Centaurs and trans-Neptunian Objects
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A trans-Neptunian object orbits the Sun at a greater average distance than Neptune, which has a semi-major axis a of 30.1 astronomical units (Figure 3). Trans-Neptunian objects are further classified according to their distance from the Sun and other orbital characteristics.

Classical KBOs (CKBOs), also called Cubewanos (after 1992 QB1), have orbits with semi-major axes between about 40 to 47 au. Classical KBOs are not moving in a mean motion resonance (MMR) with Neptune. In an MMR, the ratio of the orbital periods of both objects is a ratio of integer numbers (2:3, 3:4, etc.). There are two main groups within the classical KBOs, referred as ‘hot’ and ‘cold’ objects. This term is not related to the temperature, but rather to the orbital dynamic, specifically due to the strength of the gravitational influence of Neptune. Cold classical KBOs have low-eccentricity, low-inclination orbits, and are much less dynamically evolved by Neptune as the hot classical KBOs. The hot classical KBOs had more interactions with Neptune, resulting in more eccentric and more tilted orbits. The first example of a classical KBO was 1992 QB1= (15760) Albion. Other well-known CKBOs are: Makemake, Quaoar, and Varuna.

Resonant KBOs are in or near a mean motion resonance with Neptune. Pluto, for example, is in a 2:3 MMR, that means that Pluto completes two orbits around the Sun while Neptune has completed three revolutions at the same time. Many other resonant KBOs are located in this 2:3 MMR and this family is called ‘Plutinos’. These resonances stabilise the orbits because they avoid close encounters between both objects, resulting in a strong perturbation of the much smaller KBO. Drastic orbit changes including even an ejection from that region could be the consequence. Pluto, Ixion, Huya and Orcus are some prominent
examples of Plutinos. 

Scattered KBOs (also known as scattered-disc objects, SDO) cover the region which stretches far beyond Neptune, with perihelia beyond about 30 au. Neptune has scattered them into highly elliptical and highly inclined orbits. As they are not protected against the perturbations from Neptune (e.g. due to a resonance), at least a fraction of them is expected of being lost over time – some of them possibly scattered into the inner solar system (‘Centaurs’). Eris is an example for an SDO.

Detached KBOs (also know as detached objects) have perihelia beyond about 40 au. It seems unlikely that they have been significantly perturbed by Neptune (like in the case of SDOs). Presumably other forces are responsible for shaping their orbits, such as perturbations from an undiscovered, distant planet, the
gravity of closely passing stars, or gravitational perturbations as the Kuiper Belt was formed in the early age of the solar system (planet migration). Sedna (q ~76 au, Q ~900 au) is an example of a detached KBO.

Farther out we find objects which are called extreme-TNOs (ETNOs), with very large semi-major axes (a > 150 au according  to [16]) and perihelia well beyond Neptune (q > 40 au). At a solar distance ranging from about 2,000 to 200,000 au we find the inner Oort cloud, a disc-shaped part of the Oort cloud complex. 
The chase for ‘Planet X’ in the late 19th and early 20th century was  briefly described in the Introduction, leading to the discovery of Pluto (though more by chance and random coincidence of the predicted position of Planet X). The study of some peculiarities of  ETNOs, especially the clustering of the arguments of perihelion of Sedna and other detached objects, has, according to Konstantin Batygin and Michael Brown, only a probability of 0.007% to be due to chance, thus requiring a dynamical origin. They postulate a planet (‘Planet Nine’) far out (~400-500 au) with ~5-10 Earth masses [17], [18]. In contrast, Kevin Napier et al. found no evidence for an angular clustering by selecting a similar small sample of 14 ETNOs from three different surveys (DES, OSSOS, and the survey by Sheppard & Trujillo) under consideration of observational biases etc. of each survey [19].

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