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 disc objects (SDOs); a subset of the broader family of trans-Neptunian objects (TNOs). The scattered disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°. The perihelion distances of SDOs are greater than 30 astronomical units (AU). These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". [Maggie Masetti. (2007). " [http://heasarc.gsfc.nasa.gov/docs/cosmic/solar_system_info.html Cosmic Distance Scales - The Solar System] ". Website of NASA's High Energy Astrophysics Science Archive Research Center. Retrieved 2008 07-12.] The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.

Due to its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets observed in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send it close to Earth, transforming it into a periodic comet. Oort cloud objects are also understood to have originated in the scattered disc.

Discovery

During the 1980s, the introduction of the charge-coupled device in telescopes in combination with higher capacity computers for image analysis allowed for more efficient deep sky surveys than was practical using photography. This led to a flood of new discoveries; between 1992 and 2006, over a thousand trans-Neptunian Objects were detected. [cite conference
author=Scott S Sheppard
title=Small Bodies in the Outer Solar System
booktitle=New Horizons in Astronomy: Frank N. Bash Symposium 2005
publisher=Astronomical Society of the Pacific
pages=3–14 | date=October 16–18, 2005
location=Austin, Texas
url=http://www.ciw.edu/sheppard/pub/Sheppard06smallbodies.pdf
accessdate=2008-08-14 | isbn=1583812202 ]

The first scattered disc object to be recognised as such was mpl|(15874) 1996 TL|66, [cite journal|author= Jane Luu, Brian G. Marsden, David Jewitt, et al.|date=5 June 1997|url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/TL66_1996/TL66.pdf|title=A new dynamical class of object in the outer Solar System|journal=Nature|volume=387| doi=10.1038/42413|accessdate=2008-08-02|pages=573–575] cite book
title=Beyond Pluto: Exploring the Outer Limits of the Solar System
publisher=Cambridge University Press
author=John Keith Davies,
year=2001
url=http://assets.cambridge.org/97805218/00198/frontmatter/9780521800198_frontmatter.pdf
format=PDF
accessdate=2008-07-02
isbn=0-521-80019-6
pages=111] originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999: mp|1999 CV|118, mp|1999 CY|118 and mp|1999 CF|119. The first object presently classified as a scattered disc object to be discovered was mpl|(48639) 1995 TL|8, found in 1995 by Spacewatch. [Lutz D. Schmadel, (2003). "Dictionary of Minor Planet Names" (5th rev. and enlarged ed. edition). Berlin: Springer. Page 925 (Appendix 10). Also see McFadden, Lucy-Ann, Weissman, Paul & Johnson, Torrence (1999). "Encyclopedia of the Solar System". San Diego: Academic Press. Page 218.]

As of 2008, over 100 scattered disc objects have been identified, including mpl|2007 UK|126 (discovered by Schwamb, Brown, and Rabinowitz), [cite web|title=2007 UK126|publisher=Minor Planet Electronic Circ., 2008-D38 (2008)|url=http://adsabs.harvard.edu/abs/2008MPEC....D...38S|accessdate=2008-07-14] mpl|(84522) 2002 TC|302 (NEAT), Eris (Brown, Trujillo, and Rabinowitz) [cite web| author=Staff| date=2007-05-01| url=http://www.cfa.harvard.edu/iau/lists/NumberedMPs.html| title=Discovery Circumstances: Numbered Minor Planets| publisher=IAU: Minor Planet Center| accessdate=2007-05-05 ] Sedna (Brown, Trujillo, and Rabinowitz) [cite web
title=Discovery Circumstances: Numbered Minor Planets (90001)-(95000)
publisher=IAU: Minor Planet Center
url=http://www.cfa.harvard.edu/iau/lists/NumberedMPs090001.html
accessdate=2008-07-23 ] and mpl|2004 VN|112 (Deep Ecliptic Survey). [cite web
author=Marc W. Buie
date=2007-11-08
title=Orbit Fit and Astrometric record for 04VN112
publisher=SwRI (Space Science Department)
url=http://www.boulder.swri.edu/~buie/kbo/astrom/04VN112.html
accessdate=2008-07-17] Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer scattered disc objects have been observed to date.cite book
title = Encyclopedia of the Solar System
chapter = Comet Populations and Cometary Dynamics
author = Harold F. Levison, Luke Donnes
publisher = Academic Press
year = 2007
editor = Lucy Ann Adams McFadden, Lucy-Ann Adams, Paul Robert Weissman, Torrence V. Johnson
edition = 2^nd
publication-place = Amsterdam; Boston
isbn = 0120885891
pages = 575–588]

