The centaurs are a class of icy
named after the mythical race of
Neptune, crossing the orbits of the large
planets. The first centaur to be discovered was
2060 Chiron in
while the largest currently known is
10199 Chariklo discovered in
No centaur has yet been photographed up close by a spacecraft, although there
is evidence that Saturn's moon
Phoebe, imaged by the
Cassini probe in
2004, may be a captured centaur. In addition, the
Hubble Space Telescope has gleaned some information about the surface
Three centaurs, Chiron,
60558 Echeclus, and
166P/NEAT 2001 T4, have been found to display cometary
comas. Chiron and
60558 Echeclus are now classified as both
and comets. It is
possible that other centaurs may also be comets, but as of March 2006 no
cometary behavior has been discovered for any others.
Because the centaurs cross the orbits of the giant planets and are not
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
Belt to the
Jupiter Family of short period
may be perturbed from the
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
The relatively small size of centaurs precludes surface observations, but
spectra can indicate possible surface composition and can provide insight
into the origin of the bodies.
Centaurs display a puzzling diversity of colour that challenges any simple
model of surface composition. In the diagram on the right, the
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 enhanced colour) for all centaurs with
known colour indices. For reference, two moons:
Phoebe, and planet
Mars are plotted (yellow labels, size not to scale).
Centaurs appear to be grouped into two classes:
- very red, for example
- blue (or blue-grey, according to some authors), for example
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
- The colour difference reflects a different level of space weathering from
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 2001 T4).
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.
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
10199 Chariklo and
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
those detected on
- Pholus has been suggested to be covered by a mixture of Titan-like
- The surface of
Okyrhoe has been suggested to be a mixture of
olivines and small percentage of water ice.
- 8405 Asbolus is been suggested to be a mixture of 15% Triton-like
Titan-like tholin, 37% amorphous carbon and 40% ice tholin.
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
Similarities to comets
Observations of Chiron in 1988 and 1989 near its
found it to display a
coma (a cloud gas and dust evaporating from its surface). It is thus now
officially classified as both a
comet and an
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
60558 Echeclus, and
166P/NEAT 2001 T4 have shown such behavior.
166P/NEAT 2001 T4 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 is now accordingly also classified as both a comet and an asteroid.
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 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
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.
Well-known centaurs include:
Charles T. Kowal