Editing Centaur (small Solar System body)

{{Short description|Type of Solar System object}}
{{About|a class of Solar system objects|other uses|Centaur (disambiguation)}}
[[File:Kuiper belt plot objects of outer solar system.png|right|upright=1.5|thumb|Positions of known outer Solar System objects as of 2017.<br />The centaurs orbit generally inwards of the [[Kuiper belt]] and outside the [[Jupiter trojan]]s.
{| style="width: 100%; margin-bottom: 8px;"
|-
| valign=top width=45% |
{{legend2|#FFFF00|border=1px solid #B3B300|[[Sun]]}}<br />
{{legend2|#aaa|border=1px solid #777|[[Jupiter trojan]]s}}&nbsp;{{small|(6,178)}}<br />
{{legend2|#FFC170|border=1px solid #D67900|[[Scattered disc]]}}&nbsp;{{small|(&gt;300)}}
{{legend2|#A300A3|border=1px solid #3D003D|[[Neptune trojan]]s&nbsp;{{small|(9)}}}}
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{{legend2|#FF4D4D|border=1px solid #FF0000|[[Giant planet]]s: <br>{{·}}[[Jupiter|Jupiter (J)]]{{·}}[[Saturn|Saturn (S)]]<br>{{·}}[[Uranus|Uranus (U)]]{{·}}[[Neptune|Neptune (N)]]}}<br />
{{legend2|#009900|border=1px solid #003300|'''Centaurs'''}}&nbsp;{{small|(44,000)}}<br />
{{legend2|#66CCFF|border=1px solid #00AAFF|[[Kuiper belt]]}}&nbsp;{{small|(&gt;100,000)}}
|}
''{{small|(scale in [[Astronomical unit|AU]]; [[Epoch (astronomy)|epoch]] as of January 2015; # of objects in parentheses)}}''
]]
{{TNO}}

In [[planetary astronomy]], a '''centaur''' is a [[small Solar System body]] that orbits the Sun between [[Jupiter]] and [[Neptune]] and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable [[orbit]]s because of this; almost all their orbits have dynamic lifetimes of only a few million years,<ref name=Horner2004a>{{cite journal
  |last1=Horner |first1= J.
  |last2=Evans|first2= N.W.|last3= Bailey|first3= M. E.
  |s2cid= 16002759
 |title=Simulations of the Population of Centaurs I: The Bulk Statistics
  |year=2004
  |arxiv=astro-ph/0407400
  |doi=10.1111/j.1365-2966.2004.08240.x
  |journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=354|issue=3|pages=798–810 |doi-access= free
 |bibcode=2004MNRAS.354..798H}}</ref> but there is one 
known centaur, [[514107 Kaʻepaokaʻawela]], which may be in a [[Retrograde and prograde motion|stable (though retrograde) orbit]].<ref>{{cite journal |last1=Fathi Namouni and Maria Helena Moreira Morais |s2cid=54224209 |title=An interstellar origin for Jupiter's retrograde co-orbital asteroid |journal=Monthly Notices of the Royal Astronomical Society |volume=477 |issue=1 |pages=L117–L121 |date=May 2, 2018 |doi=10.1093/mnrasl/sly057 |doi-access=free |arxiv=1805.09013 |bibcode=2018MNRAS.477L.117N }}</ref>{{notetag|For criticism of this idea see:<ref>{{cite magazine  |last=Billings |first=Lee |date= 21 May 2018 |title=Astronomers Spot Potential 'Interstellar' Asteroid Orbiting Backward Around the Sun  |magazine=[[Scientific American]] |url= https://www.scientificamerican.com/article/astronomers-spot-potential-interstellar-asteroid-orbiting-backward-around-the-sun |access-date=1 June 2018}}</ref>}} Centaurs typically exhibit the characteristics of both [[asteroid]]s and [[comet]]s. They are named after the mythological [[centaur]]s that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult.  Estimates for the number of centaurs in the [[Solar System]] more than 1&nbsp;km in diameter range from as low as 44,000<ref name=Horner2004a/> to more than 10,000,000.<ref name=Sarid2019/><ref name=Sheppard2000>{{Cite journal
  |last=Sheppard |first= S.
  |author2=Jewitt, D.
  |author3=Trujillo, C.
  |author4=Brown, M.
  |author5=Ashley, M.
  |s2cid= 119337442
 |title=A Wide-Field CCD Survey for Centaurs and Kuiper Belt Objects
  |date=2000
  |doi= 10.1086/316805
  |journal=[[The Astronomical Journal]]
  |volume=120
  |issue=5
  |pages=2687–2694
  |bibcode =  2000AJ....120.2687S |arxiv=astro-ph/0008445
  }}</ref>

The first centaur to be discovered, under the definition of the [[Jet Propulsion Laboratory]] and the one used here, was [[944 Hidalgo]] in 1920. However, they were not recognized as a distinct population until the discovery of [[2060&nbsp;Chiron]] in 1977. The largest confirmed centaur is [[10199 Chariklo]], which at 260 kilometers in diameter is as big as a mid-sized [[main-belt]] asteroid, and is known to have a [[Rings of Chariklo|system of rings]]. It was discovered in 1997.

No centaur has been photographed up close, although there is evidence that [[Saturn]]'s moon [[Phoebe (moon)|Phoebe]], imaged by the [[Cassini–Huygens|''Cassini'' probe]] in 2004, may be a captured centaur that originated in the [[Kuiper belt]].<ref name="Jewitt2007">{{cite journal|last=Jewitt|first=David|author2=Haghighipour, Nader|s2cid=13282788|title=Irregular Satellites of the Planets: Products of Capture in the Early Solar System|journal=Annual Review of Astronomy and Astrophysics|date=2007|volume=45|issue=1|pages=261–95|url=http://www.ifa.hawaii.edu/~jewitt/papers/2007/JH07.pdf|archive-url=https://web.archive.org/web/20090919020650/http://www.ifa.hawaii.edu/~jewitt/papers/2007/JH07.pdf|archive-date=2009-09-19|doi=10.1146/annurev.astro.44.051905.092459|bibcode=2007ARA&A..45..261J|arxiv=astro-ph/0703059|url-status=dead}}</ref> In addition, the [[Hubble Space Telescope]] has gleaned some information about the surface features of [[8405 Asbolus]].

