Difference between revisions of "Kuiper belt" - New World Encyclopedia

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[[Image:Kuiper oort.jpg|350px|right|thumb|Artist's rendering of the Kuiper Belt and hypothetical more distant [[Oort cloud]].]]
 
[[Image:Kuiper oort.jpg|350px|right|thumb|Artist's rendering of the Kuiper Belt and hypothetical more distant [[Oort cloud]].]]
  
The '''Kuiper belt''' ([[IPA chart for English|pronounced]] {{IPA|/ˈkaɪpɚ/}}, rhymes with "viper") is a zone of the [[solar system]] extending from the [[orbit]] of [[Neptune]] (at 30 [[astronomical unit]]s (AU) to 50 AU from the [[Sun]].  
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The '''Kuiper belt''' ([[IPA chart for English|pronounced]] {{IPA|/ˈkaɪpɚ/}}, rhymes with "viper") is a zone of the [[solar system]] extending from the [[orbit]] of [[Neptune]] (at 30 [[astronomical unit]]s (AU) to 50 AU from the [[Sun]].  The objects within the Kuiper belt, together with the members of the [[scattered disk]] extending beyond, are collectively referred to as [[trans-Neptunian object|trans-Neptunian]], along with any hypothetical [[Hills cloud]] and [[Oort cloud]] objects.
  
The objects within the Kuiper Belt, together with the members of the [[scattered disk]] extending beyond, are collectively referred to as [[trans-Neptunian object|trans-Neptunian]], along with any hypothetical [[Hills cloud]] and [[Oort cloud]] objects.
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Together with the [[asteroid]] belt which is the source of [[meteorite]]s, Kuiper belt objects give important clues to the distribution of materials and evolution history of the [[Solar System]] through their orbital characteristics and compositions.  Kuiper belt objects coming as [[comet]]s and dusts are gift for humanity to go back in time and study the early stage of the Solar System.
 
 
The interaction with Neptune (2:1 [[orbital resonance]]) is thought to be responsible for the apparent edge at 48 AU (a sudden drop in number of objects, see ''Orbit distribution'' below)  but the current models have yet to explain this peculiar distribution in detail.
 
  
 
==Origins==
 
==Origins==
 
[[Image:Kuiper belt remote.jpg|thumb|350px|The debris disks around these two remote stars seem equivalent of our own Solar system's Kuiper Belt. The left image is a "top view," and the right image is an "edge view." 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. Observed with [[Hubble Space Telescope]].]]
 
[[Image:Kuiper belt remote.jpg|thumb|350px|The debris disks around these two remote stars seem equivalent of our own Solar system's Kuiper Belt. The left image is a "top view," and the right image is an "edge view." 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. Observed with [[Hubble Space Telescope]].]]
  
Modern [[computer]] [[simulation]]s show the Kuiper belt to have been strongly influenced by [[Jupiter (planet)|Jupiter]] and [[Neptune]].  During the early period of the [[Solar System]], Neptune's orbit is thought to have migrated outwards from the [[Sun]] due to interactions with minor bodies. In the process, Neptune swept up, or gravitationally ejected all the bodies closer to the Sun than about 40 AU (the inner edge of the region occupied by [[cubewano]]s, objects in the main Kuiper belt), apart from those which fortuitously were in a 2:3 [[orbital resonance]]. These resonant bodies formed the [[plutino]]s. The present Kuiper Belt members are thought to have largely formed in their present position, although a significant fraction may have originated in the vicinity of Jupiter, and been ejected by it to the far regions of the Solar system.
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Modern [[computer]] [[simulation]]s show the Kuiper belt to have been strongly influenced by [[Jupiter (planet)|Jupiter]] and [[Neptune]].  During the early period of the [[Solar System]], Neptune's orbit is thought to have migrated outwards from the [[Sun]] due to interactions with minor bodies. In the process, Neptune swept up, or gravitationally ejected all the bodies closer to the Sun than about 40 AU (the inner edge of the region occupied by [[cubewano]]s, objects in the main Kuiper belt), apart from those which fortuitously were in a 2:3 [[orbital resonance]]. These resonant bodies formed the [[plutino]]s. The present Kuiper Belt members are thought to have largely formed in their present position, although a significant fraction may have originated in the vicinity of Jupiter and been ejected by it to the far regions of the Solar system.
  
 
===Hypothesis===
 
===Hypothesis===
The first [[astronomer]]s to suggest the existence of this belt were [[Frederick C. Leonard]] in 1930 and [[Kenneth E. Edgeworth]] in 1943. In 1951 [[Gerard Kuiper]] suggested that the belt was the source of short period [[comet]]s (those having an [[orbital period]] of less than 200 [[year]]s). More detailed conjectures about objects in the belt were done by [[Al G. W. Cameron]] in 1962, [[Fred L. Whipple]] in 1964, and [[Julio Fernandez]] in 1980. The belt and the objects in it were named after Kuiper after the discovery of {{mpl|(15760) 1992 QB|1}}.
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The first [[astronomer]]s to suggest the existence of this belt were [[Frederick C. Leonard]] in 1930, and [[Kenneth E. Edgeworth]] in 1943. In 1951, [[Gerard Kuiper]] suggested that the belt was the source of short period [[comet]]s (those having an [[orbital period]] of less than 200 years). More detailed conjectures about objects in the belt were done by [[Al G. W. Cameron]] in 1962, [[Fred L. Whipple]] in 1964, and [[Julio Fernandez]] in 1980. The belt and the objects in it were named after Kuiper following the discovery of {{mpl|(15760) 1992 QB|1}}.
  
