Known objects in the Kuiper belt, derived from data from the Minor Planet Center. The Minor Planet Center operates at the Smithsonian Astrophysical Observatory (SAO which is part of the Center for Astrophysics (CfA along with the Harvard Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known Trojan asteroids are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as centaurs. The scale is in astronomical units. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the

The Kuiper belt (pronounced /ˈkaɪpɚ/, to rhyme with "viper"),^[1] 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. As of May 2008, there are six known Neptune trojans (named by analogy to the Trojan asteroids which have the same orbital period as Neptune. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune In Astronomy, a resonant trans-Neptunian object is a Trans-Neptunian object (TNO in mean motion Orbital resonance with Neptune In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, The Solar System consists of the Sun and those celestial objects bound to it by Gravity. In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The Sun (Sol is the Star at the center of the Solar System. ^[2] It is similar to the asteroid belt, although it is far larger; 20 times as wide and 20–200 times as massive. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. ^[3][4] Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System's formation) and at least one dwarf planet – Pluto. Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded 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. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across In Planetary science, volatiles, are that group of elements and compounds with low boiling points (see volatile) that are associated with a planet's or moon's Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Water is a common Chemical substance that is essential for the survival of all known forms of Life.

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. ^[5] The Kuiper belt was initially believed to be the main repository for periodic comets, those with orbits lasting less than 200 years. Periodic comets are defined for these purposes as those Comets having orbital periods of less than 200 years (also known as "short-period comets" or which 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. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered 5 billion years ago, that is their true place of origin. ^[6] 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 scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The centaurs, comet-like bodies that orbit among the gas giants, are believed to originate there. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily Neptune's moon Triton is believed to be a captured KBO. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek ^[7] Pluto, a dwarf planet, is the largest known member of the Kuiper belt. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded Originally considered a planet, it is similar to many other objects of the Kuiper belt, and its orbital period is identical that of the KBOs known as "Plutinos". In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune.

The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, 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). The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. ^[8]

History

Since the discovery of Pluto, many have speculated that it might not be alone. The region now called the Kuiper belt had been hypothesized in various forms for decades. It was only in 1992 that the first direct evidence for its existence was found. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it.

The first astronomer to suggest the existence of a trans-Neptunian population was Frederick C. Leonard. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena Frederick Charles Leonard ( March 12, 1896 - June 23, 1960) was an astrophysicist at the UCLA who organised the UCLA's Department of Astronomy In 1930, soon after Pluto's discovery, he pondered whether it was "not likely that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected". ^[9]

Subsequent hypotheses

Astronomer Gerard Kuiper, after whom the Kuiper belt is named

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. Gerard Peter Kuiper ( born Gerrit Pieter Kuiper ( ( December 7 1905, Harenkarspel ( Tuitjenhorn) Netherlands &ndash Kenneth Essex Edgeworth DSO, MC ( 26 February 1880 – 10 October 1972) was an Irish Astronomer, Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. The formation and evolution of the Solar System is estimated to have begun 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"^[10] 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,”^[11] becoming what we call a comet. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail —

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. Gerard Peter Kuiper ( born Gerrit Pieter Kuiper ( ( December 7 1905, Harenkarspel ( Tuitjenhorn) Netherlands &ndash 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. ^[12]

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,”^[13] 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. Alastair G W (Graham Walter Cameron ( 1925 - October 3, 2005) was a Canadian astrophysicist and space scientist who was an eminent staff member of the Fred Lawrence Whipple ( November 5, 1906 – August 30, 2004) was an American Astronomer. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[14] Observation, however, ruled out this hypothesis. ^[13]

In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. TemplateInfobox Planet. --> 2060 Chiron (, or as in Χείρων) is a Planetoid in the He used a blink comparator; the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before. A blink comparator was a viewing apparatus used by Astronomers to find differences between two photographs of the night sky shot using Optical telescopes Clyde William Tombaugh ( February 4, 1906 &ndash January 17, 1997) was an American Astronomer. ^[15] In 1992, another object 5145 Pholus, was discovered in a similar orbit. TemplateInfobox Planet. --> 5145 Pholus (ˈfoʊləs foe'-ləs from Φόλος) is a ^[16] 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. ^[17]