[[Ceres (dwarf planet)|Ceres]] may have originated in the region of the outer planets,<ref>{{cite web|url=http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf|title=Dawn at Ceres:What Have we Learned?|publisher=National Academies|work=Space Studies Board|accessdate=2023-10-11|archive-date=2018-04-13|archive-url=https://web.archive.org/web/20180413174709/http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf}}</ref> and if so might be considered an ex-centaur, but the centaurs seen today all originated elsewhere.

Of the objects known to occupy centaur-like orbits, approximately 30 have been found to display comet-like dust [[coma (cometary)|coma]]s, with three, [[2060&nbsp;Chiron]], [[60558 Echeclus]], and [[29P/Schwassmann-Wachmann]] 1, having detectable levels of volatile production in orbits entirely beyond Jupiter.<ref name=Wierzchos2017/> Chiron and Echeclus are therefore classified as both centaurs and comets, while Schwassmann-Wachmann 1 has always held a comet designation. Other centaurs, such as [[52872&nbsp;Okyrhoe]], are suspected of having shown [[Coma (cometary)|comas]]. Any centaur that is [[Perturbation (astronomy)|perturbed]] close enough to the Sun is expected to become a comet.

== Classification ==
A centaur has either a [[perihelion]] or a [[semi-major axis]] between those of the [[outer planets]] (between Jupiter and Neptune). Due to the inherent long-term instability of orbits in this region, even centaurs such as {{mp|2000 GM|137}} and {{mp|2001 XZ|255}}, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be [[Perturbation (astronomy)|perturbed]] until they start to cross the orbit of one or more of the giant planets.<ref name=Horner2004a/> Some astronomers count only bodies with semimajor axes in the region of the outer planets to be centaurs; others accept any body with a perihelion in the region, as their orbits are similarly unstable.

===Discrepant criteria===
However, different institutions have different criteria for classifying borderline objects, based on particular values of their [[orbital elements]]:
* The [[Minor Planet Center]] (MPC) defines centaurs as having a [[perihelion]] beyond the orbit of Jupiter ({{nowrap|5.2 AU < ''q''}}) and a semi-major axis less than that of Neptune ({{nowrap|''a'' < 30.1 AU}}).<ref name="MPC-centaur-def">{{cite web |url=http://www.minorplanetcenter.org/iau/lists/Unusual.html |title=Unusual Minor Planets |publisher=Minor Planet Center |access-date=2010-10-25 |df=dmy-all}}</ref> Though nowadays the MPC often lists centaurs and [[scattered disc]] objects together as a single group.{{cn|date=March 2025}}
* The [[Jet Propulsion Laboratory]] (JPL) similarly defines centaurs as having a semi-major axis, ''a'', between those of Jupiter and Neptune ({{nowrap|5.5 AU ≤ ''a'' ≤ 30.1 AU}}).<ref name=jplcent>{{cite web |title=Orbit Classification (Centaur) |publisher=JPL Solar System Dynamics |url=http://ssd.jpl.nasa.gov/sbdb_help.cgi?class=CEN |archive-url=https://web.archive.org/web/20210801091309/http://ssd.jpl.nasa.gov/sbdb_help.cgi?class=CEN |access-date=2008-10-13 |archive-date=2021-08-01 |df=dmy-all}}</ref>
* In contrast, the [[Deep Ecliptic Survey]] (DES) defines centaurs using a dynamical classification scheme. These classifications are based on the simulated change in behavior of the present orbit when extended over 10&nbsp;million years. The DES defines centaurs as [[Orbital resonance|non-resonant]] objects whose instantaneous ([[Osculating orbit|osculating]]) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation. This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit.<ref name=Elliot2005>{{cite journal |last=Elliot |first= J.L. |author2=Kern, S. D. |author3=Clancy, K. B. |author4=Gulbis, A. A. S. |author5=Millis, R. L. |author6=Buie, M. W. |author7=Wasserman, L. H. |author8=Chiang, E. I. |author9=Jordan, A. B. |author10=Trilling, D. E. |author11=Meech, K. J. |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 |issue=2 |df=dmy-all |pages=1117–1162 |year=2005 |doi=10.1086/427395 |bibcode=2005AJ....129.1117E|doi-access=free }}</ref>
* The collection ''The Solar System Beyond Neptune'' (2008) defines objects with a semi-major axis between those of Jupiter and Neptune and a Jupiter-relative [[Tisserand's parameter]] above 3.05 as centaurs, classifying the objects with a Jupiter-relative Tisserand's parameter below this and, to exclude [[Kuiper belt]] objects, an arbitrary [[Perihelion and aphelion|perihelion]] cut-off half-way to Saturn ({{nowrap|''q'' ≤ 7.35 AU}}) as [[Jupiter-family comet]]s, and classifying those objects on unstable orbits with a semi-major axis larger than Neptune's as members of the scattered disc.<ref name=ArizonaBook_Gladman2008>{{cite book |title=Nomenclature in the Outer Solar System (The Solar System Beyond Neptune) |isbn=978-0-8165-2755-7 |year=2008 |first1=B. |last1=Gladman |author-link1=Brett J. Gladman |first2=B. |last2=Marsden |author-link2=Brian G. Marsden |first3=C. |last3=Van Laerhoven |publisher=University of Arizona Press |url=http://www.lpi.usra.edu/books/ssbn2008/7002.pdf  |archive-url=https://web.archive.org/web/20121102205338/http://www.lpi.usra.edu/books/ssbn2008/7002.pdf |archive-date=2012-11-02 }}</ref>
* Other astronomers prefer to define centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the [[Hill sphere]] of a [[gas giant]] within the next 10&nbsp;million years,<ref name="Chiang2007">{{cite journal |last1=Chaing |first1= Eugene |last2=Lithwick| first2=Y. |last3=Murray-Clay |first3=R. |last4=Buie |first4= M. |last5=Grundy |first5=W. |last6=Holman |first6=M. |title=A Brief History of Transneptunian Space |journal=Protostars and Planets V |editor-first1=B. |editor-last1=Reipurth |editor-first2=D. |editor-last2=Jewitt |editor-first3=K. |editor-last3=Keil |publisher=University of Arizona Press |location=Tucson, AZ |pages=895–911 |year=2007 |bibcode=2007prpl.conf..895C |arxiv=astro-ph/0601654}}</ref> so that centaurs can be thought of as objects scattered inwards and that interact more strongly and scatter more quickly than typical scattered-disc objects.
* The JPL Small-Body Database lists 910&nbsp;centaurs.<ref name="jplcentaurlist">{{cite web |title=JPL Small-Body Database Search Engine: List of centaurs |publisher=JPL Solar System Dynamics |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=CEN;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCkCqAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AiA |archive-url=https://web.archive.org/web/20210420113542/http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=CEN;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCkCqAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AiA |access-date=2018-10-11 |archive-date=2021-04-20 |df=dmy-all}}</ref> There are an additional 223&nbsp;[[trans-Neptunian object]]s (objects with a semi-major axis further than Neptune's, i.e. {{nowrap|30.1 AU ≤ ''a''}}) with a perihelion closer than the orbit of [[Uranus]] ({{nowrap|''q'' ≤ 19.2 AU}}).<ref name="jplqUranusTNOlist">{{cite web |title=JPL Small-Body Database Search Engine: List of TNOs with perihelia closer than Uranus's orbit |publisher=JPL Solar System Dynamics |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=ast;obj_numbered=all;OBJ_field=0;ORB_field=0;combine_mode=AND;c1_group=ORB;c1_item=Bi;c1_op=%3C;c1_value=19;c2_group=ORB;c2_item=Bh;c2_op=%3E;c2_value=30.1;table_format=HTML;max_rows=200;format_option=comp;c_fields=AcBhBgBjBkBlBiBnBsCjCpCmCnCoAi;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=combine_mode;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=AiA|archive-url=https://web.archive.org/web/20210829235558/http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=ast;obj_numbered=all;OBJ_field=0;ORB_field=0;combine_mode=AND;c1_group=ORB;c1_item=Bi;c1_op=%3C;c1_value=19;c2_group=ORB;c2_item=Bh;c2_op=%3E;c2_value=30.1;table_format=HTML;max_rows=200;format_option=comp;c_fields=AcBhBgBjBkBlBiBnBsCjCpCmCnCoAi;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=combine_mode;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=AiA |access-date=2018-10-11 |archive-date=2021-08-29 |df=dmy-all}}</ref>