 
===Name===
 
===Name===
An alternative name, '''Edgeworth-Kuiper belt''' is used to credit Edgeworth. 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.
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An alternative name, '''Edgeworth-Kuiper belt''' is used to credit Edgeworth. 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.
  
 
==Discoveries thus far==
 
==Discoveries thus far==
{{TNO}}
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Over 800 '''Kuiper Belt Objects''' '''(KBOs)''' (a subset of [[trans-Neptunian object]]s) have been discovered in the belt, almost all of them since 1992. This was primarily a result of advances in computer hardware/software and [[Charge-coupled device|CCD]] of high quantum efficiency, allowing for cost effective automated KBO searching in combination with large [[telescope]]s.
Over 800 '''Kuiper belt objects''' ('''KBOs''') (a subset of [[trans-Neptunian object]]s (TNOs)) have been discovered in the belt, almost all of them since 1992. This was primarily a result of advances in computer hardware/software and [[Charge-coupled device|CCD]]-enabled [[telescope]]s allowing for cost effective automated KBO searching.{{Fact|date=February 2007}}
 
  
Among the largest are [[Pluto]] and [[Charon (moon)|Charon]], but since the year 2000 other large objects that approached and even exceeded their size were identified. [[50000 Quaoar]], discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest [[asteroid]], [[Ceres (dwarf planet)|Ceres]]. {{mpl|(136472) 2005 FY|9}} (nicknamed "Easterbunny") and {{mpl|(136108) 2003 EL|61}} (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) while smaller than Quaoar, are nonetheless quite sizable.  
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Among the largest are [[Pluto]] and [[Charon (moon)|Charon]], but since the year 2000, other large objects that approached and even exceeded their size were identified. [[50000 Quaoar]], discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest body in the [[asteroid belt]], [[Ceres (dwarf planet)|Ceres]]. {{mpl|(136472) 2005 FY|9}} (nicknamed "Easterbunny") and {{mpl|(136108) 2003 EL|61}} (nicknamed "Santa"), both announced on July 29, 2005, are larger still. Other objects, such as [[28978 Ixion]] (discovered in 2001) and [[20000 Varuna]] (discovered in 2000) while smaller than Quaoar, are nonetheless quite sizable.
  
The exact classification of these objects is unclear, since they are probably fairly different from the [[asteroid]]s of the [[asteroid belt]]. The largest recent discovery is [[136199 Eris|Eris]], which is actually larger than Pluto.  This led scientists to question the [[definition of planet|definition of the term planet]], as it is larger than [[Pluto]] and was often called a [[tenth planet]] by some sources.<ref>http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/16/</ref> Due to this discovery, on August 24, 2006, the [[IAU]] announced a first-ever definition of 'planet', and these large Kuiper belt objects accordingly became known officially as [[dwarf planet]]s.  A number of astronomers around the world came out in public disagreement with the definition in the days following it.
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The exact classification of these objects is unclear, since they are probably fairly different from the objects of the [[asteroid]] belt. The largest recent discovery was [[136199 Eris|Eris]], which is actually larger than Pluto.  This led scientists to question the [[definition of planet|definition of the term planet]], as it is larger than Pluto and was often called a [[tenth planet]] by some sources.<ref>Hubble Telescope, [http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/16/ Hubble Finds 'Tenth Planet' is Slightly Larger than Pluto.] Retrieved April 27, 2007.</ref> Due to this discovery, on August 24, 2006, the [[IAU]] announced a first-ever definition of "[[planet]]," and these large Kuiper belt objects accordingly became known officially as [[dwarf planet]]s.  A number of astronomers around the world came out in public disagreement with the definition in the days following it.
  
 
Neptune's moon [[Triton (moon)|Triton]] is commonly thought to be a captured KBO.
 
Neptune's moon [[Triton (moon)|Triton]] is commonly thought to be a captured KBO.
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[[Image:TheKuiperBelt_classes.PNG|right|thumb|250px|Orbit classification (schematic of [[semi-major axis|semi-major axes]]).]]
 
[[Image:TheKuiperBelt_classes.PNG|right|thumb|250px|Orbit classification (schematic of [[semi-major axis|semi-major axes]]).]]
  
[[Orbital resonance]] with Neptune is the major factor of the current classification of KBO, even if most of them (more than 600 objects as of October 2005) are not [[Resonant Trans-Neptunian Object|resonant]]. These objects, called Classical Kuiper Belt objects or [[cubewano]]s, are found between the 2:3 resonance (at ~39.4AU, populated by more than 150 [[plutino]]s) and the 1:2 resonance (at ~47.7AU, populated by a few [[twotino]]s). Minor resonances exist at 3:4, 3:5, 4:7 and 2:5 (this last, also fairly strongly occupied). The 1:2 resonance appears to be an edge. It is not clear whether it is actually the outer edge of the Classical Belt or just the beginning of a gap.
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[[Orbital resonance]] with Neptune is the major factor of the current classification of KBO, even if most of them (more than 600 objects as of October 2005) are not [[Resonant Trans-Neptunian Object|resonant]]. These objects, called Classical Kuiper Belt objects or [[cubewano]]s, are found between the 2:3 resonance (at ~39.4 AU, populated by more than 150 [[plutino]]s) and the 1:2 resonance (at ~47.7 AU, populated by a few [[twotino]]s). Minor resonances exist at 3:4, 3:5, 4:7 and 2:5 (this last, also fairly strongly occupied). The 1:2 resonance appears to be an edge. It is not clear whether it is actually the outer edge of the Classical Belt or just the beginning of a gap.
  
