Centaur (small Solar System body)
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 orbits because of this; almost all their orbits have dynamic lifetimes of only a few million years, but there is one
known centaur, 514107 Kaʻepaokaʻāwela, which may be in a stable 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 km in diameter range from as low as 44,000 to more than 10,000,000.
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 250 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.
Classification
A centaur has either a perihelion or a semi-major axis between those of the outer 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 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.Discrepant criteria
However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:- The Minor Planet Center defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune. The MPC sometimes lists centaurs and scattered disc objects together as a single group.
- The Jet Propulsion Laboratory similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune.
- In contrast, the Deep Ecliptic Survey 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 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 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 cut-off half-way to Saturn as 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.
- 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, 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. There are an additional 223 trans-Neptunian objects with a perihelion closer than the orbit of Uranus.
Ambiguous objects
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. It is listed as an outer centaur by the Deep Ecliptic Survey. Among the inner centaurs, 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.
A recent orbital simulation 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, 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 and are in an important evolutionary transition state that further blurs the distinction between the centaur and Jupiter-family comet populations.
Naming convention
According to the International Astronomical Union's Working Group for Small Bodies Nomenclature, centaurs with semi-major axes less than 30 AU and perihelion distances greater than 5.5 AU are to be named after mythological centaurs—creatures that are half-horse and half-human. The WGSBN technically counts Neptune-crossing trans-Neptunian objects as centaurs and reserves them for a different naming scheme, which was adopted in 2007 when the first of these objects, 65489 Ceto–Phorcys and 42355 Typhon–Echidna, were named. According to the WGSBN, Neptune-crossing trans-Neptunian objects must be named after mythological chimeras, which includes hybrid and shape-shifting mythical creatures. One example of a named object in this category is 471325 Taowu, whose namesake is said to be a hybrid of a human, tiger, and a boar.Orbits
Distribution
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.The orbits of centaurs show a wide range of eccentricity, from highly eccentric to more circular.
To illustrate the range of the orbits' parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow :
- follows an extremely eccentric orbit, leading it from inside Earth's orbit to well beyond Neptune
- follows a quasi-circular orbit
- has the lowest inclination.
- is one of a small proportion of centaurs with an extreme prograde inclination. It follows such a highly inclined orbit 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.
Seventeen of these high-inclination, retrograde centaurs were controversially claimed to have an interstellar origin.
Changing orbits
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. 2023 RB 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. 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.