ubdivisions of trans-Neptunian space

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc. A third reservoir of trans-Neptunian objects, the Oort cloud, is believed to exist, although no confirmed direct observations of the Oort cloud have been made. [cite web
title=Origin and dynamical evolution of comets and their reservoirs
author=Alessandro Morbidelli
url=http://arxiv.org/abs/astro-ph/0512256
year=2006 |accessdate=2008-08-28
date=2008-02-03 |format=PDF |publisher=arxiv] Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".

cattered disc versus Kuiper belt

The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU [cite journal
url= http://www.iop.org/EJ/abstract/1538-3881/121/5/2792
title=Thermal Evolution and Differentiation of Edgeworth-Kuiper Belt Objects
author=M. C. De Sanctis, M. T. Capria, and A. Coradini
journal=The Astronomical Journal
doi= 10.1086/320385
volume=121
pages=2792–2799
accessdate=2008-08-28
year=2001] comprising two main populations: the classical Kuiper belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper belt objects; those which Neptune has locked into a precise orbital ratio such as 3:2 (the KBO goes around twice for every three Neptune orbits) and 2:1 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the growth of the Solar System, since the objects are never close enough to Neptune to be affected by its gravity. Those in 3:2 resonances are known as "plutinos", because Pluto is the largest member of their group, whereas those in 2:1 resonances are known as "twotinos".

In contrast to the Kuiper belt, the scattered disc population can be disturbed by Neptune. Scattered disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that.cite web|title=The Scattered Disk: Origins, Dynamics and End States|author=Rodney S. Gomes, Julio A. Fernandez, Tabare Gallardo, Adrian Brunini|work=Universidad de la Republica, Uruguay|url=http://www.fisica.edu.uy/~gallardo/scatdisk.pdf|year=2008|accessdate=2008-08-10] Ongoing research [cite journal|title=The Populations of Comet-like Bodies in the Solar System|author=J Horner, NW Evans, ME Bailey, DJ Asher|journal=Monthly Notices of the Royal Astronomical Society|volume=343|pages=1057–1066|url=http://star.arm.ac.uk/preprints/396.pdf|year=2003|accessdate=2007-06-29|doi=10.1046/j.1365-8711.2003.06714.x] suggests that the centaur class of icy planetoids may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects. [Remo notes that Cis-Neptunian bodies "include terrestrial and large gaseous planets, planetary moons, asteroids, and main-belt comets within Neptune's orbit."(Remo 2007)] Some objects, like (29981) 1999 TD₁₀, blur the distinction [cite web|title=New Object in Solar System Defies Categories |author=Kenneth Silber|work=space.com|year=1999|url=http://www.space.com/scienceastronomy/solarsystem/centaur_disc_991111.html|accessdate=2008-08-12] and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects, now lists centaurs and SDOs together. The MPC also makes a clear distinction between the Kuiper belt and the scattered disc; separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs).cite web
url=http://cfa-www.harvard.edu/iau/lists/Centaurs.html
title=List Of Centaurs and Scattered-Disk Objects
publisher=Central Bureau for Astronomical Telegrams, Harvard-Smithsonian Center for Astrophysics
date=2008-07-02
author=IAU: Minor Planet Center
accessdate=2008-07-02] However, the difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper belt object" (or SKBO) for bodies of the scattered disc. [cite web |year=2005 |author=David Jewitt |title=The 1000 km Scale KBOs |work=University of Hawaii |url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html |accessdate=2006-07-16]

Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune", but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times". That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere, and the Kuiper belt as its "complement ... in the "a" > 30 AU region"; the region of the Solar System in which an object's farthest distance from the Sun is greater than 30 AU.