 
[[Image:TheKuiperBelt_75AU_Large.svg.png|right|thumb|400px|Large [[cubewano]]s, [[plutino]]s and near [[scattered disk|scattered objects]].]]
 
[[Image:TheKuiperBelt_75AU_Large.svg.png|right|thumb|400px|Large [[cubewano]]s, [[plutino]]s and near [[scattered disk|scattered objects]].]]
  
The next diagram shows the largest objects of the Kuiper belt: [[Pluto]] with the largest [[plutino]]s: [[90482 Orcus]] and [[28978 Ixion]], a few big classical objects, and two [[scattered disk|scattered objects]] (beyond the 1:2 resonance, in grey), notably [[136199 Eris|Eris]] thought to be the biggest trans-Neptunian object known. The [[Eccentricity (orbit)|eccentricity]] of the orbits is represented by the red segments (extending from [[perihelion]] to  [[apsis|aphelion)]] with [[inclination]] represented on Y axis. While eccentric orbits of many resonant KBOs bring them inside Neptune's orbit periodically, classical KBOs are in more circular orbits (short red segments, [[50000 Quaoar|Quaoar]]).
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The next diagram shows the largest objects of the Kuiper belt: [[Pluto]] with the largest [[plutino]]s: [[90482 Orcus]] and [[28978 Ixion]], a few big classical objects, and two [[scattered disk|scattered objects]] (beyond the 1:2 resonance, in grey), notably [[136199 Eris|Eris]] thought to be the biggest trans-Neptunian object known. The [[Eccentricity (orbit)|eccentricity]] of the orbits is represented by the red segments (extending from [[perihelion]] to  [[apsis|aphelion)]] with [[inclination]] represented on Y axis. While eccentric orbits of many resonant KBOs bring them inside Neptune's orbit periodically, classical KBOs are in more circular orbits (short red segments, [[50000 Quaoar|Quaoar]]).
  
Initially, the Kuiper belt was thought to be a flat belt (populated by objects on moderately  
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Initially, the Kuiper belt was thought to be a flat belt (populated by objects on moderately eccentric, low-inclination orbits), as opposed to high inclination orbits of the "scattered" disk objects. With the discovery of the large cubewanos, this belt became a thick disk or [[torus]]. It now appears that the distribution of orbit inclinations peaks around 4 and 30-40 degrees,  giving rise to a division into two groups: ''Cold'' and ''hot,'' respectively.  The ''cold'' group would have been born outside the Neptune's orbit while the ''hot'' migrated outwards due to close interactions with Neptune. The cold/hot terminology comes from analogy to particles in a gas, where, as [[temperature]] rises, so do the relative velocities between the [[Molecule|particle]]s.  
eccentric, low-inclination orbits), as opposed to high inclination orbits of the "scattered" disk objects. With the discovery of the large cubewanos, this belt became a thick disk or [[torus]]. It now appears that the distribution of orbit inclinations peaks around 4 and 30-40 degrees,  giving rise to a division into two groups: ''cold'' and ''hot'', respectively.  The ''cold'' group would have been born outside the Neptune's orbit while the ''hot'' migrated outwards due to close interactions with Neptune. The cold/hot terminology comes from analogy to particles in a gas, where, as [[temperature]] rises, so do the relative velocities between the [[Molecule|particle]]s.  
 
  
This grouping may yet be revised, however, as the current distribution of known objects is likely to be strongly affected by observational bias. Most observations have so far focused on near-ecliptic objects. Even objects with high inclinations (e.g. [[2004 XR190|2004 XR<sub>190</sub>]]) were actually found at near ecliptic positions. In addition, most of the known KBOs are detected near their closest approaches to the Sun since they appear dimmer at greater distances.
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This grouping may yet be revised, however, as the current distribution of known objects is likely to be strongly affected by observational bias. Most observations have so far focused on near-ecliptic objects. Even objects with high inclinations (for example, [[2004 XR190|2004 XR<sub>190</sub>]]) were actually found at near ecliptic positions. In addition, most of the known KBOs are detected near their closest approaches to the Sun since they appear dimmer at greater distances.
 
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The last diagram shows the distribution of known Kuiper Belt objects. The [[orbital resonance|resonant objects]]: [[Neptune#Trojan Asteroids of Neptune|Neptune Trojans]] (1:1 resonance), [[plutino]]s (2:3), [[twotino]]s (1:2) and a handful of objects occupying other resonances are shown in red. Confirmed plutinos are plotted in dark red. Beyond the 1:2 resonance, [[scattered disk object]]s are plotted for reference.  
 
The last diagram shows the distribution of known Kuiper Belt objects. The [[orbital resonance|resonant objects]]: [[Neptune#Trojan Asteroids of Neptune|Neptune Trojans]] (1:1 resonance), [[plutino]]s (2:3), [[twotino]]s (1:2) and a handful of objects occupying other resonances are shown in red. Confirmed plutinos are plotted in dark red. Beyond the 1:2 resonance, [[scattered disk object]]s are plotted for reference.  
  
