Centaur (planetoid) - AbsoluteAstronomy.com

Centaurs are an unstable orbital class of minor planet

Minor planet

An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...

s that behave with characteristics of both asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and comets. They are named after the mythological race of beings, centaur

Centaur

In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse...

s, which were a mixture of horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

and human

Human

Humans are the only living species in the Homo genus...

. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets

Gas giant

A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

, and have dynamic lifetimes of a few million years.
It has been estimated that there are around 44,000 centaurs in the Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

with diameters larger than 1 km.

The first centaur-like object to be discovered was 944 Hidalgo

944 Hidalgo

944 Hidalgo is a small Solar System body with a semi-major axis beyond Jupiter's and an orbital period of 13.77 years. This makes it a centaur, but it was discovered in 1920 and has hence traditionally been considered an asteroid....

in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron

2060 Chiron

2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

in 1977. The largest known centaur is 10199 Chariklo

10199 Chariklo

10199 Chariklo is the largest known centaur. It orbits the Sun between Saturn and Uranus, grazing Uranus.Chariklo was discovered by James V. Scotti of the Spacewatch program on February 15, 1997. Chariklo is named after the nymph Chariclo , the wife of Chiron and the daughter of Apollo.A...

, discovered in 1997, which at 260 km in diameter is as big as a mid-sized main-belt

Asteroid belt

The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

asteroid.

No centaur has been photographed up close, although there is evidence that Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moon Phoebe

Phoebe (moon)

Phoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart...

, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

has gleaned some information about the surface features of 8405 Asbolus

8405 Asbolus

8405 Asbolus is a centaur, that is, an icy asteroid that orbits between Jupiter and Neptune. It was discovered by James V. Scotti and Robert Jedicke of Spacewatch at Kitt Peak Observatory on April 5, 1995. It is named after Asbolus , a centaur in Greek mythology...

.

, three centaurs have been found to display cometary coma

Coma (cometary)

frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma In astronomy, a coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate...

s: Chiron

2060 Chiron

, 60558 Echeclus

60558 Echeclus

60558 Echeclus is a centaur in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as an asteroid with provisional designation . Research in 2001 by Rousselot and Petit at the Besançon observatory in France showed no evidence of cometary activity, but in late...

, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s. Other centaurs such as 52872 Okyrhoe

52872 Okyrhoe

52872 Okyrhoe is a centaur orbiting in the outer Solar System between Jupiter and Neptune. It was discovered on September 19, 1998, by Spacewatch.-Orbit:Centaurs have short dynamical lives due to strong interactions with the giant planets...

are suspected of showing cometary activity

Coma (cometary)

. Any centaur that is perturbed

Perturbation (astronomy)

Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

close enough to the Sun is expected to become a comet.

Classification

The generic definition of a centaur is a small body that orbits the Sun between Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

and Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

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 and , which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed

Perturbation (astronomy)

until they start to cross the orbit of one or more of the giant planets.

However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements

Orbital elements

Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...

The Minor Planet Center
Minor Planet Center
The Minor Planet Center operates at the Smithsonian Astrophysical Observatory , which is part of the Center for Astrophysics along with the Harvard College Observatory ....

(MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune.
The Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

(JPL) similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune (5.5 AU < a < 30.1 AU).
In contrast, the Deep Ecliptic Survey
Deep Ecliptic Survey
The Deep Ecliptic Survey is a project to find Kuiper belt objects , using the facilities of the National Optical Astronomy Observatory .The principal investigator is Bob Millis....

(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 as non-resonant
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

objects whose instantaneous (osculating
Osculating orbit
In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space is the gravitational Kepler orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space (at a given moment of time) is the gravitational Kepler orbit In astronomy,...

) 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.

The collection The Solar System Beyond Neptune (2008) uses the traditional definition of centaurs, limited to semi-major axes smaller than that of Neptune, classifying the objects on unstable orbits beyond this limit as members of the scattered disk. Yet, other astronomers still 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

Hill sphere

An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...

of a gas giant

Gas giant

A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

within the next 10 million years. Thus centaurs can be thought of as inward scattered objects that interact more aggressively and scatter more quickly than typical scattered disc objects.