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. Volatility in the context of Chemistry, Physics and Thermodynamics is a measure of the tendency of a substance to Vaporize. In order to still be visible over the age of the Solar System, they must be frequently replenished. ^[18] 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. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the Jan Hendrik Oort ( April 28, 1900 &ndash November 5, 1992) was a Dutch Astronomer. ^[19] It is believed to be the point of origin for long period comets, those, like Hale-Bopp, with orbits lasting thousands of years. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — Comet Hale-Bopp ( formally designated C/1995 O1) was probably the most widely observed Comet of the twentieth century and one of the brightest

There is however another comet population, known as short period or periodic comets; those with orbits lasting less than 200 years. Periodic comets are defined for these purposes as those Comets having orbital periods of less than 200 years (also known as "short-period comets" or which Periodic comets are defined for these purposes as those Comets having orbital periods of less than 200 years (also known as "short-period comets" or which 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. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, ^[20] 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. The Royal Astronomical Society (RAS is a Learned society that began as the Astronomical Society of London in 1820 to support astronomical research Dr Julio Ángel Fernández Alves (born Montevideo, 5 April 1946) is a Uruguayan astronomer and member of the department of astronomy at the He speculated that a comet belt from between 35 and 50 AU would be required to account for the observed number of comets. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the ^[21] 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. Scott Duncan Tremaine (born 1950) is a Canadian -born astrophysicist. 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. ^[22] Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named this region the "Kuiper belt. "^[23]

Discovery

The array of telescopes atop Mauna Kea, with which the Kuiper belt was discovered

In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System. Mauna Kea is a Dormant volcano in the Hawaiian Islands, one of five volcanoes which together form the Island of Hawaii. David C Jewitt is a Professor of Astronomy at the University of Hawaii Institute for Astronomy. "^[24] 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. Jane Luu (aka Jane X Luu) is a Vietnamese American Astronomer. "^[25] 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. The Kitt Peak National Observatory ( KPNO) is a United States astronomical Observatory located on a 2096 m (6880 ft peak of the The State of Arizona ( is a state located in the southwestern region of the United States. Chile, officially the Republic of Chile ( Spanish:) is a country in South America occupying a long and narrow Coastal strip wedged between the A blink comparator was a viewing apparatus used by Astronomers to find differences between two photographs of the night sky shot using Optical telescopes ^[25] Initially, examination of each pair of plates took about eight hours,^[26] 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. A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors Today, CCDs form the basis for most astronomical detectors. ^[27] In 1988, Jewitt moved to the Institute of Astronomy at the University of Hawaii. The University of Hawaii, formally the University of Hawaii System and popularly known as UH, is a public co-educational college and university system that confers He was later joined by Jane Luu to work at the University of Hawaii’s 2. 24 m telescope at Mauna Kea. ^[28] Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly. ^[29] Finally, after five years of searching, on August 30, 1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" (15760) 1992 QB₁;^[24] Six months later, they discovered a second object in the region, 1993 FW. Events 1363 - Beginning date of the Battle of Lake Poyang; the forces of two Chinese rebel leaders— Chen Youliang and Year 1992 ( MCMXCII) was a Leap year starting on Wednesday (link will display full 1992 Gregorian calendar) ^[30]

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 (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered 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). Planetary migration occurs when a Planet or other stellar satellite interacts with a disk of gas or Planetesimals resulting in the alteration of the satellite's orbital 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. ^[6]

Name

Astronomers will sometimes use alternative name Edgeworth-Kuiper belt to credit Edgeworth, and KBOs are occasionally referred to as EKOs. However, Brian Marsden claims neither deserve true credit; "Neither Edgeworth or Kuiper wrote about anything remotely like what we are now seeing, but Fred Whipple did. Brian Geoffrey Marsden (born 5 August 1937 is a British Astronomer, the longtime director of the Minor Planet Center (MPC at the Harvard-Smithsonian Center Fred Lawrence Whipple ( November 5, 1906 – August 30, 2004) was an American Astronomer. "^[31] Conversely, David Jewitt comments that, "If anything . . . Fernandez most nearly deserves the credit for predicting the Kuiper Belt. "^[12] The term trans-Neptunian object (TNO) is recommended for objects in the belt by several scientific groups because the term is less controversial than all others — it is not a synonym though, as TNOs include all objects orbiting the Sun at the outer edge of the Solar System, not just those in the Kuiper belt. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. This article deals with the general meaning of the term "synonym"

Origins

Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter

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. Pan-STARRS (an Acronym for Panoramic Survey Telescope And Rapid Response System is a planned astronomical survey that will conduct Astrometry and Photometry ^[3]

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. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star The Sun (Sol is the Star at the center of the Solar System.