Interestingly, low inclination regions which include the "cold" majority of cubewanos appear devoid of the largest objects (see diagram).
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Interestingly, low inclination regions which include the "cold" majority of [[cubewano]]s appear devoid of the largest objects (see diagram).
 
The (observed) distribution has been a challenge to the theories of the formation of the Kuiper belt as it is far too complex to be explained simply as being the remains of the original [[accretion disc]] dating back to the formation of the Solar System. Numerous competing models are being developed, typically involving so-called Neptune migration. It was suggested in the 1980s that interaction between giant planets and a massive disk of small particles would not only scatter the disk but also cause (via momentum transfer) the giants to migrate to more distant orbits. While all four giant planets would be affected, Neptune could have migrated as far as 5AU outwards to reach its current position at around 30 AU. Such models can explain for example, the ‘trapping’ of small bodies into the plutino 2:3 [[resonance]]s.
 
The (observed) distribution has been a challenge to the theories of the formation of the Kuiper belt as it is far too complex to be explained simply as being the remains of the original [[accretion disc]] dating back to the formation of the Solar System. Numerous competing models are being developed, typically involving so-called Neptune migration. It was suggested in the 1980s that interaction between giant planets and a massive disk of small particles would not only scatter the disk but also cause (via momentum transfer) the giants to migrate to more distant orbits. While all four giant planets would be affected, Neptune could have migrated as far as 5AU outwards to reach its current position at around 30 AU. Such models can explain for example, the ‘trapping’ of small bodies into the plutino 2:3 [[resonance]]s.
 
   
 
   
However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles,<ref>http://arxiv.org/abs/chao-dyn/9406004</ref> the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software."
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However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles,<ref>Renu Malhotra, [http://arxiv.org/abs/chao-dyn/9406004 Nonlinear Resonances in the Solar System.] Retrieved April 30, 2007.</ref> the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software."
  
 
The belt should not be confused with the hypothesized [[Oort cloud]], which is far more distant.
 
The belt should not be confused with the hypothesized [[Oort cloud]], which is far more distant.
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[[Image:TheKuiperBelt_PowerLaw2.svg.png|left|thumb|250px|Illustration of the power law.]]
 
[[Image:TheKuiperBelt_PowerLaw2.svg.png|left|thumb|250px|Illustration of the power law.]]
  
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<ref>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.</ref> is inversely proportional to some power of the diameter D.  
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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.  
:<math> \frac{d N}{d D} \sim D^{-q}</math>    where the current measures <ref name="Bernstein et al 2004" >Bernstein G.M., Trilling D.E., Allen R.L. , Brown K.E , Holman M., Malhotra R. ''The size Distribution of transneptunian bodies.'' The Astronomical Journal, '''128''', 1364-1390.  
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:<math> \frac{d N}{d D} \sim D^{-q}</math>    where the current measures<ref>G.M. Bernstein, D.E. Trilling, R.L. Allen, K.E. Brown, M. Holman, and R. Malhotra, The size distribution of transneptunian bodies, ''The Astronomical Journal,'' 128, 1364-1390.</ref> give q = 4 ±0.5.
[http://arxiv.org/PS_cache/astro-ph/pdf/0308/0308467.pdf preprint on arXiv (pdf)] </ref> give q = 4 ±0.5.
 
 
The relationship is illustrated on the graph for q=4.
 
The relationship is illustrated on the graph for q=4.
Less formally, there is for instance 8 (=2<sup>3</sup>) times more objects in 100-200km range than objects in 200-400km range. In other words, for a single object with the diameter of 1000 km it should be there around 1000 (=10<sup>3</sup>) objects with diameter of 100km.
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Less formally, there is for instance 8 (=2<sup>3</sup>) times more objects in 100-200 km range than objects in 200-400 km range. In other words, for a single object with the diameter of 1000 km, there should be around 1000 (=10<sup>3</sup>) objects with diameter of 100 km.
 
Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects)
 
Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects)
 
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==Unresolved issues==
 
==Unresolved issues==
 
=== Missing mass dilemma ===
 
=== Missing mass dilemma ===
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The total mass of Kuiper Belt objects can be inferred by models of the origin of the Solar System from the known mass of the planets and known distribution of mass closer to the Sun. While the estimates are model-dependent, the total mass of around 30 M<sub>Earth</sub> is expected.
 
The total mass of Kuiper Belt objects can be inferred by models of the origin of the Solar System from the known mass of the planets and known distribution of mass closer to the Sun. While the estimates are model-dependent, the total mass of around 30 M<sub>Earth</sub> is expected.
Surprisingly, the actual distribution appears to be well below that value, even accounting for the observational bias. The observed density is at least 100 times smaller than the model calls for.<ref name="Jewitt 2006">D.Jewitt,A.Delsanti ''The Solar System Beyond The Planets'',to appear in the book ''Solar System Update'', Springer-Praxis Ed., Horwood, Blondel and Mason, 2006. [http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf Preprint version (pdf)]</ref> This missing 99% of the mass can be hardly dismissed as it is required for the accretion of bigger (>100km) objects ever taking place. At the current low density these objects simply could not be created. Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent" resulting in destruction rather than accretion.  
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Surprisingly, the actual distribution appears to be well below that value, even accounting for the observational bias. The observed density is at least 100 times smaller than the model calls for.<ref>D. Jewitt and A. Delsanti, [http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf ''The Solar System Beyond The Planets.''] Retrieved May 2, 2007.</ref> This missing 99 percent of the mass can be hardly dismissed as it is required for the accretion of bigger (greater than 100 km) objects ever taking place. At the current low density, these objects simply could not be created. Moreover, the eccentricity and inclination of current orbits make 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.<ref name="Morbidelli 2005">
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Morbidelli A. ''Origin and dynamical evolution of comets and their reservoirs.''
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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.<ref>A. Morbidelli, [http://arxiv.org/abs/astro-ph/0512256 Origin and dynamical evolution of comets and their reservoirs (Preprint on arXiv).] Retrieved May 2, 2007.</ref>
[http://arxiv.org/PS_cache/astro-ph/pdf/0512/0512256.pdf Preprint on arXiv (pdf)]
 
