Inplanetary astronomy, acentaur is asmall Solar System body that orbits the Sun betweenJupiter andNeptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstableorbits because of this; almost all their orbits have dynamic lifetimes of only a few million years,[1] but there is one known centaur,514107 Kaʻepaokaʻāwela, which may be in astable (though retrograde) orbit.[2][note 1] Centaurs typically exhibit the characteristics of bothasteroids andcomets. They are named after the mythologicalcentaurs 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 theSolar System more than 1 km in diameter range from as low as 44,000[1] to more than 10,000,000.[4][5]
The first centaur to be discovered, under the definition of theJet Propulsion Laboratory and the one used here, was944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of2060 Chiron in 1977. The largest confirmed centaur is10199 Chariklo, which at 250 kilometers in diameter is as big as a mid-sizedmain-belt asteroid, and is known to have asystem of rings. It was discovered in 1997.
No centaur has been photographed up close, although there is evidence thatSaturn's moonPhoebe, imaged by theCassini probe in 2004, may be a captured centaur that originated in theKuiper belt.[6] In addition, theHubble Space Telescope has gleaned some information about the surface features of8405 Asbolus.
Ceres may have originated in the region of the outer planets,[7] 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 dustcomas, with three,2060 Chiron,60558 Echeclus, and29P/Schwassmann-Wachmann 1, having detectable levels of volatile production in orbits entirely beyond Jupiter.[8] Chiron and Echeclus are therefore classified as both centaurs and comets, while Schwassmann-Wachmann 1 has always held a comet designation. Other centaurs, such as52872 Okyrhoe, are suspected of having showncomas. Any centaur that isperturbed close enough to the Sun is expected to become a comet.
A centaur has either aperihelion or asemi-major axis between those of theouter planets (between Jupiter and Neptune). Due to the inherent long-term instability of orbits in this region, even centaurs such as2000 GM137 and2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will beperturbed until they start to cross the orbit of one or more of the giant planets.[1] 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.
However, different institutions have different criteria for classifying borderline objects, based on particular values of theirorbital elements:
TheMinor Planet Center (MPC) defines centaurs as having aperihelion beyond the orbit of Jupiter (5.2 AU <q) and a semi-major axis less than that of Neptune (a < 30.1 AU).[9] The MPC sometimes lists centaurs andscattered disc objects together as a single group.[10]
TheJet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis,a, between those of Jupiter and Neptune (5.5 AU ≤a ≤ 30.1 AU).[11]
In contrast, theDeep 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 million years. The DES defines centaurs asnon-resonant objects whose instantaneous (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.[12]
The collectionThe Solar System Beyond Neptune (2008) defines objects with a semi-major axis between those of Jupiter and Neptune and a Jupiter-relativeTisserand's parameter above 3.05 as centaurs, classifying the objects with a Jupiter-relative Tisserand's parameter below this and, to excludeKuiper belt objects, an arbitraryperihelion cut-off half-way to Saturn (q ≤ 7.35 AU) asJupiter-family comets, and classifying those objects on unstable orbits with a semi-major axis larger than Neptune's as members of the scattered disc.[13]
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 theHill sphere of agas giant within the next 10 million years,[14] 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 centaurs.[15] There are an additional 223 trans-Neptunian objects (objects with a semi-major axis further than Neptune's, i.e.30.1 AU ≤a) with a perihelion closer than the orbit ofUranus (q ≤ 19.2 AU).[16]
The Gladman & Marsden (2008)[13] criteria would make some objects Jupiter-family comets: BothEcheclus (q = 5.8 AU,TJ = 3.03) andOkyrhoe (q = 5.8 AU;TJ = 2.95) have traditionally been classified as centaurs. Traditionally considered an asteroid, but classified as a centaur by JPL,Hidalgo (q = 1.95 AU;TJ = 2.07) would also change category to a Jupiter-family comet.29P/Schwassmann-Wachmann (q = 5.72 AU;TJ = 2.99) has been categorized as both a centaur and a Jupiter-family comet depending on the definition used.[citation needed]
Other objects caught between these differences in classification methods include(44594) 1999 OX3, which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune. It is listed as an outer centaur by theDeep 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[4] 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,P/2010 TO20 LINEAR-Grauer,P/2008 CL94 Lemmon, and2016 LN8), but simulations indicate that there may of order 1000 more objects >1 km in radius that have yet to be detected. Objects in this gateway region can display significant activity[17][18] and are in an important evolutionary transition state that further blurs the distinction between the centaur and Jupiter-family comet populations.[citation needed]
According to theInternational Astronomical Union's Working Group for Small Bodies Nomenclature (WGSBN; formerly the Committee on Small Body Nomenclature[19]), centaurs with semi-major axes less than 30 AU (inside Neptune's orbit) and perihelion distances greater than 5.5 AU (outside Jupiter's orbit) are to be named aftermythological centaurs—creatures that are half-horse and half-human.[20]: 8 The WGSBN technically counts Neptune-crossing trans-Neptunian objects (semi-major axis >30 AU, perihelion <30 AU) as centaurs and reserves them for a different naming scheme,[20]: 8 which was adopted in 2007 when the first of these objects,65489 Ceto–Phorcys and42355 Typhon–Echidna, were named.[19]: 286 According to the WGSBN, Neptune-crossing trans-Neptunian objects must be named after mythologicalchimeras,[20]: 8 which includes hybrid and shape-shifting mythical creatures.[19]: 286 One example of a named object in this category is471325 Taowu,whose namesake is said to be a hybrid of a human, tiger, and a boar.[21]
The diagram illustrates the orbits of known centaurs in relation to the orbits of the planets. For selected objects, theeccentricity of the orbits is represented by red segments (extending fromperihelion to aphelion).
