View source for Scattered disc

{{short description|Collection of bodies in the extreme Solar System}}
[[File:Eris and dysnomia2.jpg|thumb|300px|[[Eris (dwarf planet)|Eris]] (center), the largest known scattered-disc object, and its moon [[Dysnomia (moon)|Dysnomia]] (left of object)]]
{{TNO}}

The '''scattered disc''' (or '''scattered disk''') is a distant [[circumstellar disc]] in the [[Solar System]] that is sparsely populated by icy [[small Solar System bodies]], which are a subset of the broader family of [[trans-Neptunian object]]s. The scattered-disc objects (SDOs) have [[orbital eccentricity|orbital eccentricities]] ranging as high as 0.8, [[inclinations]] as high as 40°, and [[perihelia]] greater than {{convert|30|AU|km mi|lk=in}}. These extreme orbits are thought to be the result of gravitational "scattering" by the [[gas giant]]s, and the objects continue to be subject to [[perturbation (astronomy)|perturbation]] by the planet [[Neptune]].

Although the closest scattered-disc objects approach the [[Sun]] at about 30–35&nbsp;AU, their orbits can extend well beyond 100&nbsp;AU. This makes scattered disc objects among the coldest and most distant objects in the Solar System.<ref>{{Cite web |last=Masetti |first=Maggie |date=2007 |title=Cosmic Distance Scales&nbsp;– The Solar System |url=https://imagine.gsfc.nasa.gov/features/cosmic/solar_system_info.html |access-date=2008-07-12 |website=Website of NASA's High Energy Astrophysics Science Archive Research Center}}</ref> The innermost portion of the scattered disc overlaps with a [[torus]]-shaped region of orbiting objects traditionally called the [[Kuiper belt]],<ref name="Morbidelli2008" /> but its outer limits reach much farther away from the Sun and farther above and below the [[ecliptic]] than the Kuiper belt proper.{{efn|The literature is inconsistent in the use of the phrases "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 a single publication.<ref>{{Cite book |title=Encyclopedia of the solar system |date=1999 |publisher=Academic Press |isbn=978-0-12-226805-2 |editor-last=Weissman |editor-first=Paul R. |location=San Diego |pages=584 (footnote) |editor-last2=McFadden |editor-first2=Lucy-Ann |editor-last3=Johnson |editor-first3=Torrence V.}}</ref> In this article, the scattered disc will be considered a separate population from the Kuiper belt.}}

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most [[periodic comet]]s in the Solar System, with the [[Centaur (minor planet)|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.<ref name="Horner2004a">{{Cite journal |last=Horner |first=J. |last2=Evans |first2=N. W. |last3=Bailey |first3=M. E. |date=2004 |title=Simulations of the population of Centaurs – I. The bulk statistics |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=354 |issue=3 |pages=798–810 |arxiv=astro-ph/0407400 |bibcode=2004MNRAS.354..798H |doi=10.1111/j.1365-2966.2004.08240.x |s2cid=16002759 |doi-access=free}}</ref> Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many objects of the proposed [[Oort cloud]] are also thought to have originated in the scattered disc. [[Detached object]]s are not sharply distinct from scattered disc objects, and some such as [[90377 Sedna|Sedna]] have sometimes been considered to be included in this group.

== Discovery ==
{{see also|Kuiper belt#History|l1=History of the Kuiper belt}}
Traditionally, devices like a [[blink comparator]] were used in astronomy to detect objects in the Solar System, because these objects would move between two exposures—this involved time-consuming steps like exposing and developing [[photographic film|photographic plates or films]], and people then using a blink comparator to manually detect prospective objects. During the 1980s, the use of [[charge-coupled device|CCD]]-based cameras in [[telescope]]s made it possible to directly produce electronic images that could then be readily [[digitizing|digitized]] and transferred to [[digital image]]s. Because the CCD captured more light than film (about 90% versus 10% of incoming light) and the blinking could now be done at an adjustable computer screen, the surveys allowed for higher throughput. A flood of new discoveries was the result: over a thousand trans-Neptunian objects were detected between 1992 and 2006.<ref>{{cite conference | first=Scott S. | last=Sheppard | title=Small Bodies in the Outer Solar System | book-title=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 | access-date=2008-08-14 | isbn=1-58381-220-2 | url-status=dead | archive-url=http://webarchive.loc.gov/all/20061012062457/http://www.ciw.edu/sheppard/pub/Sheppard06smallbodies.pdf | archive-date=2006-10-12 }}</ref>