</ref>
 
  
 
===The "Kuiper cliff"===
 
===The "Kuiper cliff"===
Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU;<ref name="Brown 1999">
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Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU; however, observation has revealed that in fact, at 50 AU, the number of observed objects in the Kuiper belt falls precipitously. This falloff is known as the "Kuiper cliff,"  and its cause is unknown, though [[Alan Stern]] of the Southwest Research Institute has claimed that a large planetary object might be responsible.<ref>M. Brooks, [http://space.newscientist.com/article.ns?id=mg18524911.600 13 things that do not make sense.] Retrieved May 2, 2007.</ref>  Bernstein and Trilling et al. have found evidence that the observed rapid decline in objects of 100 km or more in radius beyond 50 AU is a real decline in the number of objects, and not just an observational effect.<ref>{{cite journal| author = Bernstein, G.M., Trilling, D.E., Allen, R.L., Brown, M.E., Holman, M., and Malhotra, R.|title=The Size Distribution of Trans-Neptunian Bodies|url=http://www.gps.caltech.edu/~mbrown/papers/ps/bernstein.pdf|journal = The Astrophysical Journal| year = 2004}} Retrieved May 2, 2007.</ref>
E. I. Chiang and M. E. Brown, ''KECK PENCIL-BEAM SURVEY FOR FAINT KUIPER BELT
 
OBJECTS''
 
[http://arxiv.org/PS_cache/astro-ph/pdf/0512/0512256.pdf Preprint on arXiv (pdf)]
 
</ref> however, observation has revealed that in fact, at 50 AU, the number of observed objects in the Kuiper belt falls precipitously. This falloff is known as the "Kuiper cliff,"  and its cause is unknown, though [[Alan Stern]] of the [[Southwest Research Institute]] has claimed that a large planetary object might be responsible.<ref>Michael Brooks: [http://space.newscientist.com/article.ns?id=mg18524911.600 "13 Things that do not make sense"]</ref>  Bernstein and Trilling et al. have found evidence that the observed rapid decline in objects of 100 km or more in radius beyond 50 AU is a real decline in the number of objects, and not just an observational effect.<ref>{{cite journal|author = G.M. Bernstein, D.E. Trilling, R.L. Allen, M.E. Brown, M. Holman and R. Malhotra|title=The Size Distribution of Trans-Neptunian Bodies|url=http://www.gps.caltech.edu/~mbrown/papers/ps/bernstein.pdf|journal = The Astrophysical Journal|year = 2004|}}</ref>
 
  
 
===The term "Kuiper belt object" (KBO)===
 
===The term "Kuiper belt object" (KBO)===
Most models of solar system formation show icy planetoids first forming in the Kuiper belt, while later gravitational interactions displace some of them outwards into the so-named [[scattered disc]]. Strictly speaking, a KBO is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. However, in some scientific circles the term 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). Discoverer [[Michael E. Brown]], for instance, has referred to [[Eris (dwarf planet)|Eris]] as a KBO, despite it having a semi-major axis of 67 AU, well clear of the Kuiper cliff.  Other leading trans-Neptunian researchers have been more cautious in applying the KBO label to objects clearly outside the belt in the current epoch.
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Most models of solar system formation show icy planetoids first forming in the Kuiper belt, while later gravitational interactions displace some of them outwards into the so-named [[scattered disc]]. Strictly speaking, a KBO is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. However, in some scientific circles the term 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 (such as in the scattered disk region). Discoverer [[Michael E. Brown]], for instance, has referred to [[Eris (dwarf planet)|Eris]] as a KBO, despite it having a semi-major axis of 67 AU, well clear of the Kuiper cliff.  Other leading trans-Neptunian researchers have been more cautious in applying the KBO label to objects clearly outside the belt in the current epoch.
  
 
== List of the brightest KBOs ==
 
== List of the brightest KBOs ==
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| assumed [[albedo]]
 
| assumed [[albedo]]
 
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==See also==
 
*[[Planet]]''
 
*[[Asteroid]]''
 
*[[Comet]]''
 
*[[Pluto]]''
 
 
  
 
==Notes ==
 
==Notes ==
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== References ==
 
== References ==
<<We need at least 3 reliable references here, properly formatted.>>
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* Bakich, Michael E. 2000. ''The Cambridge Planetary Handbook''. Cambridge: Cambridge University Press. ISBN 0521632803
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* Beatty, J. Kelly, et al., eds. 1999. ''The New Solar System''. Cambridge: Cambridge University Press. ISBN 0-521-64587-5
 +
* Karttunen, H., et al., eds. 2003. ''Fundamental Astronomy''. Helsinki: Springer-Verlag. ISBN 3-540-00179-4
  