Detached objects

The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered disc object. Its discoverer Michael E. Brown has suggested instead that it should be considered an inner Oort cloud object rather than a member of the scattered disc, because, with a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets.cite web
title=Sedna (The coldest most distant place known in the solar system; possibly the first object in the long-hypothesized Oort cloud)
author=Michael E. Brown
publisher=California Institute of Technology, Department of Geological Sciences
url=http://www.gps.caltech.edu/~mbrown/sedna/
accessdate=2008-07-02] Thus, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc.cite journal|title=Dynamical classification of trans-Neptunian objects: Probing their origin, evolution, and interrelation|author=Patryk Sofia Lykawka, Tadashi Mukai|work=Kobe volume 189|issue=1|url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4MY0MMW-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=263a1288080d4fc7cd1d53d76f7dc580
accessdate=2008-07-24|pages=213–232]

Sedna is not the only such object: mpl|2000 CR|105 (discovered before Sedna) and mpl|2004 VN|112 have a perihelion too far away from Neptune to be influenced by it. This led to a discussion among astronomers about a new minor planet set, called the "Extended scattered disc" (E-SDO). [cite web|url=http://www.obs-nice.fr/gladman/cr105.html|title=Evidence for an Extended Scattered Disk? |work=Observatoire de la Cote d'Azur|author=Brett Gladman |accessdate=2008-08-02] mpl|2000 CR|105 may also be an inner Oort cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. More recently, these objects have been referred to as "detached",cite book| authorlink=David C. Jewitt| author=David C. Jewitt| coauthors= A. Delsanti| chapter=The Solar System Beyond The Planets| title=Solar System Update : Topical and Timely Reviews in Solar System Sciences| publisher=Springer-Praxis Ed.| isbn= 3-540-26056-0| year=2006 ( [http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf Preprint version (pdf)] )] or "Distant Detached Objects" (DDO).

There are no clear boundaries between the scattered and detached regions. Gomes et al. define scattered disc objects as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all DDOs are scattered disc objects. Since detached objects cannot be produced by Neptune scattering, alternative explanations have been put forward, including a passing starcite journal
journal=The Astronomical Journal
issue=5
volume=128
pages=2564–2576
year=2004
month=November
title=Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12
author=Alessandro Morbidelli
coauthors=Harold F. Levison
url=http://www.iop.org/EJ/abstract/1538-3881/128/5/2564/
doi=10.1086/424617
accessdate=2008-07-02 ] or a distant, planet-sized object.cite journal
url=http://adsabs.harvard.edu/abs/2006Icar..184..589G
title=A distant planetary-mass solar companion may have produced distant detached objects
author=Rodney S Gomes
coauthors= John J. Matese, and Jack J. Lissauer
journal=Icarus
volume=184
issue=2
pages=589–601
year=2006
month=October
doi=10.1016/j.icarus.2006.05.026
accessdate=2008-07-02] The classification suggested by the Deep Ecliptic Survey team introduces a formal distinction between "Scattered-Near" objects (which could be scattered by Neptune) from "Scattered-Extended" objects (such as Sedna) using Tisserand's parameter value of 3.cite journal
author=J. L. Elliot, S. D. Kern, K. B. Clancy, et. al.|title=The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population|journal=The Astronomical Journal|volume=129|year=2006|url= http://alpaca.as.arizona.edu/~trilling/des2.pdf|accessdate=2008-08-02|pages=1117–1162
doi=10.1086/427395]

Orbits

The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, scattered disc objects' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets. For this reason Gladman et al. prefer to refer to the region as the "scattering disc", rather than scattered. [cite book|title=The Solar System Beyond Neptune|editors=MA Barruci, H Boehnhardt, DF Cruikshank and A Morbidelli|chapter=Nomenclature in the Outer Solar System|author=B Gladman, BG Marsden, C Vanlaerhoven|year=2008|publisher=University of Arizona Press|pages=43–57] Unlike Kuiper belt objects (KBOs), the orbits of scattered objects can be inclined as much as 40° from the ecliptic. [cite journal|title=The trans-neptunian object UB313 is larger than Pluto.)|author=F Bertoldi, W Altenhoff, A Weiss, E Menten, MC Thum|journal=Nature|year=2006|volume=439|pages=563–4|url=http://www.ncbi.nlm.nih.gov/pubmed/16452973|accessdate=2008-07-30|doi=10.1038/nature04494]

