Kuiper belt
Sun Jupiter trojans Giant planets: Centaurs | Neptune trojans Resonant Kuiper belt Classical Kuiper belt Scattered disc |
Source: Minor Planet Center, www
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The Kuiper belt (/ˈkaɪpər/ KY-pər)[1] is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun.[2] It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive.[3][4] Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto,[5] Haumea,[6] Quaoar, and Makemake.[7] Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.[8][9]
The Kuiper belt is named in honor of the Dutch astronomer Gerard Kuiper, who conjectured the existence of the belt in 1951.[10] There were researchers before and after him who also speculated on its existence, such as Kenneth Edgeworth in the 1930s.[11] The astronomer Julio Angel Fernandez published a paper in 1980 suggesting the existence of a comet belt beyond Neptune[12][13] which could serve as a source for short-period comets.[14][15]
In 1992,
The Kuiper belt is distinct from the hypothesized Oort cloud, which is believed to be a thousand times more distant and mostly spherical. 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).[21] Pluto is the largest and most massive member of the Kuiper belt and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc.[a] Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006. It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as "plutinos," that share the same 2:3 resonance with Neptune.
The Kuiper belt and Neptune may be treated as a marker of the extent of the Solar System, alternatives being the
History
After the discovery of Pluto in 1930, many speculated that it might not be alone. The region now called the Kuiper belt was 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.[23]: 106
Hypotheses
The first astronomer to suggest the existence of a trans-Neptunian population was Frederick C. Leonard. Soon after Pluto's discovery by Clyde Tombaugh in 1930, Leonard 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".[24] That same year, astronomer Armin O. Leuschner suggested that Pluto "may be one of many long-period planetary objects yet to be discovered."[25]
In 1943, in the
In 1951, in a paper in Astrophysics: A Topical Symposium, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution and concluded that the disc consisted of "remnants of original clusterings which have lost many members that became stray asteroids, much as has occurred with open galactic clusters dissolving into stars."[10] In another paper, based upon a lecture Kuiper gave in 1950, also called On the Origin of the Solar System, Kuiper wrote about the "outermost region of the solar nebula, from 38 to 50 astr. units (i.e., just outside proto-Neptune)" where "condensation products (ices of H20, NH3, CH4, etc.) must have formed, and the flakes must have slowly collected and formed larger aggregates, estimated to range up to 1 km. or more in size." He continued to write that "these condensations appear to account for the comets, in size, number and composition." According to Kuiper "the planet Pluto, which sweeps through the whole zone from 30 to 50 astr. units, is held responsible for having started the scattering of the comets throughout the solar system."[27] It is said that Kuiper was operating on the assumption, common in his time, that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System; there would not be a Kuiper belt today if this were correct.[28]
The hypothesis took many other forms in the following decades. In 1962, physicist
In 1977,
Further evidence for the existence of the Kuiper belt 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
Discovery
In 1987, astronomer
Over one thousand bodies were found in a belt in the twenty years (1992–2012), after finding 1992 QB1 (named in 2018, 15760 Albion), showing a vast belt of bodies more than just Pluto and Albion.[40] By the 2010s the full extent and nature of Kuiper belt bodies is largely unknown.[40] Finally, in the late 2010s, two KBOs were closely flown past by an uncrewed spacecraft, providing much closer observations of the Plutonian system and another KBO.[41]
Studies conducted since the trans-Neptunian region was first charted have shown that the region now called the Kuiper belt is not the point of origin of short-period comets, but that they instead derive from a 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 that could never be affected by its orbit (the Kuiper belt proper), and a 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.[18]
Name
Astronomers sometimes use the alternative name Edgeworth–Kuiper belt to credit Edgeworth, and KBOs are occasionally referred to as EKOs.