==External links and data sources==
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==External links==
*[http://www.ifa.hawaii.edu/faculty/jewitt/kb.html Dave Jewitt's page @ University of Hawaii]
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*[http://www.solarviews.com/eng/kuiper.htm The Kuiper Belt. ''Views of the Solar System'' by  Calvin J. Hamilton.] Retrieved March 7, 2008.
**[http://www.ifa.hawaii.edu/faculty/jewitt/kb/gerard.html The belt's name]
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*[http://www.boulder.swri.edu/ekonews/ The Kuiper Belt Electronic Newsletter]. Retrieved March 7, 2008.
*[http://www.boulder.swri.edu/ekonews/ The Kuiper Belt Electronic Newsletter]
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*[http://www.johnstonsarchive.net/astro/tnos.html Wm. Robert Johnston's TNO page]. Retrieved March 7, 2008.
*[http://www.johnstonsarchive.net/astro/tnos.html Wm. Robert Johnston's TNO page]
+
*[http://cfa-www.harvard.edu/iau/lists/OuterPlot.html Plot of the Outer Solar System. ''Minor Planet Center'']. Retrieved March 7, 2008.
*[http://cfa-www.harvard.edu/iau/lists/OuterPlot.html Minor Planet Center: Plot of the Outer Solar System], illustrating Kuiper gap
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*[http://www.nature.com/nature/journal/v424/n6949/fig_tab/nature01725_F1.html Diagram showing the Kuiper belt and Oort cloud to scale with our planetary system. ''nature.com'']. Retrieved March 7, 2008.
*[http://www.iau.org/ Website of the International Astronomical Union] (debating the status of TNOs)
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* [http://www.space.com/scienceastronomy/060814_tno_found.html Discovery Hints at a Quadrillion Space Rocks Beyond Neptune. ''Space.com'']. Retrieved March 7, 2008.
*[http://www.astronomy2006.com XXVIth General Assembly 2006]
 
*[http://www.nature.com/nature/journal/v424/n6949/fig_tab/nature01725_F1.html nature.com article: diagram displaying inner solar system, Kuiper Belt, and Oort Cloud]
 
* SPACE.com: [http://www.space.com/scienceastronomy/060814_tno_found.html Discovery Hints at a Quadrillion Space Rocks Beyond Neptune] (Sara Goudarzi) 15 August 2006 06:13 am ET
 
  
 
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Revision as of 10:42, 2 April 2008

Artist's rendering of the Kuiper Belt and hypothetical more distant Oort cloud.

The Kuiper belt (pronounced /ˈkaɪpɚ/, rhymes with "viper") is a zone of the solar system extending from the orbit of Neptune (at 30 astronomical units (AU) to 50 AU from the Sun. The objects within the Kuiper belt, together with the members of the scattered disk extending beyond, are collectively referred to as trans-Neptunian, along with any hypothetical Hills cloud and Oort cloud objects.

Together with the asteroid belt which is the source of meteorites, Kuiper belt objects give important clues to the distribution of materials and evolution history of the Solar System through their orbital characteristics and compositions. Kuiper belt objects coming as comets and dusts are gift for humanity to go back in time and study the early stage of the Solar System.

Origins

The debris disks around these two remote stars seem equivalent of our own Solar system's Kuiper Belt. The left image is a "top view," and the right image is an "edge view." 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. Observed with Hubble Space Telescope.

Modern computer simulations show the Kuiper belt to have been strongly influenced by Jupiter and Neptune. During the early period of the Solar System, Neptune's orbit is thought to have migrated outwards from the Sun due to interactions with minor bodies. In the process, Neptune swept up, or gravitationally ejected all the bodies closer to the Sun than about 40 AU (the inner edge of the region occupied by cubewanos, objects in the main Kuiper belt), apart from those which fortuitously were in a 2:3 orbital resonance. These resonant bodies formed the plutinos. The present Kuiper Belt members are thought to have largely formed in their present position, although a significant fraction may have originated in the vicinity of Jupiter and been ejected by it to the far regions of the Solar system.

Hypothesis

The first astronomers to suggest the existence of this belt were Frederick C. Leonard in 1930, and Kenneth E. Edgeworth in 1943. In 1951, Gerard Kuiper suggested that the belt was the source of short period comets (those having an orbital period of less than 200 years). More detailed conjectures about objects in the belt were done by Al G. W. Cameron in 1962, Fred L. Whipple in 1964, and Julio Fernandez in 1980. The belt and the objects in it were named after Kuiper following the discovery of (15760) 1992 QB1.

Name

An alternative name, Edgeworth-Kuiper belt is used to credit Edgeworth. 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.

Discoveries thus far

Over 800 Kuiper Belt Objects (KBOs) (a subset of trans-Neptunian objects) have been discovered in the belt, almost all of them since 1992. This was primarily a result of advances in computer hardware/software and CCD of high quantum efficiency, allowing for cost effective automated KBO searching in combination with large telescopes.

Among the largest are Pluto and Charon, but since the year 2000, other large objects that approached and even exceeded their size were identified. 50000 Quaoar, discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest body in the asteroid belt, Ceres. (136472) 2005 FY9 (nicknamed "Easterbunny") and (136108) 2003 EL61 (nicknamed "Santa"), both announced on July 29, 2005, are larger still. Other objects, such as 28978 Ixion (discovered in 2001) and 20000 Varuna (discovered in 2000) while smaller than Quaoar, are nonetheless quite sizable.

