A relatively sparse population of asteroids, known as Centaurs,exists between Jupiter and Neptune. Some are dark and red,similar to the tarry (tholin-covered) D-type asteroids, but others are bluer, suggesting that much of their surfaces may be freshly exposed ice. Because their orbits cross or come close to the giant planets, they are not stable, persisting for no longer than about ten million years. Probably, Centaurs are TNOs that have been scattered inwards, perhaps by a close encounter with Neptune.Further interactions with giant planets probably nudge them inwards until they become short-period comets, spending perihelion in the inner Solar System, where they are heated by the Sun and lose their volatiles in sometimes spectacular tails.
Six trojan objects have been discovered close to Neptune’s leading Lagrangian point. Dynamical arguments suggest that vast numbers await discovery (in both Lagrangian points) and that Neptune trojans may be ten times more numerous than Jupiter’s.
Beyond Neptune, we reach the Kuiper belt and all the other TNOs. One family of Kuiper belt objects travel in 3:2 orbital resonance with Neptune. Members of the class, which includes Pluto, are known informally as ‘plutinos’, not to be confused with Plutoid, which is the offi cial IAU term for any TNO large enough to be ranked as a dwarf planet. Plutoids can be plutinos, classical Kuiper belt objects (lacking orbital resonance with Neptune), or Scattered Disk objects beyond the main belt. Classical Kuiper belt objects are known alternatively as ‘cubewanos’ (‘QB 1 -os’) because the first Kuiper belt object to be discovered after Pluto bore the provisional designation 1992 QB 1.
Pluto and Charon
The properties of most TNOs are poorly known. However, Pluto and its satellite Charon are suffi ciently large and nearby to have been studied telescopically for several decades. Frozen nitrogen,methane, and carbon dioxide have been detected spectroscopically on Pluto, and the sharpest telescope images reveal dark patches that are probably tholin-rich residues. Pluto’s density suggests that rock must be about 70% of its total mass, and most likely it is internally differentiated with a rocky core (and feasibly an iron-rich inner core) overlain by a mantle made mostly of water-ice topped by a more volatile-rich crust.