KBOs are sometimes called "kuiperoids", a name suggested by Clyde Tombaugh.[42] 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 an exact synonym though, as TNOs include all objects orbiting the Sun past the orbit of Neptune, not just those in the Kuiper belt.[43]
Structure
At its fullest extent (but excluding the scattered disc), including its outlying regions, the Kuiper belt stretches from roughly 30–55 AU. The main body of the belt is generally accepted to extend from the 2:3 mean-motion resonance (
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 that 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 have 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.[47]
Classical belt
Between the 2:3 and 1:2 resonances with Neptune, at approximately 42–48 AU, the gravitational interactions with Neptune occur over an extended timescale, and objects can exist with their orbits essentially unaltered. This region is known as the
The classical Kuiper belt appears to be a composite of two separate populations. The first, known as the "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 cold population also contains a concentration of objects, referred to as the kernel, with semi-major axes at 44–44.5 AU.[53] 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.[54] Not only are the two populations in different orbits, the cold population also differs in color and albedo, being redder and brighter, has a larger fraction of binary objects,[55] has a different size distribution,[56] and lacks very large objects.[57] The mass of the dynamically cold population is roughly 30 times less than the mass of the hot.[56] The difference in colors may be a reflection of different compositions, which suggests they formed in different regions. The hot population is proposed to have formed near Neptune's original orbit and to have been scattered out during the migration of the giant planets.[3][58] The cold population, on the other hand, has been proposed to have formed more or less in its current position because the loose binaries would be unlikely to survive encounters with Neptune.[59] Although the Nice model appears to be able to at least partially explain a compositional difference, it has also been suggested the color difference may reflect differences in surface evolution.[60]
Resonances
When an object's orbital period is an exact ratio of Neptune's (a situation called a
Additionally, there is a relative absence of objects with semi-major axes below 39 AU that 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.[26]: 107
Kuiper cliff
The
Based on estimations of the primordial mass required to form
Origin
The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of several wide-field survey telescopes such as
The Kuiper belt is thought to consist of
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 their present positions, because too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are estimated to have formed closer to Jupiter. Scattering of planetesimals early in the Solar System's history would have led to migration of the orbits of the giant planets: Saturn, Uranus, and Neptune drifted outwards, whereas Jupiter drifted inwards. Eventually, the orbits shifted to the point where Jupiter and Saturn reached an exact 1:2 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational repercussions of such a resonance ultimately destabilized the orbits of Uranus and Neptune, causing them to be scattered outward onto high-eccentricity orbits that crossed the primordial planetesimal disc.[60][76][77]
While Neptune's orbit was highly eccentric, its mean-motion resonances overlapped and the orbits of the planetesimals evolved chaotically, allowing planetesimals to wander outward as far as Neptune's 1:2 resonance to form a dynamically cold belt of low-inclination objects. Later, after its eccentricity decreased, Neptune's orbit expanded outward toward its current position. Many planetesimals were captured into and remain in resonances during this migration, others evolved onto higher-inclination and lower-eccentricity orbits and escaped from the resonances onto stable orbits.[78] Many more planetesimals were scattered inward, with small fractions being captured as Jupiter trojans, as irregular satellites orbiting the giant planets, and as outer belt asteroids. The remainder were scattered outward again by Jupiter and in most cases ejected from the Solar System reducing the primordial Kuiper belt population by 99% or more.[60]
The original version of the currently most popular model, the "Nice model", reproduces many characteristics of the Kuiper belt such as the "cold" and "hot" populations, resonant objects, and a scattered disc, but it still fails to account for some of the characteristics of their distributions. The model predicts a higher average eccentricity in classical KBO orbits than is observed (0.10–0.13 versus 0.07) and its predicted inclination distribution contains too few high inclination objects.[60] In addition, the frequency of binary objects in the cold belt, many of which are far apart and loosely bound, also poses a problem for the model. These are predicted to have been separated during encounters with Neptune,[79] leading some to propose that the cold disc formed at its current location, representing the only truly local population of small bodies in the solar system.[80]