The exact classification of these objects is unclear, since they are probably fairly different from the objects of the asteroid belt. The largest recent discovery was Eris, which is actually larger than Pluto. This led scientists to question the definition of the term planet, as it is larger than Pluto and was often called a tenth planet by some sources.[1] Due to this discovery, on August 24, 2006, the IAU announced a first-ever definition of "planet," and these large Kuiper belt objects accordingly became known officially as dwarf planets. A number of astronomers around the world came out in public disagreement with the definition in the days following it.

Neptune's moon Triton is commonly thought to be a captured KBO.

Classification and Distribution

Resonant and classical objects

Orbit classification (schematic of semi-major axes).

Orbital resonance with Neptune is the major factor of the current classification of KBO, even if most of them (more than 600 objects as of October 2005) are not resonant. These objects, called Classical Kuiper Belt objects or cubewanos, are found between the 2:3 resonance (at ~39.4 AU, populated by more than 150 plutinos) and the 1:2 resonance (at ~47.7 AU, populated by a few twotinos). Minor resonances exist at 3:4, 3:5, 4:7 and 2:5 (this last, also fairly strongly occupied). The 1:2 resonance appears to be an edge. It is not clear whether it is actually the outer edge of the Classical Belt or just the beginning of a gap.

Large cubewanos, plutinos and near scattered objects.

The next diagram shows the largest objects of the Kuiper belt: Pluto with the largest plutinos: 90482 Orcus and 28978 Ixion, a few big classical objects, and two scattered objects (beyond the 1:2 resonance, in grey), notably Eris thought to be the biggest trans-Neptunian object known. The eccentricity of the orbits is represented by the red segments (extending from perihelion to aphelion) with inclination represented on Y axis. While eccentric orbits of many resonant KBOs bring them inside Neptune's orbit periodically, classical KBOs are in more circular orbits (short red segments, Quaoar).

Initially, the Kuiper belt was thought to be a flat belt (populated by objects on moderately eccentric, low-inclination orbits), as opposed to high inclination orbits of the "scattered" disk objects. With the discovery of the large cubewanos, this belt became a thick disk or torus. It now appears that the distribution of orbit inclinations peaks around 4 and 30-40 degrees, giving rise to a division into two groups: Cold and hot, respectively. The cold group would have been born outside the Neptune's orbit while the hot migrated outwards due to close interactions with Neptune. The cold/hot terminology comes from analogy to particles in a gas, where, as temperature rises, so do the relative velocities between the particles.

This grouping may yet be revised, however, as the current distribution of known objects is likely to be strongly affected by observational bias. Most observations have so far focused on near-ecliptic objects. Even objects with high inclinations (for example, 2004 XR190) were actually found at near ecliptic positions. In addition, most of the known KBOs are detected near their closest approaches to the Sun since they appear dimmer at greater distances.

Orbit distribution

Distribution of cubewanos, plutinos and near scattered objects.

The last diagram shows the distribution of known Kuiper Belt objects. The resonant objects: Neptune Trojans (1:1 resonance), plutinos (2:3), twotinos (1:2) and a handful of objects occupying other resonances are shown in red. Confirmed plutinos are plotted in dark red. Beyond the 1:2 resonance, scattered disk objects are plotted for reference.

Interestingly, low inclination regions which include the "cold" majority of cubewanos appear devoid of the largest objects (see diagram). The (observed) distribution has been a challenge to the theories of the formation of the Kuiper belt as it is far too complex to be explained simply as being the remains of the original accretion disc dating back to the formation of the Solar System. Numerous competing models are being developed, typically involving so-called Neptune migration. It was suggested in the 1980s that interaction between giant planets and a massive disk of small particles would not only scatter the disk but also cause (via momentum transfer) the giants to migrate to more distant orbits. While all four giant planets would be affected, Neptune could have migrated as far as 5AU outwards to reach its current position at around 30 AU. Such models can explain for example, the ‘trapping’ of small bodies into the plutino 2:3 resonances.

However, the present models still fail to account for many of the characteristics of the distribution and, quoting one of the scientific articles,[2] the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software."

The belt should not be confused with the hypothesized Oort cloud, which is far more distant.

Size distribution

Illustration of the power law.

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[3] give q = 4 ±0.5.

The relationship is illustrated on the graph for q=4. Less formally, there is for instance 8 (=23) times more objects in 100-200 km range than objects in 200-400 km range. In other words, for a single object with the diameter of 1000 km, there should be around 1000 (=103) objects with diameter of 100 km. Of course, only the magnitude is actually known, the size is inferred assuming albedo (not a safe assumption for larger objects)

Unresolved issues

Missing mass dilemma

The total mass of Kuiper Belt objects can be inferred by models of the origin of the Solar System from the known mass of the planets and known distribution of mass closer to the Sun. While the estimates are model-dependent, the total mass of around 30 MEarth is expected. Surprisingly, the actual distribution appears to be well below that value, even accounting for the observational bias. The observed density is at least 100 times smaller than the model calls for.[4] This missing 99 percent of the mass can be hardly dismissed as it is required for the accretion of bigger (greater than 100 km) objects ever taking place. At the current low density, these objects simply could not be created. Moreover, the eccentricity and inclination of current orbits make 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.[5]