===Ambiguous objects===
The Gladman & Marsden (2008)<ref name=ArizonaBook_Gladman2008/> criteria would make some objects Jupiter-family comets: Both [[60558 Echeclus|Echeclus]] ({{nowrap|''q'' {{=}} 5.8 AU}}, {{nowrap|''T''{{sub|J}} {{=}} 3.03}}) and [[52872 Okyrhoe|Okyrhoe]] ({{nowrap|''q'' {{=}} 5.8 AU}}; {{nowrap|''T''{{sub|J}} {{=}} 2.95}}) have traditionally been classified as centaurs. Traditionally considered an asteroid, but classified as a centaur by JPL, [[944 Hidalgo|Hidalgo]] ({{nowrap|''q'' {{=}} 1.95 AU}}; {{nowrap|''T''{{sub|J}} {{=}} 2.07}}) would also change category to a Jupiter-family comet. [[29P/Schwassmann-Wachmann]] ({{nowrap|''q'' {{=}} 5.72 AU}}; {{nowrap|''T''{{sub|J}} {{=}} 2.99}}) has been categorized as both a centaur and a Jupiter-family comet depending on the definition used.{{cn|date=March 2025}}

Other objects caught between these differences in classification methods include {{mpl|(44594) 1999 OX|3}}, which has a semi-major axis of 32&nbsp;AU but crosses the orbits of both Uranus and Neptune. It is listed as an outer centaur by the ''Deep Ecliptic Survey'' (DES). Among the inner centaurs, [[(434620) 2005 VD]], with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.

A recent orbital simulation<ref name=Sarid2019>{{Cite journal
  |last=Sarid |first= G.
  |author2=Volk, K.
  |author3=Steckloff, J.
  |author4=Harris, W.
  |author5=Womack, M.
  |author6=Woodney, L.
  |s2cid= 199543466
 |title=29P/Schwassmann-Wachmann 1, A Centaur in the Gateway to the Jupiter-Family Comets
  |date=2019
  |doi=10.3847/2041-8213/ab3fb3
  |journal=[[The Astrophysical Journal Letters]]
  |volume=883
  |issue=1
  |pages=7
  |bibcode = 2019ApJ...883L..25S |arxiv=1908.04185
  |doi-access= free
 }}</ref> of the evolution of Kuiper Belt Objects through the centaur region has identified a short-lived "''orbital gateway''" between 5.4 and 7.8 AU through which 21% of all centaurs pass, including 72% of the centaurs that become Jupiter-family comets.  Four objects are known to occupy this region (29P/Schwassmann-Wachmann, [[List of periodic comets|P/2010 TO20 LINEAR-Grauer]], [[List of periodic comets|P/2008 CL94 Lemmon]], and {{mpl|2016 LN|8}}), but simulations indicate that there may of order 1000 more objects >1&nbsp;km in radius that have yet to be detected. Objects in this gateway region can display significant activity<ref name=Womack2017/><ref name=Lacerda2013>{{Cite journal
  |last=Lacerda |first= P.
  |s2cid= 54030926
 |title=Comet P/2010 TO20 LINEAR-Grauer as a Mini-29P/SW1
  |date=2013
  |doi=10.1093/mnras/sts164
  |journal=[[Monthly Notices of the Royal Astronomical Society]]
  |volume=883
  |issue=2
  |pages=1818–1826
  |doi-access= free
 |bibcode = 2013MNRAS.428.1818L |arxiv=1208.0598
  }}</ref> and are in an important evolutionary transition state that further blurs the distinction between the centaur and Jupiter-family comet populations.{{cn|date=March 2025}}

