Centaur (small Solar System body)
The centaurs orbit generally inwards of the Kuiper belt and outside the Jupiter trojans.
Sun Jupiter trojans (6,178) Scattered disc (>300) Neptune trojans (9) | Giant planets: · Jupiter (J) · Saturn (S) · Uranus (U) · Neptune (N) Centaurs (44,000) Kuiper belt (>100,000) |
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In
The first centaur to be discovered, under the definition of the
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.[6] 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,[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 dust
Classification
A centaur has either a
Discrepant criteria
However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:
- The perihelion beyond the orbit of Jupiter (5.2 AU < q) and a semi-major axis less than that of Neptune (a < 30.1 AU).[9] Though nowadays the MPC often lists centaurs and scattered discobjects together as a single group.
- The Jet 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).[10]
- 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 million years. The DES defines centaurs as non-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.[11]
- 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 Jupiter-family comets, and classifying those objects on unstable orbits with a semi-major axis larger than Neptune's as members of the scattered disc.[12]
- 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 million years,[13] 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 452 centaurs.[14] There are an additional 116 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 of Uranus (q ≤ 19.2 AU).[15]
Ambiguous objects
The Gladman & Marsden (2008)
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
The
Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown's website include 10199 Chariklo, (523727) 2014 NW65 and 2060 Chiron.[19]
Orbits
Distribution
The diagram illustrates the orbits of known centaurs in relation to the orbits of the planets. For selected objects, the
The orbits of centaurs show a wide range of eccentricity, from highly eccentric (
).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)
- 2001 XZ255 has the lowest inclination(i < 3°).
- 2004 YH32 is one of a small proportion of centaurs with an extreme prograde inclination (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 Dioretsa) to extreme (i < 120°; e.g. 105° for (342842) 2008 YB3[20]). Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin.[21][22][23]
Changing orbits
Because the centaurs are not protected by
will have its orbit notably changed by a close approach to Saturn in 2201.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.
Physical characteristics
The relatively small size of centaurs precludes remote observation of surfaces, but
Colours
The colours of centaurs are very diverse, which challenges any simple model of surface composition.[26] In the side-diagram, the colour indices are measures of apparent 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 and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).
Centaurs appear to be grouped into two classes:
- very red – for example 5145 Pholus
- blue (or blue-grey, according to some authors) – for example 2020 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 (see origin below)
- The colour difference reflects a different level of space-weathering from radiation and/or cometary 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).[27] 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.[28][29]
Spectra
The interpretation of
Water ice signatures have been confirmed on a number of centaurs[25] (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 amorphous carbon.
- Pholus has been suggested to be covered by a mixture of Titan-like tholins, carbon black, olivine[30] and methanol ice.
- The surface of 52872 Okyrhoe has been suggested to be a mixture of kerogens, olivines and a small percentage of water ice.
- 8405 Asbolus has been suggested to be a mixture of 15% Triton-like tholins, 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.[31][32][33]
Similarities to comets
Observations of Chiron in 1988 and 1989 near its
Carbon monoxide has been detected in 60558 Echeclus[8] and Chiron [37] 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,[16] another distantly active comet often classified as a centaur.
There is no clear orbital distinction between centaurs and comets. Both
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.[39]
Size, density, reflectivity
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.[40]
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.
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.[41] 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.
At least one centaur, 2013 VZ70, might have an origin among Saturn's irregular moon population via impact, fragmentation, or tidal disruption.[43]
Notable centaurs
Name | Year | Discoverer | Half-life[1] (forward) |
Class[a] |
---|---|---|---|---|
2060 Chiron | 1977 | Charles T. Kowal | 1.03 Ma | SU |
5145 Pholus | 1992 | Spacewatch (David L. Rabinowitz) | 1.28 Ma | SN |
7066 Nessus | 1993 | Spacewatch (David L. Rabinowitz) | 4.9 Ma | SK |
8405 Asbolus | 1995 | Spacewatch (James V. Scotti) | 0.86 Ma | SN |
10199 Chariklo | 1997 | Spacewatch | 10.3 Ma | U |
10370 Hylonome | 1995 | Mauna Kea Observatory |
6.3 Ma | UN |
54598 Bienor | 2000 | Marc W. Buie et al. |
? | U |
55576 Amycus | 2002 | NEAT at Palomar |
11.1 Ma | UK |
- ^ 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.
See also
Explanatory notes
References
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- ^ "Dawn at Ceres:What Have we Learned?" (PDF). Space Studies Board. National Academies. Archived from the original (PDF) on 2018-04-13. Retrieved 2023-10-11.
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- ^ 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 from the original (PDF) on 29 May 2008. Retrieved 20 March 2008.
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External links
- List of centaurs and scattered-disk objects
- Centaurs from The Encyclopedia of Astrobiology Astronomy and Spaceflight
- Horner, Jonathan; Lykawka, Patryk Sofia (2010). "Planetary Trojans – the main source of short period comets?". International Journal of Astrobiology. 9 (4): 227–234. S2CID 53982616.
- NASA's WISE Finds Mysterious Centaurs May Be Comets (2013)