Centaurs (minor planets)

Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets.

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.

It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.

Naming

They are named after the mythological race of beings, centaurs, which were a mixture of horse and human.

In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse. In early Attic and Boeotian vase-paintings, they are depicted with the hindquarters of a horse attached to human; in later renderings centaurs are given the torso of a human joined at the waist to the horse's withers, where the horse's neck would be.

The centaurs were usually said to have been born of Ixion and Nephele (the cloud made in the image of Hera).

Orbits

Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus, Asbolus, Amicus, Nessus) to more circular (Chariklo and the Saturn-crossers: Thereus, Okyrhoe).

Since the centaurs cross the orbits of the giant planets and are not protected by orbital resonances, their orbits are unstable within a timescale of 10⁶ – 10⁷ years.

Dynamical studies of their orbits indicate that centaurs are 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. 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. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

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 surface observations, but colour indices and spectra can indicate possible surface composition and can provide insight into the origin of the bodies.

Color

Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition.

Centaurs appear to be grouped into two classes:
1. Very red, for example 5145 Pholus
2. Blue (or blue-grey), for example 2060 Chiron

There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:
1. The colour difference results from a difference in the origin and/or composition of the centaur.
2. The colour difference reflects a different level of space-weathering from radiation and/or cometary activity.

Spectra

The interpretation of spectra is often ambiguous, related to particle sizes and other factors. For example, if there is cometary activity, 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.

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 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. 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 KBOs.