The first scattered-disc object (SDO) to be recognised as such was {{mpl-|15874|1996 TL|66}},<ref>{{cite journal |first1=Jane X. |last1=Luu |first2=Brian G. |last2=Marsden |first3=David C. |last3=Jewitt |date=5 June 1997 |url=http://www2.ess.ucla.edu/~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|access-date=2008-08-02|pages=573–575|archive-url = https://web.archive.org/web/20070812095854/http://www.ifa.hawaii.edu/faculty/jewitt/papers/TL66_1996/TL66.pdf |archive-date = August 12, 2007|bibcode = 1997Natur.387..573L|issue=6633|s2cid=4370529 }}</ref><ref name="Davies1">{{cite book |title=Beyond Pluto: Exploring the Outer Limits of the Solar System |publisher=Cambridge University Press |first=John Keith |last=Davies |date=2001 |url=https://archive.org/details/beyondplutoexplo00davi/page/111 |access-date=2008-07-02 |isbn=978-0-521-80019-8 |page=[https://archive.org/details/beyondplutoexplo00davi/page/111 111] |url-access=registration }}</ref> originally identified in 1996 by [[astronomer]]s 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}}.<ref name="jewitt2000" /> The first object presently classified as an SDO to be discovered was {{mpl-|48639|1995 TL|8}}, found in 1995 by [[Spacewatch]].<ref>{{Cite book |last=Schmadel |first=Lutz D. |title=Dictionary of minor planet names |date=2012 |publisher=Springer |isbn=978-3-642-29717-5 |edition=Sixth revised and enlarged |location=Heidelberg; New York |pages=925 (Appendix 10)}} Also see {{Cite book |title=Encyclopedia of the solar system |date=1999 |publisher=Academic Press |isbn=978-0-12-226805-2 |editor-last=Weissman |editor-first=Paul Robert |location=San Diego |pages=218 |editor-last2=McFadden |editor-first2=Lucy-Ann Adams |editor-last3=Johnson |editor-first3=T. V.}}</ref>

As of 2011, over 200 SDOs have been identified,<ref name="MPC" /> including [[229762 Gǃkúnǁʼhòmdímà|Gǃkúnǁʼhòmdímà]] (discovered by Schwamb, Brown, and Rabinowitz), [[225088 Gonggong|Gonggong]] (Schwamb, Brown, and Rabinowitz)<ref>{{cite journal|title=2007 UK126|bibcode=2008MPEC....D...38S|last1=Schwamb|first1=M. E.|last2=Brown |first2=Michael E. |last3=Rabinowitz |first3=Davdi |last4=Marsden |first4=Brian G. |date=2008|pages=38|journal=Minor Planet Electronic Circ.|volume=2008-D38}}</ref> {{mpl-|84522|2002 TC|302}} ([[Near Earth Asteroid Tracking|NEAT]]), [[Eris (dwarf planet)|Eris]] (Brown, Trujillo, and Rabinowitz),<ref>{{cite web| date=2007-05-01| url=http://www.minorplanetcenter.org/iau/lists/NumberedMPs.html| title=Discovery Circumstances: Numbered Minor Planets| publisher=Minor Planet Center| access-date=2010-10-25 }}</ref> [[90377 Sedna|Sedna]] (Brown, Trujillo, and Rabinowitz),<ref>{{cite web |title=Discovery Circumstances: Numbered Minor Planets (90001)-(95000) |publisher=Minor Planet Center |url=http://www.minorplanetcenter.org/iau/lists/NumberedMPs090001.html |access-date=2010-10-25}}</ref> and [[474640 Alicanto]] ([[Deep Ecliptic Survey]]).<ref>{{Cite web |last=Buie |first=Marc W. |author-link=Marc W. Buie |date=2007-11-08 |title=Orbit Fit and Astrometric record for 04VN112 |url=http://www.boulder.swri.edu/~buie/kbo/astrom/04VN112.html |url-status=dead |archive-url=https://web.archive.org/web/20100818145946/http://www.boulder.swri.edu/~buie/kbo/astrom/04VN112.html |archive-date=2010-08-18 |access-date=2008-07-17 |publisher=SwRI (Space Science Department)}}</ref> 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 SDOs have been observed to date.<ref name="dynam">{{cite book |title=Encyclopedia of the Solar System |chapter=Comet Populations and Cometary Dynamics |first1=Harold F. |last1=Levison |first2=Luke |last2=Donnes |publisher=Academic Press |date=2007 |editor-first1=Lucy Ann |editor-last1=Adams McFadden |editor-first2=Paul Robert |editor-last2=Weissman |editor-first3=Torrence V. |editor-last3=Johnson |edition=2nd |location=Amsterdam; Boston |isbn=978-0-12-088589-3 |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588] |chapter-url=http://astropolis.pl/index.php?app=core |url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 }}</ref>