The [[Committee on Small Body Nomenclature]] of the [[International Astronomical Union]] has not formally weighed in on any side of the debate. Instead, it has adopted the following naming convention for such objects: Befitting their centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only the binary objects [[65489 Ceto|Ceto and Phorcys]] and [[42355 Typhon|Typhon and Echidna]] have been named according to the new policy.<ref name="Grundy2007">{{cite journal |last1=Grundy |first1= Will |first2=J.A. |last2=Stansberry |first3=K |last3=Noll |first4=D.C. |last4=Stephens |first5=D.E. |last5=Trilling |first6=S.D. |last6=Kern |first7=J.R. |last7=Spencer |first8=D.P. |last8=Cruikshank |first9=H.F. |last9=Levison |s2cid= 1532765 |title=The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur |journal=Icarus |volume=191 |issue=1 |pages=286–297 |year=2007 |arxiv=0704.1523 |doi=10.1016/j.icarus.2007.04.004 |bibcode=2007Icar..191..286G}}</ref>

Centaurs with measured diameters listed as possible [[dwarf planet]]s according to [[Michael E. Brown|Mike Brown]]'s website include [[10199 Chariklo]], {{mpl|(523727) 2014 NW|65}} and [[2060 Chiron]].<ref name="Brown-dplist">{{cite web |title=How many dwarf planets are there in the outer solar system? (updates daily) |publisher=California Institute of Technology |author-link=Michael E. Brown |last=Brown |first=Michael E. |url=http://web.gps.caltech.edu/~mbrown/dps.html |access-date=13 February 2021}}</ref>

== Orbits ==

=== Distribution ===
[[Image:TheKuiperBelt 42AU Centaurs.svg|right|thumb|upright=1.5|Orbits of known centaurs<ref group=note>For the purpose of this diagram, an object is classified as a centaur if its [[semi-major axis]] lies between Jupiter and Neptune</ref>]]

The diagram illustrates the orbits of known centaurs in relation to the orbits of the planets. For selected objects, the [[orbital eccentricity|eccentricity]] of the orbits is represented by red segments (extending from [[perihelion]] to aphelion).

The orbits of centaurs show a wide range of eccentricity, from highly eccentric ([[5145 Pholus|Pholus]], [[8405 Asbolus|Asbolus]], [[55576 Amycus|Amycus]], [[7066 Nessus|Nessus]]) to more circular ([[10199 Chariklo|Chariklo]] and the [[Saturn-crosser asteroid|Saturn-crossers]] [[32532 Thereus|Thereus]] and [[52872 Okyrhoe|Okyrhoe]]).

To illustrate the range of the orbits' parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow :
*{{mpl|1999 XS|35}} ([[Apollo asteroid]]) follows an extremely eccentric orbit ({{nowrap|1=''e'' = 0.947}}), leading it from inside Earth's orbit (0.94&nbsp;AU) to well beyond Neptune ({{nowrap|> 34 AU}})
*{{mp|2007 TB|434}} follows a quasi-circular orbit ({{nowrap|''e'' < 0.026}})
*{{mp|2001 XZ|255}} has the lowest [[inclination]] ({{nowrap|''i'' < 3°}}).
* {{mpl-|144908|2004 YH|32}} is one of a small proportion of centaurs with an extreme [[Retrograde and prograde motion#Inclination|prograde inclination]] ({{nowrap|''i'' > 60°}}). It follows such a highly inclined orbit (79°) that, while it crosses from the distance of the [[asteroid belt]] from the Sun to past the distance of Saturn, if its orbit is projected onto the plane of Jupiter's orbit, it does not even go out as far as Jupiter.

Over a dozen known centaurs follow retrograde orbits. Their inclinations range from modest (''e.g''., 160° for [[20461 Dioretsa|Dioretsa]]) to extreme ({{nowrap|''i'' < 120°}}; ''e.g''. 105° for {{mpl|(342842) 2008 YB|3}}<ref name="Marcos">{{Cite journal
 | author = C. de la Fuente Marcos
 | author2 = R. de la Fuente Marcos
 | s2cid = 119255885
 | title = Large retrograde Centaurs: visitors from the Oort cloud?
 | journal = Astrophysics and Space Science
 | date = 2014 | doi = 10.1007/s10509-014-1993-9 
 | volume = 352
 | issue = 2
 | pages = 409–419
 | arxiv=1406.1450 | bibcode = 2014Ap&SS.352..409D
 }}
</ref>).
Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin.<ref>{{cite journal |last1=Fathi Namouni and Maria Helena Moreira Morais |s2cid=216056648 |title=An interstellar origin for high-inclination Centaurs  |journal=Monthly Notices of the Royal Astronomical Society |volume=494 |issue=2  |pages=2191–2199 |date=May 2020 |doi=10.1093/mnras/staa712 |doi-access=free |arxiv=2004.10510 |bibcode=2020MNRAS.494.2191N }}</ref><ref>{{cite journal|arxiv=2006.04534|doi=10.1093/mnrasl/slaa111|title=No evidence for interstellar planetesimals trapped in the Solar system|year=2020|last1=Raymond|first1=S. N.|last2=Brasser|first2=R.|last3=Batygin|first3=K.|last4=Morbidelli|first4=A.|s2cid=219531537|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=497|issue=1|pages=L46–L49|doi-access=free |bibcode=2020MNRAS.497L..46M}}</ref><ref name="Namouni_Moreira Morias_2020b">{{cite journal |last1=Namouni |first1=Fathi |title=Inclination pathways of planet-crossing asteroids |journal=Monthly Notices of the Royal Astronomical Society |year=2022 |volume=510 |pages=276–291 |doi=10.1093/mnras/stab3405 |doi-access=free |arxiv=2111.10777
}}</ref>

