle 2438 616 155 (136108) Haumea
circle 342 1305 137 (90377) Sedna
circle 1088 1305 114 (90482) Orcus
circle 1784 1305 97 (50000) Quaoar
circle 2420 1305 58 (20000) Varuna
Since the year 2000, a number of KBOs with diameters of between 500 and 1200 km (about half that of Pluto) have been discovered. 50000 Quaoar, a classical KBO discovered in 2002, is over 1200 km across. (originally , nicknamed "Easterbunny") and (originally , nicknamed "Santa"), both announced on 29 July 2005, are larger still. Other objects, such as 28978 Ixion (discovered in 2001) and 20000 Varuna (discovered in 2000) measure roughly 500 km across.
The issue was brought to a head by the discovery of Eris, an object in the scattered disc far beyond the Kuiper belt, that is now known to be 27 percent more massive than Pluto. In response, the International Astronomical Union (IAU), was forced to define a planet for the first time, and in so doing included in their definition that a planet must have "cleared the neighbourhood around its orbit. As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt.
Though Pluto is the largest KBO, a number of objects outside the Kuiper belt which may have begun their lives as KBOs are larger. Eris is the most obvious example, but Neptune's moon Triton, which, as explained above, is probably a captured KBO, is also larger than Pluto.
As of 2008, only five objects in the Solar System, Ceres, Pluto, Eris, Makemake and Haumea, are considered dwarf planets. However, a number of other Kuiper belt objects are also large enough to be spherical and could be classified as dwarf planets in the future.
The scattered disc is a sparsely populated region beyond the Kuiper belt, extending as far as 100 AU and farther. Scattered disc objects (SDOs) travel in highly elliptical orbits, usually also highly inclined to the ecliptic. Most models of solar system formation show both KBOs and SDOs first forming in a primordial comet belt, while later gravitational interactions, particularly with Neptune, sent the objects spiraling outward; some into stable orbits (the KBOs) and some into unstable orbits, becoming the scattered disc. Due to its unstable nature, the scattered disc is believed to be the point of origin for many of the Solar System's short-period comets.
According to the Minor Planet Center, which officially catalogues all trans-Neptunian objects, a KBO, strictly speaking, is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. Objects found outside the belt are classed as scattered objects. However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy planetoid native to the outer solar system believed to have been part of that initial class, even if its orbit during the bulk of solar system history has been beyond the Kuiper belt (e.g. in the scattered disk region). They often describe scattered disc objects as "scattered Kuiper belt objects. Eris, the recently discovered object now known to be larger than Pluto, is often referred to as a KBO, but is technically an SDO. A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved.
The centaurs, which are not normally considered part of the Kuiper belt, are also believed to be scattered objects, the only difference being that they were scattered inward, rather than outward. The Minor Planet Center groups the centaurs and the SDOs together as scattered KBOs.
During its period of migration, Neptune is thought to have captured one of the larger KBOs and set it in orbit around itself. This is its moon Triton, which is the only large moon in the Solar System to have a retrograde orbit; it orbits in the opposite direction to Neptune's rotation. This suggests that, unlike the large moons of Jupiter and Saturn, which are thought to have coalesced from spinning discs of material encircling their young parent planets, Triton was a fully formed body that was captured from surrounding space. Gravitational capture of an object is not easy; it requires that some force act upon the object to slow it down enough to be snared by the larger object's gravity. How this happened to Triton is not well understood, though it does suggest that Triton formed as part of a large population of similar objects whose gravity could impede its motion enough to be captured. Triton is only slightly larger than Pluto, and spectral analysis of both worlds shows that they are largely composed of similar materials, such as methane and carbon monoxide. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its outward migration.