The scattered disc objects are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.cite journal
author= Chadwick A. Trujillo
coauthors=David C. Jewitt and Jane X. Luu
title=Population of the Scattered Kuiper Belt
journal=The Astrophysical Journal
volume=529
pages=L103–L106
date=2000-02-01
url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/SKBO/tjl2000.pdf
format=PDF
accessdate=2008-07-02
doi=10.1086/312467 ] Having a perihelion greater than 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence.cite web
month=July | year=2000
title=Scattered Kuiper Belt Objects (SKBOs)
publisher=Institute for Astronomy
author=David Jewitt
url=http://www.ifa.hawaii.edu/~jewitt/kb/kb-scattered.html
accessdate=2008-05-13]

The classical objects (cubewanos) are very different from the scattered objects: more than 30% of all cubewanos are on low–inclination, near–circular orbits whose eccentricities peak at 0.25. [cite web|title=The formation of the Kuiper belt by the outward transport of bodies during Neptune’s migration|author=Harold F. Levison, Alessandro Morbidelli|url=http://www.obs-nice.fr/morby/stuff/NATURE.pdf|year=2003|accessdate=2007-06-25|format=pdf] Classical objects seldom possess eccentricities over 0.2; scattered objects possess eccentricities ranging from 0.2 to 0.8. Though the inclinations of scattered objects are similar to the more extreme KBOs, very few scattered objects have orbits as close to the ecliptic as much of the KBO population.

Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of resonant orbits within the scattered disc include 1:3, 2:7 3:11, 5:22 and 4:79.

Formation

The scattered disc is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties.cite book|author=Alessandro Morbidelli|coauthors=ME Brown|title=Comets II|editor= MC Festou, HU Keller, HA Weaver|publisher=University of Arizona Press|location=Tucson|date=2004-11-01|pages=175–91|chapter=The Kuiper Belt and the Primordial Evolution of the Solar System|isbn=0816524505|oclc=56755773|url=http://www.lpi.usra.edu/books/CometsII/7004.pdf|accessdate=2008-07-27]

According to contemporary models, the scattered disc formed when Kuiper belt objects (KBOs) were "scattered" into eccentric and inclined orbits by gravitational interaction with Neptune and the other outer planets.cite journal
author=Martin J. Duncan, Harold F. Levison
title=A Disk of Scattered Icy Objects and the Origin of Jupiter-Family Comets
journal=Science
volume=276 |issue=5319 |pages=1670–1672 |year=1997
doi=10.1126/science.276.5319.1670
pmid=9180070
accessdate=2008-07-12 ] The amount of time for this process to occur remains uncertain. One hypothesis estimates a period equal to the entire age of the Solar System; [cite journal|title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets|author=Harold F. Levison, Martin J Duncan|journal=Icarusissue=1|year=1997|pages=13–32|doi=10.1006/icar.1996.5637 |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45M91DF-24&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=6fa927eab9338038f6678e6fd538d2f5|accessdate=2008-07-18|volume=127] a second posits that the scattering took place relatively quickly, during Neptune's early migration epoch.

Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak gravitational instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time, the scattered disc would gradually form from these isolated events.

Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed "in situ" beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during the early evolution of the Solar System, perhaps through exchanges of angular momentum with scattered objects. [cite journal|title=Neptune’s Migration into a Stirred–Up Kuiper Belt: A Detailed Comparison of Simulations to Observations|author=Joseph M. Hahn, Renu Malhotra|journal=Astronomical Journal|url=http://arxiv.org/abs/astro-ph/0507319v1|date=13 July 2005|accessdate=2007-06-23 (subscription required)] Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt.cite web|title=Orbital shuffle for early solar system|author=Kathryn Hansen|work=Geotimes|url=http://www.geotimes.org/june05/WebExtra060705.html|date= 2005-06-07|accessdate=2007-08-26] As Neptune traveled outward, it scattered many Trans-Neptunian objects into higher and more eccentric orbits.cite journal
author=Martin J Duncan, Harold F Levison
title=A Disk of Scattered Icy Objects and the Origin of Jupiter-Family Comets
journal=Science
volume=276 |issue=5319 |pages=1670–1672 |year=1997
doi=10.1126/science.276.5319.1670
pmid=9180070
accessdate=2008-07-12 ] [cite journal|title=The Formation of Uranus and Neptune Among Jupiter and Saturn|author=E. W.Thommes, MJ Duncan, HF Levison, |url=http://www.iop.org/EJ/abstract/1538-3881/123/5/2862/|publisher=American Astronomical Society|doi=10.1086/339975|journal=The Astromonical Journal|location=Chicago, IL|volume=123|month=May | year=2002|accessdate=2007-06-24|pages=2862–83] This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch... [therefore] the scattered disc might not be so scattered." [cite journal|author=Joseph M Hahn, Renu Malhotra|month=November | year=2005|title=Neptune's Migration into a Stirred-Up Kuiper Belt: A Detailed Comparison of Simulations to Observations|journal=The Astronomical Journal|publisher=American Astronomical Society|location=Chicago, IL|volume=130|issue=5|pages=2392–414|issn=0004-6256|oclc=83709353|url=http://www.iop.org/EJ/article/1538-3881/130/5/2392/204509.web.pdf|accessdate=2008-07-27|doi= 10.1086/452638]