To illustrate the range of the orbits' parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow :
1999 XS35 (Apollo asteroid) follows an extremely eccentric orbit (e = 0.947), leading it from inside Earth's orbit (0.94 AU) to well beyond Neptune (> 34 AU)
2007 TB434 follows a quasi-circular orbit (e < 0.026)
2004 YH32 is one of a small proportion of centaurs with an extremeprograde inclination (i > 60°). It follows such a highly inclined orbit (79°) that, while it crosses from the distance of theasteroid 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° forDioretsa) to extreme (i < 120°;e.g. 105° for(342842) 2008 YB3[22]).Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin.[23][24][25]
Thesemi-major axis ofAsbolus 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.[26]
Because the centaurs are not protected byorbital resonances, their orbits are unstable within a timescale of 106–107 years.[27] For example,55576 Amycus is in an unstable orbit near the 3:4 resonance of Uranus.[1] Dynamical studies of their orbits indicate that being a centaur is probably an intermediate orbital state of objects transitioning from theKuiper belt to theJupiter family of short-periodcomets.(679997) 2023 RB will have its orbit notably changed by a close approach to Saturn in 2201.
Objects may beperturbed from the Kuiper belt, whereupon they becomeNeptune-crossing and interact gravitationally with that planet (seetheories 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 activecomets in the Jupiter family if they display cometary activity. Centaurs will thus ultimatelycollide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularlyJupiter.[citation needed]
Compared to dwarf planets and asteroids, the relatively small size and distance of centaurs precludes remote observation of surfaces, butcolour indices andspectra can provide clues about surface composition and insight into the origin of the bodies.[27]
The colours of centaurs are very diverse, which challenges any simple model of surface composition.[28] In the side-diagram, thecolour indices are measures ofapparent magnitude of an object through blue (B), visible (V) (i.e. green-yellow) and red (R) filters. The diagram illustrates these differences (in exaggerated colours) for all centaurs with known colour indices. For reference, two moons:Triton andPhoebe, and planetMars are plotted (yellow labels, size not to scale).
blue (or blue-grey, according to some authors) – for example2060 Chiron or2020 MK4
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 (seeorigin below)
The colour difference reflects a different level of space-weathering fromradiation and/orcometary 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).[29] 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.[30][31]
The interpretation ofspectra 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[27] (including2060 Chiron,10199 Chariklo and5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:
The surface of52872 Okyrhoe has been suggested to be a mixture ofkerogens, olivines and a small percentage of water ice.
8405 Asbolus has been suggested to be a mixture of 15% Triton-liketholins, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.
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.[33][34][35]
Comet38P exhibits centaur-like behavior by making close approaches to Jupiter, Saturn, and Uranus between 1982 and 2067.[36]
Observations of Chiron in 1988 and 1989 near itsperihelion found it to display acoma (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, and166P/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. Echeclus was discovered without a coma but recently became active,[37] 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.[38]
Carbon monoxide has been detected in Echeclus[8] and Chiron[39] 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 Echeclus and Chiron is substantially lower than what is typically observed for29P/Schwassmann–Wachmann,[17] another distantly active comet often classified as a centaur.
There is no clear orbital distinction between centaurs and comets. Both29P/Schwassmann–Wachmann and39P/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.[40] The faint comet38P/Stephan–Oterma would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5 AU. By the year 2200, comet78P/Gehrels will probably migrate outwards into a centaur-like orbit.[citation needed]
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.[41]
Comparison of sizes, albedos, and colors of various large centaurs with measured diameters.
Centaurs can reach diameters up to hundreds of kilometers. The largest centaurs have diameters in excess of 300 km, and primarily reside beyond 20AU.[42]
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.
Simulations indicate that the orbit of someKuiper 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.Plutinos 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 toperturbation byPluto.[43]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.[44] The orbits of centaurs2020 MK4, P/2008 CL94 (Lemmon), and P/2010 TO20 (LINEAR-Grauer) pass close to that of comet29P/Schwassmann–Wachmann, the first discovered centaur and close encounters are possible in which one of the objects traverses the coma of 29P when active.[44]
At least one centaur, 2013 VZ70, might have an origin among Saturn's irregular moon population via impact, fragmentation, or tidal disruption.[45]
^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.
^Chaing, Eugene; Lithwick, Y.; Murray-Clay, R.; Buie, M.; Grundy, W.; Holman, M. (2007). Reipurth, B.; Jewitt, D.; Keil, K. (eds.). "A Brief History of Transneptunian Space".Protostars and Planets V. Tucson, AZ: University of Arizona Press:895–911.arXiv:astro-ph/0601654.Bibcode:2007prpl.conf..895C.
^Barucci, M. A.; Doressoundiram, A.; Cruikshank, D. P. (2003)."Physical Characteristics of TNOs and Centaurs"(PDF). Laboratory for Space Studies and Astrophysics Instrumentation, Paris Observatory. Archived fromthe original(PDF) on 29 May 2008. Retrieved20 March 2008.
^Galiazzo, M. A.; de la Fuente Marcos, C.; de la Fuente Marcos, R.; Carraro, G.; Maris, M.; Montalto, M. (2016). "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)".Astrophysics and Space Science.361 (3):212–218.arXiv:1605.08251.Bibcode:2016Ap&SS.361..212G.doi:10.1007/s10509-016-2801-5.S2CID119204060.
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