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Jupiter trojans (6,178)
Scattered disc (>300) Neptune trojans (9)
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Kuiper belt (>100,000)
In planetary astronomy, a centaur is a small Solar System body with either a perihelion or a semi-major axis between those of the outer planets (between Jupiter and Neptune). Centaurs generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years, km in diameter range from as low as 44,000 to more than 10,000,000.but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable (though retrograde) orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs 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
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 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 system of rings. It was discovered in 1997.
No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur that originated in the Kuiper belt.In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus.
Ceres may have originated in the region of the outer planets,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 comas, with three, 2060 Chiron, 60558 Echeclus, and 29P/Schwassmann-Wachmann 1, having detectable levels of volatile production in orbits entirely beyond Jupiter.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 Okyrhoe, are suspected of having shown comas. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even centaurs such as 2000 GM137 and 2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets. 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 their orbital elements:
The Gladman & Marsden (2008) q = 5.8 AU, TJ = 3.03) and Okyrhoe (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. Schwassmann-Wachmann 1 (q = 5.72 AU; TJ = 2.99) has been categorized as both a centaur and a Jupiter-family comet depending on the definition used.criteria would make some objects Jupiter-family comets: Both Echeclus (
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 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 km in radius that have yet to be detected. Objects in this gateway region can display significant activity and are in an important evolutionary transition state that further blurs the distinction between the centaur and Jupiter-family comet populations.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, including 29P/Schwassmann-Wachmann, P/2010 TO20 LINEAR-Grauer, P/2008 CL94 Lemmon, and 2016 LN8, but the simulations indicate that there may of order 1000 more objects >1
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 Ceto and Phorcys and Typhon and Echidna have been named according to the new policy.
Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown's website include 10199 Chariklo, (523727) 2014 NW65 and 2060 Chiron.
The diagram illustrates the orbits of known centaurs in relation to the orbits of the planets. For selected objects, the 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 (Pholus, Asbolus, Amycus, Nessus) to more circular (Chariklo and the Saturn-crossers Thereus and Okyrhoe).
To illustrate the range of the orbits' parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow :
Over a dozen known centaurs follow retrograde orbits. Their inclinations range from modest (e.g., 160° for Dioretsa) to extreme (i < 120°; e.g. 105° for (342842) 2008 YB3 ). Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin.
Because the centaurs are not protected by orbital resonances, their orbits are unstable within a timescale of 106–107 years. For example, 55576 Amycus is in an unstable orbit near the 3:4 resonance of Uranus. 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 of short-period comets.
Objects may be perturbed from the Kuiper belt, whereupon they become Neptune-crossing and interact gravitationally with that planet (see 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 comets in the Jupiter family if they display cometary activity. Centaurs will thus ultimately 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.
The relatively small size of centaurs precludes remote observation of surfaces, but colour indices and spectra can provide clues about surface composition and insight into the origin of the bodies.
The colours of centaurs are very diverse, which challenges any simple model of surface composition. (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 and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).In the side-diagram, the colour indices are measures of apparent magnitude of an object through blue
Centaurs appear to be grouped into two classes:
There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:
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).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.
The interpretation of 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(including 2060 Chiron, 10199 Chariklo and 5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:
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.
Observations of Chiron in 1988 and 1989 near its perihelion found it to display a 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,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.
Carbon monoxide has been detected in 60558 Echeclusand Chiron 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 Chiron is substantially lower than what is typically observed for 29P/Schwassmann–Wachmann, 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. AU. By the year 2200, comet 78P/Gehrels will probably migrate outwards into a centaur-like orbit.The faint comet 38P/Stephan–Oterma would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5
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.
Centaurs can reach diameters up to hundreds of kilometers. The largest centaurs have diameters in excess of 300 km, and primarily reside beyond 20 AU.
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 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. 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 to perturbation by Pluto.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.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.
At least one centaur, 2013 VZ70, might have an origin among Saturn's irregular moon population via impact, fragmentation, or tidal disruption.
|55576 Amycus||2002||NEAT at Palomar||11.1 Ma||UK|
|54598 Bienor||2000||Marc W. Buie et al.||?||U|
|10370 Hylonome||1995||Mauna Kea Observatory||6.3 Ma||UN|
|10199 Chariklo||1997||Spacewatch||10.3 Ma||U|
|8405 Asbolus||1995||Spacewatch (James V. Scotti)||0.86 Ma||SN|
|7066 Nessus||1993||Spacewatch (David L. Rabinowitz)||4.9 Ma||SK|
|5145 Pholus||1992||Spacewatch (David L. Rabinowitz)||1.28 Ma||SN|
|2060 Chiron||1977||Charles T. Kowal||1.03 Ma||SU|
The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the largest member as well as the namesake of this group. The next largest members are Orcus, (208996) 2003 AZ84, and Ixion. Plutinos are named after mythological creatures associated with the underworld.
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au).
2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orbiting between the asteroid belt and the Kuiper belt.