== Subdivisions of trans-Neptunian space ==
{{main|Trans-Neptunian object}}
[[File:TheKuiperBelt Projections 100AU Classical SDO.svg|thumb|210px|The eccentricity and inclination of the scattered-disc population compared to the classical and [[Resonant trans-Neptunian object|5:2 resonant]] Kuiper-belt objects]]

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc.<ref name= MorbidBrown /> A third reservoir of trans-Neptunian objects, the [[Oort cloud]], has been hypothesized, although no confirmed direct observations of the Oort cloud have been made.<ref name= Morbidelli2008>{{cite arXiv |title=Origin and dynamical evolution of comets and their reservoirs |first=Alessandro |last= Morbidelli |eprint=astro-ph/0512256 |date=2005}}</ref> Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "[[detached object]]s".<ref name= gomes />

=== Scattered disc versus Kuiper belt ===
{{see also|Kuiper belt}}
The Kuiper belt is a relatively thick [[torus]] (or "doughnut") of space, extending from about 30&nbsp;to 50&nbsp;AU<ref>{{cite journal |title=Thermal Evolution and Differentiation of Edgeworth-Kuiper Belt Objects|first1=M. C. |last1=De Sanctis|first2=M. T. |last2= Capria|first3=A. |last3= Coradini|journal=The Astronomical Journal| doi= 10.1086/320385 |volume=121 |pages=2792–2799|date=2001 |bibcode=2001AJ....121.2792D |issue=5|doi-access=free}}</ref> comprising two main populations of [[Kuiper belt object]]s (KBOs): the [[classical Kuiper-belt object]]s (or "cubewanos"), which lie in orbits untouched by Neptune, and the [[resonant trans-Neptunian object|resonant Kuiper-belt object]]s, those which Neptune has locked into a precise orbital ratio such as 2:3 (the object goes around twice for every three Neptune orbits) and 1:2 (the object goes around once for every two Neptune orbits). These ratios, called [[orbital resonance]]s, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 2:3 resonances are known as "[[plutino]]s", because [[Pluto]] is the largest member of their group, whereas those in 1:2 resonances are known as "[[resonant trans-Neptunian object#1:2 resonance ("twotinos", period ~330 years)|twotinos]]".