The JPL Small-Body Database lists 183 centaurs. There are an additional 40 trans-Neptunian object

Trans-Neptunian object

A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

s with a semi-major axis further than Neptune (a > 30.1 AU) and perihelion closer than the orbit of Uranus

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

(q < 19 AU). The Committee on Small Body Nomenclature of the International Astronomical Union

International Astronomical Union

The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

has not formally weighed in on either 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

65489 Ceto

65489 Ceto , provisionally known as 2003 FX128, is a binary trans-Neptunian object discovered on March 22, 2003 by C. A. Trujillo and M. Brown at Palomar. It is named after the sea goddess Ceto from Greek mythology. The object was identified as a binary on April 11, 2006 by K. Noll, H. Levison, W....

and Typhon and Echidna

42355 Typhon

42355 Typhon is a scattered disc object that was discovered on February 5, 2002, by the NEAT program. It measures 134±13 km in diameter, and is named after Typhon, a monster in Greek mythology....

have been named according to the new policy.

Other objects caught between these differences in classification methods include , which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune. Among the inner centaurs, 2005 VD

2005 VD

-Classification:2005 VD has a semi-major axis greater than Jupiter and almost crosses the orbit of Jupiter when near perihelion. JPL, the Deep Ecliptic Survey , and the Minor Planet Center all currently classify it as a centaur ....

, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.

Distribution

The diagram at right illustrates the orbits of all known centaurs in relation to the orbits of the planets. For selected objects, the eccentricity

Orbital eccentricity

The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

of the orbits is represented by red segments (extending from perihelion to aphelion).

Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus

5145 Pholus

5145 Pholus is a centaur in an eccentric orbit, with a perihelion less than Saturn's and aphelion greater than Neptune's. Pholus has not come within one astronomical unit of a planet since 764 BC, and will not until 5290. It is believed that Pholus originated as a Kuiper belt object.It was...

, Asbolus

8405 Asbolus

, Amicus, Nessus

7066 Nessus

7066 Nessus is a centaur that was discovered by David L. Rabinowitz, working with Spacewatch, at Kitt Peak on 26 April 1993. It was the second centaur found by him , and the third centaur discovery...

) to more circular (Chariklo and the Saturn-crossers

Saturn-crosser asteroid

A Saturn-crosser is a minor planet whose orbit crosses that of Saturn. The known numbered Saturn-crossers are listed below. There is only one inner-grazer and no outer-grazers or co-orbitals known; most if not all of the crossers are centaurs.Notes: † inner-grazer.* 944 Hidalgo †* 2060 Chiron*...

: Thereus, Okyrhoe

52872 Okyrhoe

).

To illustrate the range of the orbits' parameters, a few objects with very unusual orbits are plotted in yellow on the diagram:
(Apollo asteroid

Apollo asteroid

The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...

) follows an extremely eccentric orbit (e=0.947), leading it from inside Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's orbit (0.94 AU) to well beyond Neptune (>34 AU) follows a quasi-circular orbit (e<0.026) has the lowest inclination (i<3°).

Damocles
5335 Damocles
5335 Damocles is the archetype of the Damocloids, asteroids that are inactive nuclei of the Halley Family and long-period comets. It was discovered in 1991 and named after Damocles, a figure of Greek mythology....

is among a few centaurs on orbits with extreme inclination (prograde i>70°, e.g. , , retrograde i<120° e.g. ; not shown) follows such a highly inclined orbit (nearly 80°) that, while it crosses from the distance of the asteroid belt from the Sun to past the distance of Saturn, it does not even cross Jupiter relative to the plane of Jupiter's orbit.

A dozen known centaurs, including Dioretsa

20461 Dioretsa

Dioretsa is a centaur discovered in 1999 notable for its unusual orbit, which is highly eccentric and retrograde. Its name is the word 'asteroid' spelled backwards to reflect this fact...

("asteroid" spelled backwards), follow retrograde orbits.

Changing orbits

Since the centaurs cross the orbits of the giant planets and are not protected by orbital resonance

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

s, their orbits are unstable within a timescale of 10⁶–10⁷ years. For example, 55576 Amycus

55576 Amycus

55576 Amycus , provisionally known as 2002 GB10, is a centaur discovered on April 8, 2002, by the NEAT at Palomar.55576 Amycus was named for Amycus, a male centaur in Greek mythology.It came to perihelion in February 2003....

is in an unstable orbit near the 3:4 resonance of Uranus. Dynamical studies of their orbits indicate that centaurs are probably an intermediate orbital state of objects transitioning from the Kuiper belt

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

to the Jupiter family of short-period comet

Comet

s. Objects may be perturbed

Perturbation (astronomy)

from the Kuiper belt, whereupon they become Neptune

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 comet

Comet

s in the Jupiter family if they display cometary activity. Centaurs will thus ultimately collide

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

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

Jupiter

Physical characteristics

The relatively small size of centaurs precludes surface observations, but colour indices

Color index

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

and spectra can indicate possible surface composition and can provide insight into the origin of the bodies.