The "Kuiper cliff"

Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU; however, observation has revealed that in fact, at 50 AU, the number of observed objects in the Kuiper belt falls precipitously. This falloff is known as the "Kuiper cliff," and its cause is unknown, though Alan Stern of the Southwest Research Institute has claimed that a large planetary object might be responsible.[6] Bernstein and Trilling et al. have found evidence that the observed rapid decline in objects of 100 km or more in radius beyond 50 AU is a real decline in the number of objects, and not just an observational effect.[7]

The term "Kuiper belt object" (KBO)

Most models of solar system formation show icy planetoids first forming in the Kuiper belt, while later gravitational interactions displace some of them outwards into the so-named scattered disc. Strictly speaking, a KBO is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. However, in some scientific circles the term 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 (such as in the scattered disk region). Discoverer Michael E. Brown, for instance, has referred to Eris as a KBO, despite it having a semi-major axis of 67 AU, well clear of the Kuiper cliff. Other leading trans-Neptunian researchers have been more cautious in applying the KBO label to objects clearly outside the belt in the current epoch.

List of the brightest KBOs

The brightest known KBOs (with absolute magnitudes less than 4.0), are:

Permanent
Designation
Provisional
Designation
Absolute magnitude Albedo Equatorial diameter
(km)
Semimajor axis
(AU)
Date found Discoverer Diameter method
Pluto −1.0 0.6 2320 39.4 1930 C. Tombaugh occultation
136472 2005 FY9 −0.3 0.8 ± 0.2 1800 ± 200 45.7 2005 M. Brown, C. Trujillo & D. Rabinowitz assumed albedo
136108 2003 EL61 0.1 0.6 (assumed) ~1500 (1 43.3 2005 M. Brown, C. Trujillo & D. Rabinowitz assumed albedo
Charon S/1978 P 1 1 0.4 1205 39.4 1978 J. Christy occultation
Orcus 2004 DW 2.3 0.1 (assumed) ~1500 39.4 2004 M. Brown, C. Trujillo & D. Rabinowitz assumed albedo
Quaoar 2002 LM60 2.6 0.10 ± 0.03 1260 ± 190 43.5 2002 C. Trujillo & M. Brown disk resolved
Ixion 2001 KX76 3.2 0.25 – 0.50 400 – 550 39.6 2001 DES thermal
55636 2002 TX300 3.3 > 0.19 < 709 43.1 2002 NEAT thermal
55565 2002 AW197 3.3 0.14 – 0.20 650 – 750 47.4 2002 C. Trujillo, M. Brown, E. Helin, S. Pravdo,
K. Lawrence & M. Hicks / Palomar Observatory
thermal
55637 2002 UX25 3.6 0.08? ~910 42.5 2002 A. Descour / Spacewatch assumed albedo
Varuna 2000 WR106 3.7 0.12 – 0.30 450 – 750 43.0 2000 R. McMillan thermal
2002 MS4 3.8 0.1 (assumed) 730? 41.8 2002 C. Trujillo, M. Brown assumed albedo
2003 AZ84 3.9 0.1 (assumed) 700? 39.6 2003 C. Trujillo, M. Brown, E. Helin, S. Pravdo,
K. Lawrence & M. Hicks [1]
assumed albedo

Notes

  1. Hubble Telescope, Hubble Finds 'Tenth Planet' is Slightly Larger than Pluto. Retrieved April 27, 2007.
  2. Renu Malhotra, Nonlinear Resonances in the Solar System. Retrieved April 30, 2007.
  3. G.M. Bernstein, D.E. Trilling, R.L. Allen, K.E. Brown, M. Holman, and R. Malhotra, The size distribution of transneptunian bodies, The Astronomical Journal, 128, 1364-1390.
  4. D. Jewitt and A. Delsanti, The Solar System Beyond The Planets. Retrieved May 2, 2007.
  5. A. Morbidelli, Origin and dynamical evolution of comets and their reservoirs (Preprint on arXiv). Retrieved May 2, 2007.
  6. M. Brooks, 13 things that do not make sense. Retrieved May 2, 2007.
  7. Bernstein, G.M., Trilling, D.E., Allen, R.L., Brown, M.E., Holman, M., and Malhotra, R. (2004). The Size Distribution of Trans-Neptunian Bodies. The Astrophysical Journal. Retrieved May 2, 2007.

References
ISBN links support NWE through referral fees

  • Bakich, Michael E. 2000. The Cambridge Planetary Handbook. Cambridge: Cambridge University Press. ISBN 0521632803
  • Beatty, J. Kelly, et al., eds. 1999. The New Solar System. Cambridge: Cambridge University Press. ISBN 0-521-64587-5
  • Karttunen, H., et al., eds. 2003. Fundamental Astronomy. Helsinki: Springer-Verlag. ISBN 3-540-00179-4

External links


The minor planetsedit
Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
Large trans-Neptunian objects
Kuiper belt: Orcus | Pluto | Ixion | 2002 UX25 | Varuna | 2002 TX300 | 2003 EL61 | Quaoar | 2005 FY9 | 2002 AW197
Scattered disc: 2002 TC302 | Eris | 2004 XR190 | Sedna
 See also Triton, astronomical objects and the solar system's list of objects, sorted by radius or mass. 
For pronunciation, see: Centaur and TNO pronunciation.
 The Solar System
Solar System Template Final.png
The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
SSSBs: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.

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