28978 Ixion, provisional designation 2001 KX76, is a large trans-Neptunian object and a possible dwarf planet. It is located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Ixion is classified as a plutino, a dynamical class of objects in a 2:3 orbital resonance with Neptune. It was discovered in May 2001 by astronomers of the Deep Ecliptic Survey at the Cerro Tololo Inter-American Observatory, and was announced in July 2001. The object is named after the Greek mythological figure Ixion, who was a king of the Lapiths.
Comet 29P/Schwassmann–Wachmann, also known as Schwassmann–Wachmann 1, was discovered on November 15, 1927, by Arnold Schwassmann and Arno Arthur Wachmann at the Hamburg Observatory in Bergedorf, Germany. It was discovered photographically, when the comet was in outburst and the magnitude was about 13. Precovery images of the comet from March 4, 1902, were found in 1931 and showed the comet at 12th magnitude.
8405 Asbolus is a centaur orbiting in the outer Solar System between the orbits of Jupiter and Neptune. It was discovered on 5 April 1995, by James Scotti and Robert Jedicke of Spacewatch (credited) at Kitt Peak Observatory in Arizona, United States. It is named after Asbolus, a centaur in Greek mythology and measures approximately 80 kilometers in diameter.
(55565) 2002 AW197 is a classical, non-resonant trans-Neptunian object from the Kuiper belt in the outermost region of the Solar System, also known as a cubewano. With a likely diameter of at least 700 kilometers (430 miles), it is approximately tied with 2002 MS4 and 2013 FY27 (to within measurement uncertainties) as the largest unnamed object in the Solar System. It was discovered at Palomar Observatory in 2002.
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 objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.
(55637) 2002 UX25 is a trans-Neptunian object that orbits the Sun in the Kuiper belt beyond Neptune. This TNO briefly garnered scientific attention when it was found to have an unexpectedly low density of about 0.82 g/cm3.
7066 Nessus is a very red centaur on an eccentric orbit, located beyond Saturn in the outer Solar System. It was discovered on 26 April 1993, by astronomers of the Spacewatch program at the Kitt Peak National Observatory in Tucson, Arizona. The dark and reddish minor planet is likely elongated and measures approximately 60 kilometers in diameter. It was named after Nessus from Greek mythology.
10199 Chariklo is the largest confirmed centaur. It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings around Chariklo by observing a stellar occultation, making it the first minor planet known to have rings.
10370 Hylonome (; prov. designation: 1995 DW2) is a minor planet orbiting in the outer Solar System. The dark and icy body belongs to the class of centaurs and measures approximately 72 kilometers (45 miles) in diameter. It was discovered on 27 February 1995, by English astronomer David C. Jewitt and Vietnamese American astronomer Jane Luu at the U.S. Mauna Kea Observatory in Hawaii, and later named after the mythological creature Hylonome.
60558 Echeclus is a centaur, approximately 84 kilometers (52 miles) in diameter, located in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as a minor planet with provisional designation 2000 EC98 (also written 2000 EC98). Research in 2001 by Rousselot and Petit at the Besançon observatory in France indicated that it was not a comet, but in December 2005 a cometary coma was detected. In early 2006 the Committee on Small Bodies Nomenclature (CSBN) gave it the cometary designation 174P/Echeclus. It last came to perihelion in April 2015, and was expected to reach about apparent magnitude 16.7 near opposition in September 2015.
Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are only moderately affected by Neptune and the other known planets: This makes them appear to be "detached" from the rest of the Solar System, except for their attraction to the Sun.
2014 FC72 is a trans-Neptunian object, classified as a scattered and detached object, located in the outermost region of the Solar System. It was first observed on 24 March 2014 by astronomers with the Pan-STARRS survey at Haleakala Observatory, Hawaii, United States. With its perihelion distant from Neptune, it belongs to a small and poorly understood group of objects with moderate eccentricities. It is estimated to measure 500 kilometers (300 miles) in diameter, assuming a low albedo.
Centaurus is a mission concept to flyby the centaurs 2060 Chiron and Schwassmann-Wachmann 1. It was submitted in response to the NASA Discovery program call for proposals in 2019 but ultimately was not among the four missions selected for further development by NASA in February 2020. If it had been selected, Centaurus would have been the first mission to attempt a flyby of a centaur.
Chimera is a NASA mission concept to orbit and explore 29P/Schwassmann-Wachmann 1 (SW1), an active, outbursting small icy body in the outer solar system. The concept was developed in response to the 2019 NASA call for potential missions in the Discovery-class, and it would have been the first spacecraft encounter with a Centaur and the first orbital exploration of a small body in the outer solar system. The Chimera proposal was ranked in the first tier of submissions, but was not selected for further development for the programmatic reason of maintaining scientific balance.
(523676) 2013 UL10 (prov. designation:2013 UL10) is a reddish centaur with cometary activity orbiting the Sun between Jupiter and Uranus. It was discovered on 18 August 2010, by a team of astronomers with the Pan-STARRS survey at the Haleakalā Observatory, Hawaii. It is the first centaur known to have both comet-like activity and red surface colors. It is also one of the smallest centaurs, with a nucleus of no more than 10 kilometers (6.2 miles) in diameter. As of 2021, it has not been named.