A recent modification of the Nice model has the Solar System begin with five giant planets, including an additional ice giant, in a chain of mean-motion resonances. About 400 million years after the formation of the Solar System the resonance chain is broken. Instead of being scattered into the disc, the ice giants first migrate outward several AU.[81] This divergent migration eventually leads to a resonance crossing, destabilizing the orbits of the planets. The extra ice giant encounters Saturn and is scattered inward onto a Jupiter-crossing orbit and after a series of encounters is ejected from the Solar System. The remaining planets then continue their migration until the planetesimal disc is nearly depleted with small fractions remaining in various locations.[81]
As in the original Nice model, objects are captured into resonances with Neptune during its outward migration. Some remain in the resonances, others evolve onto higher-inclination, lower-eccentricity orbits, and are released onto stable orbits forming the dynamically hot classical belt. The hot belt's inclination distribution can be reproduced if Neptune migrated from 24 AU to 30 AU on a 30 Myr timescale.[82] When Neptune migrates to 28 AU, it has a gravitational encounter with the extra ice giant. Objects captured from the cold belt into the 1:2 mean-motion resonance with Neptune are left behind as a local concentration at 44 AU when this encounter causes Neptune's semi-major axis to jump outward.[83] The objects deposited in the cold belt include some loosely bound 'blue' binaries originating from closer than the cold belt's current location.[84] If Neptune's eccentricity remains small during this encounter, the chaotic evolution of orbits of the original Nice model is avoided and a primordial cold belt is preserved.[85] In the later phases of Neptune's migration, a slow sweeping of mean-motion resonances removes the higher-eccentricity objects from the cold belt, truncating its eccentricity distribution.[86]
Composition
Being distant from the Sun and major planets, Kuiper belt objects are thought to be relatively unaffected by the processes that have shaped and altered other Solar System objects; thus, determining their composition would provide substantial information on the makeup of the earliest Solar System.
Analysis indicates that Kuiper belt objects are composed of a mixture of rock and a variety of ices such as water, methane, and ammonia. The temperature of the belt is only about 50 K,[88] so many compounds that would be gaseous closer to the Sun remain solid. The densities and rock–ice fractions are known for only a small number of objects for which the diameters and the masses have been determined. The diameter can be determined by imaging with a high-resolution telescope such as the Hubble Space Telescope, by the timing of an occultation when an object passes in front of a star or, most commonly, by using the albedo of an object calculated from its infrared emissions. The masses are determined using the semi-major axes and periods of satellites, which are therefore known only for a few binary objects. The densities range from less than 0.4 to 2.6 g/cm3. The least dense objects are thought to be largely composed of ice and have significant porosity. The densest objects are likely composed of rock with a thin crust of ice. There is a trend of low densities for small objects and high densities for the largest objects. One possible explanation for this trend is that ice was lost from the surface layers when differentiated objects collided to form the largest objects.[87]
Initially, detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their color.[90] These first data showed a broad range of colors among KBOs, ranging from neutral grey to deep red.[91] This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons.[91] This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of the volatile ices from their surfaces to the effects of cosmic rays.[26]: 118 Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing.[90] Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in color was too extreme to be easily explained by random impacts.[92] The radiation from the Sun is thought to have chemically altered methane on the surface of KBOs, producing products such as tholins. Makemake has been shown to possess a number of hydrocarbons derived from the radiation-processing of methane, including ethane, ethylene and acetylene.[87]
Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition.[88] In 1996, Robert H. Brown et al. acquired spectroscopic data on the KBO 1993 SC, which revealed that its surface composition is markedly similar to that of Pluto, as well as Neptune's moon Triton, with large amounts of methane ice.[93] For the smaller objects, only colors and in some cases the albedos have been determined. These objects largely fall into two classes: gray with low albedos, or very red with higher albedos. The difference in colors and albedos is hypothesized to be due to the retention or the loss of hydrogen sulfide (H2S) on the surface of these objects, with the surfaces of those that formed far enough from the Sun to retain H2S being reddened due to irradiation.[94]