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. A computer is a Machine that manipulates data according to a list of instructions. Simulation is the imitation of some real thing state of affairs or process Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. 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. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position ^[32] 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. ^[33] As Neptune traveled outward, it excited and scattered many TNOs into higher and more eccentric orbits. ^[34]

However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles,^[35] the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".

Structure

At its fullest extent, including its outlying regions, the Kuiper belt stretches from roughly 30 to 55 AU. However, the main body of the belt is generally accepted to extend from the 2:3 resonance (see below) at 39. 5 AU to the 1:2 resonance at roughly 48 AU. The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the ecliptic plane and a more diffuse distribution of objects extending several times farther. The ecliptic is the apparent path that the Sun traces out in the sky during the year Overall it more resembles a torus or doughnut than a belt. In Geometry, a torus (pl tori) is a Surface of revolution generated by revolving a Circle in three dimensional space about an axis Coplanar ^[36] Its mean position is inclined to the ecliptic by 1. 86 degrees. ^[37]

Orbit classification (schematic of semi-major axes). In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae

The presence of Neptune has a profound effect on the Kuiper belt's structure due to orbital resonances. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their 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. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered This causes the Kuiper belt to possess pronounced gaps in its current layout, similar to the Kirkwood gaps in the Asteroid belt. Kirkwood gaps are gaps or dips in the distribution of Main belt Asteroids with Semi-major axis (or equivalently their Orbital period) as seen The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. 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. ^[38]

Classical belt

Main article: Classical Kuiper belt object

Between ~42 ~48 AU, however, the gravitational influence of Neptune is negligible, and objects can exist with their orbits pretty much unmolested. In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune This region is known as the classical Kuiper belt, and its members comprise roughly two thirds of KBOs observed to date. In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune ^[39][40] Because the first modern KBO discovered, 1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as cubewanos ("Q-B-1-os"). In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune ^[41][42]

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. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 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. ^[43] 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. ^[3][44]

Resonances

Main article: Resonant trans-Neptunian object

Distribution of cubewanos, plutinos and near scattered objects. In Astronomy, a resonant trans-Neptunian object is a Trans-Neptunian object (TNO in mean motion Orbital resonance with Neptune In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered

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. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their 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 ~39. In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae 4AU. This 2:3 resonance is populated by about 200 known objects,^[45] including Pluto together with its moons. In recognition of this, the other members of this family are known as Plutinos. In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. 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 dwarf planets when more is known about them. TemplateInfobox Planet. --> 90482 Orcus (, Orcus originally known by the Provisional designation TemplateInfobox Planet. --> 28978 Ixion (ɪkˈsaɪən ik-SYE-ən, sometimes ^[46][47] 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. ^[48] 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. ^[49] Its residents are sometimes referred to as twotinos. In Astronomy, a resonant trans-Neptunian object is a Trans-Neptunian object (TNO in mean motion Orbital resonance with Neptune Minor resonances also exist at 3:4, 3:5, 4:7 and 2:5. ^[50] Neptune possesses a number of trojan objects, which occupy its L₄ and L₅ points; gravitationally stable regions leading and trailing it in its orbit. As of May 2008, there are six known Neptune trojans (named by analogy to the Trojan asteroids which have the same orbital period as Neptune. 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. ^[48]

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. ^[51]

"Kuiper cliff"

Graph showing the numbers of KBOs for a given distance from the Sun

The 1:2 resonance appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the Classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation. ^[48]

Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU;^[52] 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. ^[53] 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. also known in the Kansai region as,is a leading National university located in Kobe, Japan. ^[54][55]

Composition

The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines

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. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[56] The temperature of the belt is only about 50K,^[57] 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. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ 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. A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. 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. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared 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. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass.

