Centaur (small Solar System body)

The largest confirmed centaur is 10199 Chariklo, which at 260 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings.

However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements: The Gladman & Marsden (2008)[12] criteria would make some objects Jupiter-family comets: Both Echeclus (q = 5.8 AU, TJ = 3.03) and Okyrhoe (q = 5.8 AU; TJ = 2.95) have traditionally been classified as centaurs.

Traditionally considered an asteroid, but classified as a centaur by JPL, Hidalgo (q = 1.95 AU; TJ = 2.07) would also change category to a Jupiter-family comet.

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.

Objects in this gateway region can display significant activity[16][17] and are in an important evolutionary transition state that further blurs the distinction between the centaur and Jupiter-family comet populations.

The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on any 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.

[18] Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown's website include 10199 Chariklo, (523727) 2014 NW65 and 2060 Chiron.

[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.

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

In addition to the water ice signature, a number of other models have been put forward: Chiron appears to be the most complex.

[31][32][33] Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma (a cloud of gas and dust evaporating from its surface).

[36] 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.

[citation needed] 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.

[40] The study of centaurs’ origins is rich in recent developments, but any conclusions are still hampered by limited physical data.