Composition

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane. Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane.

Astronomers originally supposed that the trans-Neptunian population would show a similar red surface colour, as they were believed to have originated in the same region and subjected to the same physical processes.cite book
title = Encyclopedia of the Solar System
chapter = Kuiper Belt Objects: Physical Studies
author = Stephen C. Tegler
publisher = Academic Press
year = 2007
editor = Lucy Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
edition = 2^nd
publication-place = Amsterdam; Boston
isbn = 0120885891
pages = 605–620] Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into complex organic molecules by energy from the Sun. This would absorb blue light, creating a reddish hue. Most classical objects display this colour, but scattered objects do not; instead, they present a white or greyish appearance.

One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets. Mike Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen to the surface, creating an inches-thick layer of bright white ice. Pluto, conversely, is closer to the Sun, and so will retain its nitrogen atmosphere until its orbit takes it far enough from the Sun for it to freeze.cite journal
title = Discovery of a Planetary-sized Object in the Scattered Kuiper Belt
author = Michael E. Brown, Chadwick A. Trujillo, David L. Rabinowitz
journal = The Astrophysical Journal
year = 2005
volume =635
issue = 1
pages = L97–L100
doi = 10.1086/499336
url =http://arxiv.org/abs/astro-ph/0508633
accessdate=2008-08-02
format = abstract page]

Comets

The Kuiper belt was initially believed to be the source of the Solar System's ecliptic comets. However, studies of the region since 1992 have revealed that the orbits within what is now called the Kuiper belt are relatively stable, and that these comets originate from the more dynamic scattered disc. [cite journal|title=The Kuiper Belt and the Solar System's Comet Disk|author=Gladman, Brett|year=2005|journal=Science|volume=307|issue=5706|pages=pp. 71–75|doi=10.1126/science.1100553|pmid=15637267]

Comets can loosely be divided into two categories: short-period and long period—the latter being believed to originate in the Oort cloud. There are two major 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. The former type, the Jupiter family, are believed to have originated from the scattered disc.cite book
title = Encyclopedia of the Solar System
chapter = Kuiper Belt Dynamics
author =Alessandro Morbidelli, Harold F. Levison.
publisher = Academic Press
year = 2007
editor = Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
edition = 2^nd
publication-place = Amsterdam; Boston
isbn = 0120885891
pages = 589–604] The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. [cite journal|title=The Populations of Comet-like Bodies in the Solar System|author=J Horner, NW Evans, ME Bailey, DJ Asher|journal=Monthly Notices of the Royal Astronomical Society|volume=343|pages=1057–1066|url=http://star.arm.ac.uk/preprints/396.pdf|year=2003|accessdate=2007-06-29|doi=10.1046/j.1365-8711.2003.06714.x]

There are many differences between SDOs and Jupiter-family comets, even though many of the latter may have originated in the scattered disc. Although the centaurs share a reddish or neutral coloration with many SDOs, their nuclei are bluer, indicating a fundamental chemical or physical difference. One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material.cite journal|title=From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter|author= David C Jewitt|volume=123|pages=1039–1049|year=2001 |journal=The Astronomical Journal|url=http://www.iop.org/EJ/article/1538-3881/123/2/1039/201410.text.html|accessdate=2007-06-26|doi=10.1086/338692]

ee also

* List of trans-Neptunian objects
* List of plutoid candidates

References