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. ^[58] These first data showed a broad range of colours among KBOs, ranging from neutral grey to deep red. ^[59] This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons. ^[59] 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. ^[60] Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing. ^[58] 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. ^[61]

Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition. ^[57] 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. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. ^[62]

Water ice has been detected in several KBOs, including 1996 TO66,^[63] 2000 EB173 and 2000 WR106. TemplateInfobox Planet.--> This article is about 20000 Varuna, a minor planet For other uses of the word see Varuna (disambiguation 20000 Varuna ( ^[64] 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. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Hydrate is a term used in Inorganic chemistry and Organic chemistry to indicate that a substance contains Water. 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. ^[57]

Mass and size distribution

Illustration of the power law.

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. ^[3] Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses. ^[3] 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. ^[65]

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.

where the current measures^[66] give q = 4 ±0. 5.

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)

Largest KBOs

Main article: List of the brightest KBOs

The relative sizes of the largest trans-Neptunian objects as compared to Earth. The albedo of an object is the extent to which it diffusely reflects light from the sun See also Kuiper belt Since the year 2000 a number of KBOs with diameters of between 500 and 1200 km (about half that of Pluto have been discovered

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. (136472) 2005 FY₉ (nicknamed "Easterbunny") and (136108) 2003 EL₆₁ (nicknamed "Santa"), both announced on 29 July 2005, are larger still. Events 1014 - Byzantine-Bulgarian Wars: Battle of Kleidion: Byzantine emperor Basil II inflicts a decisive defeat Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Other objects, such as 28978 Ixion (discovered in 2001) and 20000 Varuna (discovered in 2000) measure roughly 500 km across. TemplateInfobox Planet. --> 28978 Ixion (ɪkˈsaɪən ik-SYE-ən, sometimes This article is about 20000 Varuna, a minor planet For other uses of the word see Varuna (disambiguation 20000 Varuna ( ^[3]

Pluto

The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size, Pluto was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs^[3]). Ceres was considered a planet before the discovery of its fellow asteroids, and, based on this precedent, many astronomers concluded that Pluto should also be reclassified. Ceres (ˈsɪəriːz Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but

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. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered ^[67] 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. From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity "^[68] 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. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek

As of 2007, only three objects in the Solar System, Ceres, Pluto and Eris, 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. ^[69]

Satellites

Of the four largest TNOs, three (Eris, Pluto, and 2003 EL61) possess satellites, and two have more than one. A higher percentage of the largest KBOs possess satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible. ^[70] There are also a high number of binaries (two objects close enough in mass to be orbiting "each other") in the Kuiper belt. The most notable example is the Pluto-Charon binary, but it is estimated that over 1 percent of KBOs (a high percentage) exist in binaries. ^[71]

Scattered objects

Main articles: Scattered disc and Centaur (planetoid)

The orbits of objects in the scattered disc; the classical KBOs are blue, while the 2:5 resonant objects are green. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered

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. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered 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. ^[6] 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. ^[6]

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. The Minor Planet Center operates at the Smithsonian Astrophysical Observatory (SAO which is part of the Center for Astrophysics (CfA along with the Harvard Objects found outside the belt are classed as scattered objects. ^[72] 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. "^[73] 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. The Minor Planet Center operates at the Smithsonian Astrophysical Observatory (SAO which is part of the Center for Astrophysics (CfA along with the Harvard ^[72]

Comets

Main article: Comet

Comet Tempel 1, a Jupiter-family comet

Comets in the Solar System can be loosely divided into two categories: short-period and long period. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — 9P/Tempel, also known as Tempel 1 or 9P/Tempel 1, is a periodic Comet. Long period comets are believed to originate in the Oort cloud. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, There are two recognised categories of short-period comets: Jupiter-family comets and Halley-family comets. The latter group, which is named for its prototype, Halley's Comet, are believed to have emerged from the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every ^[18] It is the former type, the Jupiter family, that are believed to have originated from the scattered disc. ^[6] The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. ^[74]

Despite the fact that many are universally thought to have hailed from the scattered disc, there exist a wide array of differences between KBOs and Jupiter-family comets. Although the centaurs share a reddish colouration with many KBOs, the nuclei of comets are far bluer, indicating a fundamental chemical or physical difference. ^[18] The current hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older reddish material. ^[18]

Triton

Main article: Triton (moon)