=== Changing orbits ===
[[File:AsbolA.png|upright=1.5|thumb|right|The [[semi-major axis]] of [[8405 Asbolus|Asbolus]] during the next 5500 years, using two slightly different estimates of present-day orbital elements. After the Jupiter encounter of year 4713 the two calculations diverge.<ref name=AsbolusClones>{{cite web 
 |title=Three clones of centaur 8405 Asbolus making passes within 450Gm 
 |url=http://home.surewest.net/kheider/astro/AsbolusClones.txt 
 |access-date=2009-05-02 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20150913215558/http://home.surewest.net/kheider/astro/AsbolusClones.txt 
 |archive-date=2015-09-13 
 }} ({{cite web|url=http://chemistry.unina.it/~alvitagl/solex/|title=Solex 10|url-status=dead|archive-url=https://web.archive.org/web/20081220235836/http://chemistry.unina.it/~alvitagl/solex/|archive-date=2008-12-20 }})</ref>]]

Because the centaurs are not protected by [[orbital resonance]]s, their orbits are unstable within a timescale of 10<sup>6</sup>&ndash;10<sup>7</sup>&nbsp;years.<ref name=Jewitt2006/> For example, [[55576 Amycus]] is in an unstable orbit near the 3:4 resonance of Uranus.<ref name=Horner2004a/> Dynamical studies of their orbits indicate that being a centaur is probably an intermediate orbital state of objects transitioning from the [[Kuiper belt]] to the [[Jupiter-family comet|Jupiter family]] of short-period [[comet]]s. [[(679997) 2023 RB]] will have its orbit notably changed by a close approach to Saturn in 2201.

Objects may be [[Perturbation (astronomy)|perturbed]] from the Kuiper belt, whereupon they become [[Neptune]]-crossing and interact gravitationally with that planet (see [[#Hypotheses of origin|theories of origin]]). They then become classed as centaurs, but their orbits are chaotic, evolving relatively rapidly as the centaur makes repeated close approaches to one or more of the outer planets. Some centaurs will evolve into Jupiter-crossing orbits whereupon their perihelia may become reduced into the inner Solar System and they may be reclassified as active [[comet]]s in the Jupiter family if they display cometary activity. Centaurs will thus ultimately [[Impact event|collide]] with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly [[Jupiter]].{{cn|date=March 2025}}

== Physical characteristics ==
[[File:Phoebe_cassini_full.jpg|right|thumb|[[Phoebe (moon)|Phoebe]], a moon of [[Saturn]] imaged by the ''[[Cassini-Huygens]]'' probe in 2004, is theorized to be a captured centaur.]]

Compared to dwarf planets and asteroids, the relatively small size and distance of centaurs precludes remote observation of surfaces, but [[color index|colour indices]] and [[Optical spectrum|spectra]] can provide clues about surface composition and insight into the origin of the bodies.<ref name=Jewitt2006>{{cite book| author-link=David C. Jewitt| first=David C.| last= Jewitt|author2=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= 978-3-540-26056-1| year=2006}} ([https://web.archive.org/web/20081217010309/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf Preprint version (pdf)])</ref>

=== Colours ===
[[File:TheKuiperBelt Albedo and Color.svg|right|thumb|Colour distribution of centaurs]]

The colours of centaurs are very diverse, which challenges any simple model of surface composition.<ref name="Barucci">{{cite web| first1= M. A.| last1= Barucci| first2= A.| last2= Doressoundiram| first3= D. P.| last3= Cruikshank| title= Physical Characteristics of TNOs and Centaurs| year= 2003| url= http://www.lesia.obspm.fr/~alaind/TNO/Barucci2003_comet2.pdf| publisher= Laboratory for Space Studies and Astrophysics Instrumentation, Paris Observatory| access-date= 20 March 2008| url-status= dead| archive-url= https://web.archive.org/web/20080529193555/http://www.lesia.obspm.fr/~alaind/TNO/Barucci2003_comet2.pdf| archive-date= 29 May 2008}}</ref> In the side-diagram, the [[color index|colour indices]] are measures of [[apparent magnitude]] of an object through blue&nbsp;(B), visible&nbsp;(V) (i.e. green-yellow) and red&nbsp;(R) filters. The diagram illustrates these differences (in exaggerated colours) for all centaurs with known colour indices. For reference, two moons: [[Triton (moon)|Triton]] and [[Phoebe (moon)|Phoebe]], and planet [[Mars]] are plotted (yellow labels, size not to scale).

Centaurs appear to be grouped into two classes:
* very red &ndash; for example [[5145 Pholus]]
* blue (or blue-grey, according to some authors) &ndash; for example [[2060 Chiron]] or {{mpl|2020 MK|4}}

There are numerous theories to explain this colour difference, but they can be broadly divided into two categories:
*The colour difference results from a difference in the origin and/or composition of the centaur (see [[#Theories of origin|origin]] below)
* The colour difference reflects a different level of space-weathering from [[radiation]] and/or [[comet]]ary activity.

As examples of the second category, the reddish colour of Pholus has been explained as a possible mantle of irradiated red organics, whereas Chiron has instead had its ice exposed due to its periodic cometary activity, giving it a blue/grey index. The correlation with activity and color is not certain, however, as the active centaurs span the range of colors from blue (Chiron) to red (166P/NEAT).<ref name="Baueretal2003">{{cite journal | doi=10.1086/377012 | title=An Optical Survey of the Active Centaur C/NEAT (2001 T4) | date=2003 | last1=Bauer | first1=James M. | last2=Fernández | first2=Yanga R. | last3=Meech | first3=Karen J. | journal=Publications of the Astronomical Society of the Pacific | volume=115 | issue=810 | pages=981–989 | bibcode=2003PASP..115..981B | s2cid=122502310 | doi-access=free }}</ref> Alternatively, Pholus may have been only recently expelled from the Kuiper belt, so that surface transformation processes have not yet taken place.

Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions.<ref name="Peixinho 2003">{{cite journal
|first1=N.|last1=Peixinho|first2= A.|last2=Doressoundiram|first3= A.|last3=Delsanti
|first4=H.|last4=Boehnhardt|first5=M. A.|last5=Barucci|first6=I.|last6=Belskaya
|s2cid=8515984|title=Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality
|journal=Astronomy and Astrophysics|volume=410
|issue=3|pages= L29–L32|year=2003
|doi=10.1051/0004-6361:20031420|arxiv=astro-ph/0309428|bibcode=2003A&A...410L..29P}}</ref><ref name="Hainaut, Delsanti 2002">
Hainaut & Delsanti (2002) ''Color of Minor Bodies in the Outer Solar System'' Astronomy & Astrophysics, '''389''', 641 [http://www.eso.org/~ohainaut/MBOSS/ datasource]
</ref>

=== Spectra ===
The interpretation of [[optical spectrum|spectra]] is often ambiguous, related to particle sizes and other factors, but the spectra offer an insight into surface composition. As with the colours, the observed spectra can fit a number of models of the surface.

Water ice signatures have been confirmed on a number of centaurs<ref name=Jewitt2006/> (including [[2060 Chiron]], [[10199 Chariklo]] and [[5145 Pholus]]). In addition to the water ice signature, a number of other models have been put forward:
*Chariklo's surface has been suggested to be a mixture of [[tholin]]s (like those detected on [[Titan (moon)|Titan]] and [[Triton (moon)|Triton]]) with [[amorphous]] [[carbon]].
*Pholus has been suggested to be covered by a mixture of Titan-like [[tholin]]s, [[carbon black]], [[olivine]]<ref>A class of Magnesium Iron Silicates (Mg, Fe)<sub>2</sub>SiO<sub>4</sub>, common components of [[igneous]] rocks.</ref> and [[methanol]] ice.
*The surface of [[52872 Okyrhoe]] has been suggested to be a mixture of [[kerogen]]s, olivines and a small percentage of water ice.
*[[8405 Asbolus]] has been suggested to be a mixture of 15% Triton-like [[tholin]]s, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.
[[2060 Chiron|Chiron]] appears to be the most complex. The spectra observed vary depending on the period of the observation. Water ice signature was detected during a period of low activity and disappeared during high activity.<ref name="Dotto 2003">
{{cite journal| last1=Dotto| first1= E| last2= Barucci| first2=M A| last3= De Bergh| first3=C| s2cid= 189905595| title=Colours and composition of the Centaurs| journal=Earth, Moon, and Planets| volume=92| number= 1–4| pages= 157–167| date=June 2003| doi=10.1023/b:moon.0000031934.89097.88| bibcode=2003EM&P...92..157D}}</ref><ref name="Water on Chiron">{{cite journal
|first1=Jane X.|last1=Luu|first2=David|last2=Jewitt|author2-link=David Jewitt
|first3=C. A.|last3=Trujillo|s2cid=9946112|author3-link=Chadwick A. Trujillo
|title=Water Ice on 2060 Chiron and its Implications for Centaurs and Kuiper Belt Objects
|journal=The Astrophysical Journal|volume=531|year=2000 |issue=2
|pmid=10688775|pages=L151–L154
|doi=10.1086/312536|arxiv=astro-ph/0002094|bibcode=2000ApJ...531L.151L}}</ref><ref name="Jewitt_Asbolus_Chiron2002">{{cite journal
|first1=Y. R.|last1=Fernandez|author2-link=David Jewitt|first2=D. C.|last2=Jewitt|first3=S. S.|last3=Sheppard
|s2cid=11266670|title=Thermal Properties of Centaurs Asbolus and Chiron
|journal=The Astronomical Journal|volume=123|issue=2|year=2002|pages=1050–1055
|arxiv=astro-ph/0111395|doi=10.1086/338436|bibcode=2002AJ....123.1050F}}</ref>

===Similarities to comets===
[[Image:Comet38P2067.png|thumb|right|Comet [[38P/Stephan–Oterma|38P]] exhibits centaur-like behavior by making close approaches to Jupiter, Saturn, and Uranus between 1982 and 2067.<ref name=hybrid>{{cite web
  |date=1981-04-04 |at=last obs
  |title=JPL Close-Approach Data: 38P/Stephan-Oterma
  |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=38P
  |publisher= NASA
  |archive-url=https://web.archive.org/web/20210921212628/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=38P
 |access-date=2009-05-07|archive-date=2021-09-21
 }}</ref>]]
Observations of Chiron in 1988 and 1989 near its [[perihelion]] found it to display a [[coma (cometary)|coma]] (a cloud of gas and dust evaporating from its surface). It is thus now officially classified as both a minor planet and a comet, although it is far larger than a typical comet and there is some lingering controversy. Other centaurs are being monitored for comet-like activity: so far two, [[60558 Echeclus]], and [[166P/NEAT]] have shown such behavior. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet, though its orbit is that of a centaur. [[60558 Echeclus]] was discovered without a coma but recently became active,<ref name="Choietal2006">{{cite journal| first1= Y-J.|last1= Choi|first2= P.R.| last2= Weissman| first3=D. |last3= Polishook |title=(60558) 2000 EC_98| journal= IAU Circ.| number=8656| date=January 2006| page= 2}}</ref> and so it too is now classified as both a comet and an asteroid. Overall, there are ~30 centaurs for which activity has been detected, with the active population biased toward objects with smaller perihelion distances.<ref name=Jewitt2009>{{cite journal
  |last=Jewitt |first= D.
  |s2cid= 119093318
 |title=The Active Centaurs
  |date=2009
  |doi=10.3847/1538-3881/aa689c
  |arxiv= 0902.4687
 |journal=[[The Astronomical Journal]]
  |volume=137
  |issue=5
  |pages=4295–4312
  |bibcode = 2009AJ....137.4296J
  |doi-access= free
 }}</ref>