In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune.<ref name= dynam2 /> Scattered-disc objects come within gravitational range of Neptune at their closest approaches (~30&nbsp;AU) but their farthest distances reach many times that.<ref name=gomes>{{cite web|title=The Scattered Disk: Origins, Dynamics and End States|first1=Rodney S. |last1=Gomes |first2=Julio A. |last2=Fernandez |first3=Tabare |last3=Gallardo |first4=Adrian |last4=Brunini |work=Universidad de la Republica, Uruguay |url=http://www.fisica.edu.uy/~gallardo/scatdisk.pdf |date= 2008 |access-date=2008-08-10 }}</ref> Ongoing research<ref name=horner2003>{{cite journal|title=The Populations of Comet-like Bodies in the Solar System|last1=Horner|first1=J.|last2=Evans|first2=N. W.|last3=Bailey|first3=Mark E.|last4=Asher|first4=D. J.|journal=Monthly Notices of the Royal Astronomical Society|volume=343|pages=1057–1066|date=2003|doi=10.1046/j.1365-8711.2003.06714.x|arxiv=astro-ph/0304319|bibcode=2003MNRAS.343.1057H|issue=4|doi-access=free |s2cid=2822011}}</ref> suggests that the [[Centaur (minor planet)|centaurs]], a class of icy [[Minor planet|planetoid]]s that orbit between Jupiter and Neptune, 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.<ref>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)</ref> Some objects, like (29981) 1999 TD<sub>10</sub>, blur the distinction<ref>{{cite web|title=New Object in Solar System Defies Categories |first= Kenneth | last= Silber |work=space.com |date=1999 |url=http://www.space.com/scienceastronomy/solarsystem/centaur_disc_991111.html |access-date=2008-08-12 |url-status=dead |archive-url=https://web.archive.org/web/20050921034710/http://www.space.com/scienceastronomy/solarsystem/centaur_disc_991111.html |archive-date= September 21, 2005 }}</ref> and the [[Minor Planet Center]] (MPC), which officially catalogues all [[trans-Neptunian object]]s, now lists centaurs and SDOs together.<ref name= MPC />

The MPC, however, 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).<ref name= MPC>{{cite web
|url= http://www.minorplanetcenter.org/iau/lists/Centaurs.html |title= List Of Centaurs and Scattered-Disk Objects |publisher= Central Bureau for Astronomical Telegrams, Harvard-Smithsonian Center for Astrophysics|date= 2011-01-03 |author= IAU: Minor Planet Center
|access-date=2011-01-03}}</ref> However, the difference between the Kuiper belt and the scattered disc is not clear-cut, 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.<ref>{{cite web
|date=2008 |first=David C. |last=Jewitt |title= The 1000&nbsp;km Scale KBOs |url= http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html|access-date=2010-01-23}}</ref>

Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune,"<ref name= MorbidBrown /> 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."<ref name= MorbidBrown /> 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&nbsp;... in the ''a'' > 30&nbsp;AU region"; the region of the Solar System populated by objects with semi-major axes greater than 30&nbsp;AU.<ref name= MorbidBrown />

=== Detached objects ===
{{main|Detached object}}
{{See also|Sednoid}}
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.<ref name="Sedna">{{cite web|title= Sedna (The coldest most distant place known in the solar system; possibly the first object in the long-hypothesized Oort cloud) |first= Michael E. |last= Brown |publisher= California Institute of Technology, Department of Geological Sciences |url=http://www.gps.caltech.edu/~mbrown/sedna/ |access-date=2008-07-02}}</ref> Under this definition, an object with a perihelion greater than 40&nbsp;AU could be classified as outside the scattered disc.<ref name="Kobe">{{cite journal
|title= Dynamical classification of trans-Neptunian objects: Probing their origin, evolution, and interrelation 
|first1 =Patryk Sofia | last1= Lykawka |first2=Tadashi | last2= Mukai
|place=Kobe |volume=189 |issue=1|pages=213–232
|doi=10.1016/j.icarus.2007.01.001
|bibcode=2007Icar..189..213L
|date=2007
|journal=Icarus}}</ref>

Sedna is not the only such object: {{mpl|(148209) 2000 CR|105}} (discovered before Sedna) and [[474640 Alicanto]] 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''').<ref>{{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 |first=Brett J. |last=Gladman |access-date=2008-08-02}}</ref> {{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"'',<ref name= Jewitt2006>{{cite book| author-link=David C. Jewitt| first1= David C. |last1= Jewitt |first2=A. |last2=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= 978-3-540-26056-1| date=2006 |url=http://www2.ess.ucla.edu/~jewitt/papers/2006/DJ06.pdf}}</ref> or ''distant detached objects'' ('''DDO''').<ref name="Gomez 2006" />

