Centaurs and trans-Neptunian Objects (TNOs)
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Centaurs are small bodies of our solar system moving around the Sun between Jupiter and Neptune (Figure 3), gravitationally influenced by the giant planets. There is no ‘official’ (dynamical) definition; several are in use. The Minor Planet Center (MPC) for example defines Centaurs as objects having perihelia q beyond the orbit of Jupiter and semi-major axes a inside the orbit of Neptune, whereas the Jet Propulsion Laboratory (JPL) just restricts the semi-major axis a to be between those of Jupiter and Neptune (5.5 au ≤ a ≤ 30.1 au). Jewitt [10] defines Centaurs as bodies having orbital perihelia and semi-major axes between the orbits of Jupiter and Neptune and which are not in 1:1 mean-motion resonance with any planet. Because they strongly interact with the giant planets, their dynamical mean lifetime is short (~105-7 yr) [11], [12].

Centaurs are assumed to be in a transition phase between trans-Neptunian Objects (TNOs) and other populations of the inner solar system, particularly Jupiter-Family Comets (JFCs), though this relationship and the transition process is yet not well studied (Figure 1, 4 and 5). They can also be transported outward or even
ejected from the solar system [12]. Currently several hundred Centaurs are known3. The whole population of Centaurs more than 1 km in diameter might range from 104 up to 107 [12], [13].

Beside their dynamical life history, Centaurs are remarkable and interesting objects, because they can show cometary-like activity (even beyond the snow-line, probably triggered by the crystallisation of the amorphous water ice), have (sometimes) brightness outbursts, and can have satellites and even rings. The largest currently known Centaur is (10199) Chariklo [14], with a mean diameter of about 250 km (q = 13 au, e = 0.17). It is also remarkable, that Chariklo has a ring system, which was discovered by a stellar occultation [6]. 

The first Centaur (which was recognised as such) was (2060) Chiron (q = 8.5 au, e = 0.38, diameter ~220 km), discovered in 1977 by Charles Kowal at Palomar Observatory. At the time of discovery it was the most distant known minor planet – the first Trans-Neptunian Object (TNO) was discovered 15 years later.
Nevertheless, it should be noted that (2060) Chiron is not the first ever discovered Centaur. (920) Hidalgo (q = 1.9 au, a = 5.7 au, e = 0.66), discovered in 1920 by Walter Baade at Bergedorf Observatory, belongs to the Centaurs per JPL definition [15]. But at that time, Hidalgo was considered to be a (maybe somehow
special) asteroid, because this population of objects was not recognized as a distinct class until the discovery of Chiron. 

Centaurs (especially the inactive ones) can also serve as proxies for trans-Neptunian objects, because they are dynamically young and not so much evolved, but they are brighter (because of their smaller distance to Earth) and thus can be better studied with smaller telescopes than it is necessary for distant and weak TNOs. 29P/Schwassmann-Wachmann 1 is another well-known Centaur beside (10199) Chariklo and (2060) Chiron = 95P/Chiron.

 

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