[[Carbon monoxide]] has been detected in [[60558 Echeclus]]<ref name=Wierzchos2017>{{cite journal
  |last=Wierzchos |first= K.
  |author2=Womack, M.
  |author3=Sarid, G.
  |s2cid= 119093318
 |title=Carbon Monoxide in the Distantly Active Centaur (60558) 174P/Echeclus at 6 au
  |date=2017
  |doi=10.3847/1538-3881/aa689c
  |journal=[[The Astronomical Journal]]
  |volume=153
  |issue=5
  |pages=8
  |bibcode = 2017AJ....153..230W |arxiv=1703.07660
  |doi-access= free
 }}</ref> 
and [[2060 Chiron|Chiron]]<ref name=womack1999>{{Cite web
 | last1 = Womack | first1 = M. | last2 = Stern | first2 = A.
 | title = Observations of Carbon Monoxide in (2060) Chiron.
 | publisher=Lunar and Planetary Science XXVIII 
 | date=1999
 | url= http://www.lpi.usra.edu/meetings/lpsc97/pdf/1492.PDF
 | access-date=2017-07-11 }}</ref> in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma. The calculated CO production rate from both [[60558 Echeclus]] and [[2060 Chiron|Chiron]] is substantially lower than what is typically observed for [[29P/Schwassmann–Wachmann]],<ref name=Womack2017>{{cite journal
  |last=Womack |first= M.
  |author2=Wierzchos, K.
  |author3=Sarid, G.
  |s2cid= 118507805
 |title= CO in Distantly Active Comets
  |date=2017
  |doi=10.1088/1538-3873/129/973/031001
  |journal=[[Publications of the Astronomical Society of the Pacific]]
  |volume=129
  |number=973
  |pages=031001
  |bibcode = 2017PASP..129c1001W |arxiv=1611.00051
  }}</ref> another distantly active comet often classified as a centaur.

There is no clear orbital distinction between centaurs and comets. Both [[29P/Schwassmann-Wachmann]] and [[39P/Oterma]] have been referred to as centaurs since they have typical centaur orbits. The comet 39P/Oterma is currently inactive and was seen to be active only before it was perturbed into a centaur orbit by Jupiter in 1963.<ref name=Mazzotta2006>{{cite journal |last=Mazzotta Epifani |first=E. |author2=Palumbo, P. |author3=Capria, M. T. |author4=Cremonese, G. |author5=Fulle, M. |author6=Colangeli, L. |title=The dust coma of the active Centaur P/2004 A1 (LONEOS): a CO-driven environment? |journal=Astronomy & Astrophysics |volume=460 |pages=935–944 |issue=3 |year=2006 |doi=10.1051/0004-6361:20065189 |bibcode=2006A&A...460..935M |doi-access=free }}</ref> The faint comet [[38P/Stephan–Oterma]] would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5&nbsp;AU.  By the year 2200, comet [[78P/Gehrels]] will probably migrate outwards into a centaur-like orbit.{{citation needed|date=March 2024}}

===Rotational periods===
A periodogram analysis of the light-curves of these Chiron and Chariklo gives respectively the following rotational periods: 5.5±0.4~h and 7.0± 0.6~h.<ref name="Galiazzo et al. 2016">{{cite journal
|first1=M. A.|last1=Galiazzo|first2= C.|last2=de la Fuente Marcos|first3= R.|last3=de la Fuente Marcos
|first4=G.|last4=Carraro|first5=M.|last5=Maris|first6=M.|last6=Montalto
|s2cid=119204060|title=Photometry of Centaurs and trans-Neptunian objects: 2060 Chiron (1977 UB), 10199 Chariklo (1997 CU26), 38628 Huya (2000 EB173), 28978 Ixion (2001 KX76), and 90482 Orcus (2004 DW)
|journal=Astrophysics and Space Science|volume=361
|issue=3|pages=212–218|year=2016
|doi=10.1007/s10509-016-2801-5|
arxiv=1605.08251|bibcode= 2016Ap&SS.361..212G}}</ref>

===Size, density, reflectivity===
Centaurs can reach diameters up to hundreds of kilometers. The largest centaurs have diameters in excess of 300&nbsp;km, and primarily reside beyond 20 [[Astronomical unit|AU]].<ref>{{cite journal
  |author1=Galiazzo, M. A. |author2=Wiegert, P. |author3=Aljbaae, S. |s2cid=118898917 |name-list-style=amp | title=Influence of the Centaurs and TNOs on the main belt and its families
  | journal=Astrophysics and Space Science
  | date=2016
  | volume=361
  | issue=12
  | pages=361–371
  | bibcode=2016Ap&SS.361..371G  | doi = 10.1007/s10509-016-2957-z
|arxiv=1611.05731}}</ref> <!-- Needs a word about reflectivity (albedo?) and density since those are in the section title. -->

== Hypotheses of origin ==
The study of centaurs’ origins is rich in recent developments, but any conclusions are still hampered by limited physical data. Different models have been put forward for possible origin of centaurs.<!-- This sentence needs more explanation, it's too hard to understand: It is widely accepted, however, that the model of origin of centaurs must account for the peculiar distribution of [[Kuiper belt]] objects, the existence of the edge at 48 AU, the missing-mass puzzle (see [[Kuiper belt]]) etc. -->