The largest KBOs, such as Pluto and
Mass and size distribution
Despite its vast extent, the collective mass of the Kuiper belt is relatively low. The total mass of the dynamically hot population is estimated to be 1% the mass of the Earth. The dynamically cold population is estimated to be much smaller with only 0.03% the mass of the Earth.[56][97] While the dynamically hot population is thought to be the remnant of a much larger population that formed closer to the Sun and was scattered outward during the migration of the giant planets, in contrast, the dynamically cold population is thought to have formed at its current location. The most recent estimate (2018) puts the total mass of the Kuiper belt at (1.97±0.30)×10−2 Earth masses based on the influence that it exerts on the motion of planets.[98]
The small total mass of the dynamically cold population presents some problems for models of the Solar System's formation because a sizable mass is required for accretion of KBOs larger than 100 km (62 mi) in diameter.[3] If the cold classical Kuiper belt had always had its current low density, these large objects simply could not have formed by the collision and mergers of smaller planetesimals.[3] Moreover, the eccentricity and inclination of current orbits make the encounters quite "violent" resulting in destruction rather than accretion. The removal of a large fraction of the mass of the dynamically cold population is thought to be unlikely. Neptune's current influence is too weak to explain such a massive "vacuuming", and the extent of mass loss by collisional grinding is limited by the presence of loosely bound binaries in the cold disk, which are likely to be disrupted in collisions.[99] Instead of forming from the collisions of smaller planetesimals, the larger object may have formed directly from the collapse of clouds of pebbles.[100]
The size distributions of the Kuiper belt objects follow a number of power laws. A power law describes the relationship between N(D) (the number of objects of diameter greater than D) and D, and is referred to as brightness slope. The number of objects is inversely proportional to some power of the diameter D:
- which yields (assuming q is not 1):
(The constant may be non-zero only if the power law doesn't apply at high values of D.)
Early estimates that were based on measurements of the apparent magnitude distribution found a value of q = 4 ± 0.5,[65] which implied that there are 8 (=23) times more objects in the 100–200 km range than in the 200–400 km range.
Recent research has revealed that the size distributions of the hot classical and cold classical objects have differing slopes. The slope for the hot objects is q = 5.3 at large diameters and q = 2.0 at small diameters with the change in slope at 110 km. The slope for the cold objects is q = 8.2 at large diameters and q = 2.9 at small diameters with a change in slope at 140 km.
The smallest known Kuiper belt objects with radii below 1 km have only been detected by
Observations made by NASA's New Horizons Venetia Burney Student Dust Counter showed "higher than model-predicted dust fluxes" as far as 55 au, not explained by any existing model.[106]
Scattered objects
The scattered disc is a sparsely populated region, overlapping with the Kuiper belt but extending to beyond 100 AU. Scattered disc objects (SDOs) have very elliptical orbits, often also very inclined to the ecliptic. Most models of Solar System formation show both KBOs and SDOs first forming in a primordial belt, with later gravitational interactions, particularly with Neptune, sending the objects outward, some into stable orbits (the KBOs) and some into unstable orbits, the scattered disc.
According to the Minor Planet Center, which officially catalogues all trans-Neptunian objects, a KBO 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.[107] In some scientific circles the term "Kuiper belt object" has become synonymous with any icy minor planet native to the outer Solar System assumed 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-disc region). They often describe scattered disc objects as "scattered Kuiper belt objects".[108] Eris, which is known to be more massive than Pluto, is often referred to as a KBO, but is technically an SDO.[107] 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 thought 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 objects.[107]
Triton
During its period of migration, Neptune is thought to have captured a large KBO,
Largest KBOs
Since 2000, a number of KBOs with diameters of between 500 and 1,500 km (932 mi), more than half that of Pluto (diameter 2370 km), have been discovered. 50000 Quaoar, a classical KBO discovered in 2002, is over 1,200 km across. Makemake and Haumea, 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 600–700 km (373–435 mi) across.[3]
Pluto
The discovery of these large KBOs in orbits similar to Pluto's led many to conclude that, aside from its relative size,
The issue was brought to a head by the discovery of
It is not clear how many KBOs are large enough to be dwarf planets. Consideration of the surprisingly low densities of many dwarf-planet candidates suggests that not many are.