There are no clear boundaries between the scattered and detached regions.<ref name="Kobe" /> Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs.<ref name="gomes" /> Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star<ref name="Morbidelli2004">{{cite journal |journal=The Astronomical Journal |issue=5 |volume=128 |pages=2564–2576 |date=November 2004 |title=Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR<sub>105</sub> and 2003 VB<sub>12</sub> |first1=Alessandro |last1=Morbidelli |first2=Harold F. |last2=Levison |doi=10.1086/424617 |bibcode=2004AJ....128.2564M |arxiv = astro-ph/0403358 |s2cid=119486916 }}</ref><ref>{{Cite journal|last1=Pfalzner|first1=Susanne|last2=Bhandare|first2=Asmita|last3=Vincke|first3=Kirsten|last4=Lacerda|first4=Pedro|date=2018-08-09|title=Outer Solar System Possibly Shaped by a Stellar Fly-by|journal=The Astrophysical Journal|volume=863|issue=1|pages=45|doi=10.3847/1538-4357/aad23c|issn=1538-4357|arxiv=1807.02960|bibcode=2018ApJ...863...45P|s2cid=119197960 |doi-access=free }}</ref> or a distant, [[Planets beyond Neptune|planet-sized object]].<ref name="Gomez 2006" >{{cite journal|bibcode=2006Icar..184..589G|title=A distant planetary-mass solar companion may have produced distant detached objects|first1=Rodney S. |last1=Gomes |first2=John J. |last2=Matese |first3=Jack J. |last3=Lissauer |journal=Icarus |volume=184 |issue=2 |pages=589–601 |date=October 2006 |doi=10.1016/j.icarus.2006.05.026 }}</ref> Alternatively, it has been suggested that these objects have been captured from a passing star.<ref>{{Cite journal|last1=Jílková|first1=Lucie|last2=Portegies Zwart|first2=Simon|last3=Pijloo|first3=Tjibaria|last4=Hammer|first4=Michael|date=2015-11-01|title=How Sedna and family were captured in a close encounter with a solar sibling|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=453|issue=3|pages=3158–3163|doi=10.1093/mnras/stv1803|doi-access=free |arxiv=1506.03105|bibcode=2015MNRAS.453.3157J|s2cid=119188358|issn=0035-8711}}</ref>

A scheme introduced by a 2005 report from the Deep Ecliptic Survey by J. L. Elliott et al. distinguishes between two categories: ''scattered-near'' (i.e. typical SDOs) and ''scattered-extended'' (i.e. detached objects).<ref name="DES_Elliot2006">{{cite journal|first1=J. L. |last1= Elliot|first2= S. D. |last2= Kern| first3=K. B. | last3= Clancy|display-authors=etal|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|date=2005| pages=1117–1162|doi=10.1086/427395|bibcode=2005AJ....129.1117E|issue=2|s2cid= 19385887|doi-access=free}}</ref> Scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a [[Tisserand parameter]] (relative to Neptune) less than 3.<ref name="DES_Elliot2006" /> Scattered-extended objects have a [[Tisserand parameter]] (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2.<ref name="DES_Elliot2006" />

An alternative classification, introduced by [[Brett J. Gladman|B. J. Gladman]], [[Brian G. Marsden|B. G. Marsden]] and <!-- Christa -->C. Van Laerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter.<ref name="ArizonaBook_Gladman2007">{{cite book |first1=Brett J. |last1=Gladman |author-link=Brett J. Gladman |first2=Brian G. |last2=Marsden |author2-link=Brian G. Marsden |first3=Christa |last3=Van Laerhoven |chapter= Nomenclature in the Outer Solar System |title= The Solar System Beyond Neptune| isbn= 978-0-8165-2755-7 |date= 2008 |bibcode=2008ssbn.book...43G |pages=43 |publisher=University of Arizona Press }}</ref> An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during the integration, its semi-major axis shows an excursion of 1.5 AU or more.<ref name="ArizonaBook_Gladman2007" /> Gladman et al. suggest the term ''scattering disk object'' to emphasize this present mobility.<ref name="ArizonaBook_Gladman2007" /> If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a ''detached TNO''.<ref name="ArizonaBook_Gladman2007" /> (Objects with smaller eccentricity are considered classical.) In this scheme, the disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud.