Simulations indicate that the orbit of some [[Kuiper belt]] objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. [[Scattered disc]] objects would be dynamically the best candidates (For instance, the centaurs could be part of an "inner" scattered disc of objects perturbed inwards from the Kuiper belt.) for such expulsions, but their colours do not fit the bicoloured nature of the centaurs. [[Plutino]]s are a class of Kuiper belt object that display a similar bicoloured nature, and there are suggestions that not all plutinos' orbits are as stable as initially thought, due to [[Perturbation (astronomy)|perturbation]] by [[Pluto]].<ref name=wan2001>{{cite journal |author=Wan, X.-S. |author2=Huang, T.-Y. |title=The orbit evolution of 32&nbsp;plutinos over 100&nbsp;million years |journal=Astronomy and Astrophysics |volume=368 |issue=2 |pages=700–705 |year=2001 |bibcode=2001A&A...368..700W |doi=10.1051/0004-6361:20010056|doi-access=free }}</ref>
<!-- This sentence also needs better explaining: [[Cubewano|classical objects]] are other candidates as the differences in color distribution between hot and cold have been confirmed. -->
Further developments are expected with more physical data on Kuiper belt objects.

Some centaurs may have their origin in fragmentation episodes, perhaps triggered during close encounters with Jupiter.<ref name="Marcos2021">{{cite journal
  |title      = The active centaur 2020 MK4
  |first1     = C. |last1 = de la Fuente Marcos
  |first2     = R. |last2 = de la Fuente Marcos
  |first3     = J. |last3 = Licandro
  |first4     = M. |last4 = Serra-Ricart
  |first5     = S. |last5 = Martino
  |first6     = J. |last6 = de Leon
  |first7     = F. |last7 = Chaudry
  |first8     = M. R. |last8 = Alarcón
  |doi=10.1051/0004-6361/202039117
  |url=https://www.aanda.org/articles/aa/abs/2021/05/aa39117-20/aa39117-20.html
  |journal=[[Astronomy & Astrophysics]]
  |date= 13 May 2021 
  |volume= 649
  |issue= 1
  |pages=A85 (15 pages)
  |arxiv=2104.01668
  |bibcode=2021A&A...649A..85D |s2cid = 233024896 }}</ref> The orbits of centaurs [[2020 MK4]], P/2008 CL94 (Lemmon), and P/2010 TO20 (LINEAR-Grauer) pass close to that of comet [[29P/Schwassmann–Wachmann]], the first discovered centaur and close encounters are possible in which one of the objects traverses the coma of 29P when active.<ref name="Marcos2021"/>

At least one centaur, 2013 VZ<sub>70</sub>, might have an origin among Saturn's irregular moon population via impact, fragmentation, or tidal disruption.<ref name="Marcos2022">{{cite journal
  |title      = Centaur 2013 VZ<sub>70</sub>: Debris from Saturn's irregular moon population?
  |first1     = C. |last1 = de la Fuente Marcos
  |first2     = R. |last2 = de la Fuente Marcos
  |doi=10.1051/0004-6361/202142166
  |url=https://www.aanda.org/articles/aa/abs/2022/01/aa42166-21/aa42166-21.html
  |journal=[[Astronomy & Astrophysics]]
  |date= 10 January 2022 
  |volume=657 
  |issue=1 
  |pages=A59 (10 pp)
  |arxiv=2110.04264
  |bibcode=2022A&A...657A..59D|s2cid = 238856647 }}</ref>

==Notable centaurs==
{{main list|List of centaurs (small Solar System bodies)}}
<!-- Note: REVERSE NUMERICAL ORDER, to match authoritative external link below, for easy comparison -->
{| class="wikitable"
! Name !! Year !! Discoverer !! Half-life<ref name=Horner2004a/><br />(forward) !! Class{{efn|the class is defined by the perihelion and aphelion distance of the object: S indicates a perihelion/aphelion near Saturn, U near Uranus, N near Neptune, and K in the Kuiper belt.}}
|-
| [[2060 Chiron]] || 1977 || [[Charles T. Kowal]] || 1.03&nbsp;Ma || SU
|-
| [[5145 Pholus]] || 1992 || [[Spacewatch]] ([[David L. Rabinowitz]]) || 1.28&nbsp;Ma || SN
|-
| [[7066 Nessus]] || 1993 || [[Spacewatch]] ([[David L. Rabinowitz]]) || 4.9&nbsp;Ma || SK
|-
| [[8405 Asbolus]] || 1995 || [[Spacewatch]] ([[James V. Scotti]]) || 0.86&nbsp;Ma || SN
|-
| [[10199 Chariklo]] || 1997 || [[Spacewatch]] || 10.3&nbsp;Ma || U
|-
| [[10370 Hylonome]] || 1995 || [[Mauna Kea Observatory]] || 6.3&nbsp;Ma || UN
|-
| [[54598 Bienor]] || 2000 || [[Marc W. Buie]] et al. || ? || U
|-
| [[55576 Amycus]] || 2002 || [[Near Earth Asteroid Tracking|NEAT]] at [[Palomar Observatory|Palomar]] || 11.1&nbsp;[[Year#SI prefix multipliers|Ma]] || UK
|}
{{Notelist}}

==See also==
* [[Asteroid]]
* [[Dwarf planet]]

== Explanatory notes ==
{{Reflist|group=note}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Centaurs (minor planets)}}
* [https://minorplanetcenter.net//iau/lists/Centaurs.html List of centaurs and scattered-disk objects]
* Centaurs from [http://www.daviddarling.info/encyclopedia/C/Centaur.html The Encyclopedia of Astrobiology Astronomy and Spaceflight]
* {{cite journal|first1=Jonathan|last1=Horner|first2=Patryk Sofia|last2=Lykawka|s2cid=53982616|title=Planetary Trojans – the main source of short period comets?|journal=International Journal of Astrobiology|arxiv=1007.2541|doi=10.1017/S1473550410000212|year=2010|volume=9|issue=4|pages=227–234|bibcode = 2010IJAsB...9..227H }}
* [http://www.jpl.nasa.gov/news/news.php?release=2013-234 NASA's WISE Finds Mysterious Centaurs May Be Comets (2013)]

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