Colours

Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition. In the side-diagram, the colour indices

Color index

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

are measures of apparent magnitude

Apparent magnitude

The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

of an object through blue

Blue

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

(B), visible (V) i.e. green-yellow and red

Red

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

(R) filters. The diagram illustrates these differences (in enhanced colour) for all centaurs with known colour indices. For reference, two moons: Triton

Triton (moon)

Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

and Phoebe

Phoebe (moon)

, and planet Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

are plotted (yellow labels, size not to scale).

Centaurs appear to be grouped into two classes:

very red, for example 5145 Pholus
5145 Pholus
5145 Pholus is a centaur in an eccentric orbit, with a perihelion less than Saturn's and aphelion greater than Neptune's. Pholus has not come within one astronomical unit of a planet since 764 BC, and will not until 5290. It is believed that Pholus originated as a Kuiper belt object.It was...
blue (or blue-grey, according to some authors), for example 2060 Chiron
2060 Chiron
2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:

The colour difference results from a difference in the origin and/or composition of the centaur (see origin below)
The colour difference reflects a different level of space-weathering from radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

and/or comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the 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). Alternatively, Pholus may have been only recently expelled from the Kuiper belt, so that surface transformation processes have not yet taken place.

A. Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions.

Spectra

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

2060 Chiron

, 10199 Chariklo

10199 Chariklo

and 5145 Pholus

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
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s (like those detected on Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

and Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

) with amorphous carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

.
Pholus has been suggested to be covered by a mixture of Titan-like tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s, carbon black
Carbon black
Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

, olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

and methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

ice.
The surface of 52872 Okyrhoe
52872 Okyrhoe
52872 Okyrhoe is a centaur orbiting in the outer Solar System between Jupiter and Neptune. It was discovered on September 19, 1998, by Spacewatch.-Orbit:Centaurs have short dynamical lives due to strong interactions with the giant planets...

has been suggested to be a mixture of kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

s, olivines and small percentage of water ice.
8405 Asbolus
8405 Asbolus
8405 Asbolus is a centaur, that is, an icy asteroid that orbits between Jupiter and Neptune. It was discovered by James V. Scotti and Robert Jedicke of Spacewatch at Kitt Peak Observatory on April 5, 1995. It is named after Asbolus , a centaur in Greek mythology...

has been suggested to be a mixture of 15% Triton-like tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.

Chiron

2060 Chiron

, the only centaur with known cometary activity, 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.

Similarities to comets

Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma

Coma (cometary)

(a cloud of gas and dust evaporating from its surface). It is thus now officially classified as both a comet

Comet

and an asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

, 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

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

60558 Echeclus

was discovered without a coma but recently became active, and so it is now accordingly also classified as both a comet and an asteroid.

There is no clear orbital distinction between centaurs and comets. Both 29P/Schwassmann-Wachmann

29P/Schwassmann-Wachmann

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...

and 39P/Oterma

39P/Oterma

39P/Oterma is a periodic comet in our solar system. It has a centaur like orbit with a perihelion greater than Jupiter and a semi-major axis less than Neptune....

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. The faint comet 38P/Stephan-Oterma

38P/Stephan-Oterma

38P/Stephan–Oterma is a periodic comet discovered in January 1867, by Jerome Coggia at Marseilles Observatory, France.-Orbit:It has perihelion near the orbit of Mars and has aphelion near the orbit of Uranus...

would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5 AU. By the year 2200, comet 78P/Gehrels

78P/Gehrels

78P/Gehrels, also known as Gehrels 2, is a periodic comet in the Solar System.- Outward migration :Comet 78P/Gehrels' aphelion of 5.4AU is in the zone of control of the giant planet Jupiter and the orbit of the comet is frequently perturbed by Jupiter...

will probably migrate outwards into a centaur-like orbit.

Theories of origin

The study of centaur development is rich in recent developments but 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

Kuiper belt

objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. Scattered disk objects would be dynamically the best candidates for such expulsions, but their colours do not fit the bicoloured nature of the centaurs. Plutino

Plutino

In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...

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

Perturbation (astronomy)

by Pluto.