== Orbits ==
[[File:KBOs and resonances.png|right|thumb|660px|Distribution of trans-Neptunian objects, with semi-major axis on the horizontal, and inclination on the vertical axis. Scattered disc objects are shown in grey, objects that are in resonance with [[Neptune]] in red. Classical Kuiper belt objects (cubewanos) and [[sednoid]]s are blue and yellow, respectively.]]

The scattered disc is a very dynamic environment.<ref name="dynam" /> Because they are still capable of being perturbed by Neptune, SDOs' 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.<ref name="dynam" /> For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered.<ref name="ArizonaBook_Gladman2007" /> Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the [[ecliptic]].<ref name="Bertoldi Altenhoff et al. 2006">{{cite journal| doi = 10.1038/nature04494| last1 = Bertoldi | first1 = F.| last2 = Altenhoff | first2 = W.| last3 = Weiss | first3 = A.| last4 = Menten | first4 = K. M.| last5 = Thum | first5 = C.| date = 2 February 2006| title = The trans-Neptunian object {{mp|UB|313}} is larger than Pluto| journal = Nature| volume = 439| issue = 7076| pages = 563–564| pmid = 16452973| bibcode = 2006Natur.439..563B| s2cid = 4369483 | ref = {{sfnRef|Bertoldi Altenhoff et al.|2006}}
}}</ref>

SDOs are typically characterized by orbits with medium and high eccentricities with a [[semi-major axis]] greater than 50&nbsp;AU, but their perihelia bring them within influence of Neptune.<ref name="tjl2000">{{cite journal |first1=Chadwick A. |last1=Trujillo |first2=David C. |last2=Jewitt |author2-link=David C. Jewitt |first3=Jane X. |last3=Luu |author3-link=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://www2.ess.ucla.edu/~jewitt/papers/SKBO/tjl2000.pdf  |access-date=2008-07-02 |doi=10.1086/312467 |pmid= 10622765 |issue= 2 |archive-url = https://web.archive.org/web/20070812095910/http://www.ifa.hawaii.edu/faculty/jewitt/papers/SKBO/tjl2000.pdf |archive-date = August 12, 2007 |bibcode=2000ApJ...529L.103T|arxiv = astro-ph/9912428 |s2cid=8240136 }}</ref> Having a perihelion of roughly 30&nbsp;AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence.<ref name="jewitt2000">{{cite web |date=August 2009 |title=Scattered Kuiper Belt Objects (SKBOs) |publisher=Institute for Astronomy |first=David C. |last=Jewitt |author-link=David C. Jewitt |url=http://www2.ess.ucla.edu/~jewitt/kb/kb-scattered.html |access-date=2010-01-23 }}</ref>

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.<ref name="Levison2003">{{cite journal
 | last1 = Levison | first1 = Harold F. | author-link = Harold F. Levison | last2 = Morbidelli | first2 = Alessandro 
 | title = The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration
 | journal = [[Nature (journal)|Nature]]
 | volume = 426 | issue = 6965 | pages = 419–421
 | date = 2003-11-27 
 | doi = 10.1038/nature02120 | pmid = 14647375
 |bibcode = 2003Natur.426..419L | s2cid = 4395099 }}</ref> Classical 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.<ref name="dynam" />

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 possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.<ref name="gomes" />

== Formation ==
{{see also|Formation and evolution of the Solar System}}
[[File:Lhborbits.png|thumb|400px|Simulation showing [[Outer planets|Outer Planets]] and Kuiper Belt: a)&nbsp;Before [[Jupiter]]/[[Saturn]] 2:1 resonance b)&nbsp;Scattering of Kuiper-belt objects into the Solar System after the orbital shift of [[Neptune]] c)&nbsp;After ejection of Kuiper-belt bodies by Jupiter]]
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.<ref name="MorbidBrown">{{cite book |first1=Alessandro |last1=Morbidelli |first2=Michael E. |last2=Brown |title=Comets II |editor=M. C. Festou |editor2=H. U. Keller |editor3=H. A. Weaver |publisher=University of Arizona Press |location=Tucson (AZ) |date=2004-11-01 |pages=175–91 |chapter=The Kuiper Belt and the Primordial Evolution of the Solar System |isbn=978-0-8165-2450-1 |oclc=56755773 |chapter-url=http://www.lpi.usra.edu/books/CometsII/7004.pdf |access-date=2008-07-27 |archive-date=2009-06-21 |archive-url=https://web.archive.org/web/20090621084121/http://www.lpi.usra.edu/books/CometsII/7004.pdf |url-status=bot: unknown }}</ref>