Satellites
The six largest TNOs (Eris, Pluto, Gonggong, Makemake, Haumea and Quaoar) are all known to have satellites, and two of them have more than one. A higher percentage of the larger KBOs have satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible.[116] 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 around 11% of KBOs exist in binaries.[117]
Exploration
On 19 January 2006, the first spacecraft to explore the Kuiper belt, New Horizons, was launched, which flew by Pluto on 14 July 2015. Beyond the Pluto flyby, the mission's goal was to locate and investigate other, farther objects in the Kuiper belt.[118]
On 15 October 2014, it was revealed that Hubble had uncovered three potential targets, provisionally designated PT1 ("potential target 1"), PT2 and PT3 by the New Horizons team.
On 26 August 2015, the first target, 2014 MU69 (nicknamed "Ultima Thule" and later named 486958 Arrokoth), was chosen. Course adjustment took place in late October and early November 2015, leading to a flyby in January 2019.[127] On 1 July 2016, NASA approved additional funding for New Horizons to visit the object.[128]
On 2 December 2015, New Horizons detected what was then called
On 1 January 2019, New Horizons successfully flew by Arrokoth, returning data showing Arrokoth to be a contact binary 32 km long by 16 km wide.[130] The Ralph instrument aboard New Horizons confirmed Arrokoth's red color. Data from the fly-by will continue to be downloaded over the next 20 months.
No follow-up missions for New Horizons are planned, though at least two concepts for missions that would return to orbit or land on Pluto have been studied.[131][132] Beyond Pluto, there exist many large KBOs that cannot be visited with New Horizons, such as the dwarf planets Makemake and Haumea. New missions would be tasked to explore and study these objects in detail. Thales Alenia Space has studied the logistics of an orbiter mission to Haumea,[133] a high priority scientific target due to its status as the parent body of a collisional family that includes several other TNOs, as well as Haumea's ring and two moons. The lead author, Joel Poncy, has advocated for new technology that would allow spacecraft to reach and orbit KBOs in 10–20 years or less.[134] New Horizons Principal Investigator Alan Stern has informally suggested missions that would flyby the planets Uranus or Neptune before visiting new KBO targets,[135] thus furthering the exploration of the Kuiper belt while also visiting these ice giant planets for the first time since the Voyager 2 flybys in the 1980s.
Design studies and concept missions
Quaoar has been considered as a flyby target for a probe tasked with exploring the interstellar medium, as it currently lies near the heliospheric nose; Pontus Brandt at Johns Hopkins Applied Physics Laboratory and his colleagues have studied a probe that would flyby Quaoar in the 2030s before continuing to the interstellar medium through the heliospheric nose.[136][137] Among their interests in Quaoar include its likely disappearing methane atmosphere and cryovolcanism.[136] The mission studied by Brandt and his colleagues would launch using SLS and achieve 30 km/s using a Jupiter flyby. Alternatively, for an orbiter mission, a study published in 2012 concluded that Ixion and Huya are among the most feasible targets.[138] For instance, the authors calculated that an orbiter mission could reach Ixion after 17 years cruise time if launched in 2039.
Extrasolar Kuiper belts
By 2006, astronomers had resolved dust discs thought to be Kuiper belt-like structures around nine stars other than the Sun. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (tentatively like that of the Solar System) with radii of between 20 and 30 AU and relatively sharp boundaries.
See also
Notes
- ^ a b The literature is inconsistent in the usage of the terms scattered disc and Kuiper belt. For some, they are distinct populations; for others, the scattered disc is part of the Kuiper belt. Authors may even switch between these two uses in one publication.[19] Because the International Astronomical Union's Minor Planet Center, the body responsible for cataloguing minor planets in the Solar System, makes the distinction,[20] the editorial choice for Wikipedia articles on the trans-Neptunian region is to make this distinction as well. On Wikipedia, Eris, the most massive known trans-Neptunian object, is not part of the Kuiper belt and this makes Pluto the most massive Kuiper belt object.
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External links
- Dave Jewitt's page @ UCLA
- 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
- List of TNOS