Further developments are expected with more physical data on KBOs

Kuiper belt

Notable centaurs

Well-known centaurs include:

Name	Year	Discoverer	Half-life (forward)	Class
55576 Amycus 55576 Amycus 55576 Amycus , provisionally known as 2002 GB10, is a centaur discovered on April 8, 2002, by the NEAT at Palomar.55576 Amycus was named for Amycus, a male centaur in Greek mythology.It came to perihelion in February 2003....	2002	NEAT Near Earth Asteroid Tracking Near-Earth Asteroid Tracking is a program run by NASA and Jet Propulsion Laboratory to discover near-Earth objects. The NEAT project began in December 1995 and ran until April 2007.-History:... at Palomar Palomar Observatory Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...	11.1 Myr Myr The symbol myr was formerly used in English-language geology, and remains as the standard usage in astronomy, as a unit of one million years.It is an abbreviation for 'million years' and lower case is used in geology, while upper case is used in astronomy....	UE
10370 Hylonome 10370 Hylonome 10370 Hylonome is a Small Solar System Body orbiting in the outer Solar System. It belongs to the class of icy objects that are known as centaurs, with an orbit that crosses the orbit of Neptune and grazes that of Uranus...	1995	Mauna Kea Observatory Mauna Kea Observatory The Observatories at Mauna Kea, , are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawai'i, USA. The facilities are located in a special land use zone known as the "Astronomy Precinct," which is located in the Mauna Kea...	6.3 Myr	UN
10199 Chariklo 10199 Chariklo 10199 Chariklo is the largest known centaur. It orbits the Sun between Saturn and Uranus, grazing Uranus.Chariklo was discovered by James V. Scotti of the Spacewatch program on February 15, 1997. Chariklo is named after the nymph Chariclo , the wife of Chiron and the daughter of Apollo.A...	1997	Spacewatch Spacewatch Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets...	10.3 Myr	U
8405 Asbolus 8405 Asbolus 8405 Asbolus is a centaur, that is, an icy asteroid that orbits between Jupiter and Neptune. It was discovered by James V. Scotti and Robert Jedicke of Spacewatch at Kitt Peak Observatory on April 5, 1995. It is named after Asbolus , a centaur in Greek mythology...	1995	Spacewatch Spacewatch Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets... (James V. Scotti James V. Scotti James Vernon Scotti is an American astronomer. He was born in Bandon, Oregon and graduated from Woodway Senior High in Edmonds, Washington in 1978. He received his B.Sc. in Astronomy from the University of Arizona in Tucson in 1983. Ever since that time, he has worked on the Spacewatch project,... )	0.86 Myr	SN
7066 Nessus 7066 Nessus 7066 Nessus is a centaur that was discovered by David L. Rabinowitz, working with Spacewatch, at Kitt Peak on 26 April 1993. It was the second centaur found by him , and the third centaur discovery...	1993	Spacewatch Spacewatch Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets... (David L. Rabinowitz David L. Rabinowitz David Lincoln Rabinowitz is a researcher at Yale University. He has built CCD cameras and software for the detection of near-Earth asteroids and Kuiper Belt Objects, and his research has helped reduce the assumed number of near-Earth asteroids by half, from 1,000-2,000 to 500-1,000 He has also... )	4.9 Myr	SE
5145 Pholus 5145 Pholus 5145 Pholus is a centaur in an eccentric orbit, with a perihelion less than Saturn's and aphelion greater than Neptune's. Pholus has not come within one astronomical unit of a planet since 764 BC, and will not until 5290. It is believed that Pholus originated as a Kuiper belt object.It was...	1992	Spacewatch Spacewatch Spacewatch is a project at the University of Arizona led by Robert S. McMillan that specializes in the study of minor planets, including various types of asteroids and comets... (David L. Rabinowitz David L. Rabinowitz David Lincoln Rabinowitz is a researcher at Yale University. He has built CCD cameras and software for the detection of near-Earth asteroids and Kuiper Belt Objects, and his research has helped reduce the assumed number of near-Earth asteroids by half, from 1,000-2,000 to 500-1,000 He has also... )	1.28 Myr	SN
2060 Chiron 2060 Chiron 2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...	1977	Charles T. Kowal Charles T. Kowal Charles Thomas Kowal was an American astronomer.He discovered two moons of Jupiter: Leda in 1974 and Themisto in 1975, although the latter was lost and not rediscovered until 2000....	1.03 Myr	SU

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