According to contemporary models, the scattered disc formed when [[Kuiper belt]] objects (KBOs) were "scattered" into [[eccentricity (orbit)|eccentric]] and [[inclination|inclined]] orbits by gravitational interaction with Neptune and the other [[outer planets]].<ref name="Duncan Levison 1997">{{cite journal | first1 = Martin J. | last1 = Duncan | first2 = Harold F. | last2 = Levison  | author-link2 = 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 | date = 1997 | doi = 10.1126/science.276.5319.1670 | pmid = 9180070 | bibcode = 1997Sci...276.1670D }}</ref> 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;<ref>{{cite journal|title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets|first1= Harold F. |last1= Levison| first2= Martin J. |last2= Duncan|journal=Icarus|issue=1|date=1997|pages=13–32|doi=10.1006/icar.1996.5637 |volume=127|bibcode=1997Icar..127...13L}}</ref> a second posits that the scattering took place relatively quickly, during Neptune's early [[Neptune#Formation and migration|migration]] epoch.<ref name="shuffle" />

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 orbital 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.<ref name="gomes" />

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.<ref name=migration>{{cite journal| title= Neptune's Migration into a Stirred–Up Kuiper Belt: A Detailed Comparison of Simulations to Observations|first1= Joseph M. |last1= Hahn| first2= Renu |last2= Malhotra|journal=Astronomical Journal|arxiv=astro-ph/0507319|date=13 July 2005|bibcode = 2005AJ....130.2392H |doi = 10.1086/452638|volume=130|issue=5|pages=2392–414 |s2cid= 14153557 }}</ref> 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.<ref name="shuffle">{{cite web|title=Orbital shuffle for early solar system| first= Kathryn |last= Hansen|work=Geotimes|url=http://www.geotimes.org/june05/WebExtra060705.html|date= 2005-06-07|access-date=2007-08-26}}</ref> As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits.<ref name="Duncan Levison 1997" /><ref>{{cite journal|title=The Formation of Uranus and Neptune Among Jupiter and Saturn|first1= E. W. |last1= Thommes| first2= M. J. |last2= Duncan| first3= H. F. |last3= Levison|doi=10.1086/339975|journal=The Astronomical Journal|volume=123|date=May 2002|pages=2862–83| bibcode= 2002AJ....123.2862T|arxiv = astro-ph/0111290|issue=5 |s2cid= 17510705 }}</ref> 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."<ref name=migration />

== Composition ==
[[File:2003 UB313 near-infrared spectrum.png|thumb|200px|The infrared spectra of both [[Eris (dwarf planet)|Eris]] and [[Pluto]], highlighting their common [[methane]] absorption lines]]

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen [[Volatile (astrogeology)|volatiles]] such as water and [[methane]].<ref name="dynam3" /> 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.<ref name="methane" />

Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour, as they were thought to have originated in the same region and subjected to the same physical processes.<ref name="dynam3">{{cite book | title = Encyclopedia of the Solar System | chapter = Kuiper Belt Objects: Physical Studies | first = Stephen C. | last = Tegler | publisher = Academic Press | date = 2007 | editor-first1 = Lucy Ann | editor-last1 = Adams McFadden | editor-first2 = Paul Robert | editor-last2 = Weissman | editor-first3 = Torrence V. | editor-last3 = Johnson | edition = 2nd | location = Amsterdam; Boston | isbn = 978-0-12-088589-3 | pages = [https://archive.org/details/encyclopediaofso0000unse_u6d1/page/605 605–620] | chapter-url = https://archive.org/details/encyclopediaofso0000unse_u6d1/page/605 }}</ref> Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into [[tholins]] by sunlight from the Sun. This would absorb blue light, creating a reddish hue.<ref name="dynam3" /> Most classical objects display this colour, but scattered objects do not; instead, they present a white or greyish appearance.<ref name="dynam3" />