Neptune's moon Triton

During its period of migration, Neptune is thought to have captured one of the larger KBOs and set it in orbit around itself. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek 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. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek Direct motion is the motion of a Planetary body in a direction similar to that of other bodies within its system and is sometimes called prograde motion. 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. ^[75] 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. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its outward migration. ^[76]

Exploration

Main article: New Horizons

Artist's conception of New Horizons at Pluto

On January 19, 2006, the first spacecraft mission to explore the Kuiper belt, New Horizons, was launched. New Horizons is a Robotic spacecraft mission by NASA currently underway Events 1419 - Hundred Years' War: Rouen surrenders to Henry V of England completing his reconquest of Normandy. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. New Horizons is a Robotic spacecraft mission by NASA currently underway The mission, headed by Alan Stern of the Southwest Research Institute, will arrive at Pluto on July 14, 2015 and, circumstances permitting, will continue on to study another as-yet undetermined KBO. S Alan Stern is an American planetary scientist, born 22 November 1957 New Orleans Louisiana married (three children Southwest Research Institute ( SwRI) headquartered in San Antonio Texas, is one of the oldest and largest independent Nonprofit, applied Research Events 1223 - Louis VIII becomes King of France upon the death of his father Philip II of France. 2015 ( MMXV) will be a Common year starting on Thursday of the Gregorian calendar. Any KBO chosen will be between 25 and 55 miles (40 to 90 km) in diameter and, ideally, white or grey, to contrast with Pluto's reddish colour. ^[77] John Spencer, an astronomer on the New Horizons mission team, says that no target for a post-Pluto Kuiper belt encounter has yet been selected, as they are awaiting data from the Pan-STARRS survey project to ensure as wide a field of options as possible. Pan-STARRS (an Acronym for Panoramic Survey Telescope And Rapid Response System is a planned astronomical survey that will conduct Astrometry and Photometry ^[78] The Pan-STARRS project, due to come fully online by 2009,^[79] will survey the entire sky with four 1. 4 gigapixel digital cameras to detect any moving objects, from near-earth objects to KBOs. Near-Earth objects ( NEOs) are Asteroids Comets and large Meteoroids whose orbits bring them close ( Perihelion distance AU ^[80]

The debris disks around two stars (HD 139664 and HD 53143)

Other Kuiper belts

As of 2006, astronomers have resolved dust disks believed to be Kuiper belt-like structures around nine stars other than the Sun. HD 53143 is an orange Main sequence Star that is located about 60 Light years from the Sun. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (like our own Kuiper belt) with diameters of between 20 and 30 AU and relatively sharp boundaries. Most known debris discs around other stars are fairly young, but the two imaged at right, taken by the Hubble Space Telescope in January, 2006, are old enough (roughly 300 million years) to have settled into stable configurations. A debris disk is a ring-shaped Circumstellar disk of dust and debris in orbit around a Star. The left image is a "top view" of a wide belt, and the right image is an "edge view" of a narrow belt. The black central circle is produced by the camera's coronagraph which hides the central star to allow the much fainter disks to be seen. A coronagraph is a telescopic attachment designed specifically to block out the harsh direct light from a Star, so that nearby objects can be resolved without burning ^[81][82] Beyond this, 15-20% of solar-type stars have observed infrared excess which is believed to indicate massive Kuiper Belt like structures. An infrared excess is a measurement of an astronomical source typically a Star, that has a greater measured Infrared flux than expected by assuming the ^[83]

See also

• List of dwarf planet candidates
• List of trans-Neptunian objects

References

External links and data sources

• Dave Jewitt's page @ University of Hawaii
  • The belt's name
• List of short period comets by family
• Kuiper Belt Profile by NASA's Solar System Exploration
• The Kuiper Belt Electronic Newsletter
• Wm. Robert Johnston's TNO page
• Minor Planet Center: Plot of the Outer Solar System, illustrating Kuiper gap
• Website of the International Astronomical Union (debating the status of TNOs)
• XXVIth General Assembly 2006
• nature.com article: diagram displaying inner solar system, Kuiper Belt, and Oort Cloud
• SPACE. com: Discovery Hints at a Quadrillion Space Rocks Beyond Neptune (Sara Goudarzi) 15 August 2006 06:13 a. m. ET
• The Outer Solar System Astronomy Cast episode #64, includes full transcript. Astronomy Cast is an educational Nonprofit Podcast discussing various topics in the field of Astronomy.