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.<ref name="dynam3" /> Michael E. 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 over its entire surface, creating an inches-thick layer of bright white ice. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high-[[albedo]] regions, leaving low-albedo [[tholin]]-covered regions bare of ice.<ref name="methane">{{cite journal | title = Discovery of a Planetary-sized Object in the Scattered Kuiper Belt| first1=Michael E. |last1=Brown |author-link=Michael E. Brown |first2=Chadwick A. |last2=Trujillo |author2-link=Chadwick A. Trujillo |first3=David L. |last3=Rabinowitz |author3-link=David L. Rabinowitz |journal=The Astrophysical Journal |date=2005 |volume=635 |issue=1 |pages=L97–L100 |doi=10.1086/499336 |arxiv=astro-ph/0508633 |bibcode=2005ApJ...635L..97B | s2cid=1761936 }}</ref>

== Comets ==
{{main|Comet#Short period}}
[[File:Tempel 1 (PIA02127).jpg|thumb|[[Tempel 1]], a [[Jupiter-family comet]]]]

The Kuiper belt was initially thought to be the source of the Solar System's [[ecliptic comet]]s. However, studies of the region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable.<ref>{{cite journal |title=The Kuiper Belt and the Solar System's Comet Disk |last=Gladman |first=Brett J. |date=2005 |journal=Science |volume=307 |issue=5706 |pages=71–75 |doi=10.1126/science.1100553 |pmid=15637267 |bibcode=2005Sci...307...71G |s2cid=33160822 }}</ref>

Comets can loosely be divided into two categories: short-period and long-period—the latter being thought to originate in the Oort cloud. The two major categories of short-period comets are [[Jupiter-family comet]]s (JFCs) and [[Halley-type comet]]s.<ref name="dynam" /> Halley-type comets, which are named after their prototype, [[Halley's Comet]], are thought to have originated in the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets,<ref name="matter" /> whereas the JFCs are thought to have originated in the scattered disc.<ref name="dynam2">{{cite book | title = Encyclopedia of the Solar System | chapter = Kuiper Belt Dynamics | first1 = Alessandro | last1 = Morbidelli | first2 = Harold F. | last2 = Levison | publisher = Academic Press | date = 2007 | editor = Lucy-Ann Adams McFadden | editor2 = Paul Robert Weissman | editor3 = Torrence V. Johnson | edition = 2nd | location = Amsterdam; Boston | isbn = 978-0-12-088589-3 | pages = [https://archive.org/details/encyclopediaofso0000unse_u6d1/page/589 589–604] | chapter-url = https://archive.org/details/encyclopediaofso0000unse_u6d1/page/589 }}</ref> The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family.<ref name="horner2003" />

There are many differences between SDOs and JFCs, even though many of the Jupiter-family comets 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.<ref name="matter" /> One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material.<ref name="matter">{{cite journal|title=From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter|first=David C. |last=Jewitt |volume=123 |pages=1039–1049 |date=2001 |journal=The Astronomical Journal |doi=10.1086/338692 |bibcode=2002AJ....123.1039J |issue=2 |s2cid=122240711 |url=http://pdfs.semanticscholar.org/9b63/4b7bf2b08f6bafae8bbe61fec60d36de6346.pdf|archive-url=https://web.archive.org/web/20200503023241/http://pdfs.semanticscholar.org/9b63/4b7bf2b08f6bafae8bbe61fec60d36de6346.pdf|url-status=dead|archive-date=2020-05-03}}</ref>

== See also ==
* [[List of possible dwarf planets]]
* [[List of trans-Neptunian objects]]
{{Clear}}

== Notes ==
{{notelist}}

== References ==
{{Reflist}}

{{Dwarf planets}}
{{Small Solar System bodies}}
{{Solar System}}
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[[Category:Astronomical objects discovered in 1996|19961009]]
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