Trans-Neptunian Object - Distribution and Classification

Distribution and Classification

According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:

• The (classical) Kuiper belt contains objects with an average distance to the Sun of 30 to about 55 AU, usually having close-to-circular orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:
  • Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1:2 resonance are called twotinos, and objects with a 2:3 resonance are called plutinos, after their most prominent member, Pluto.
  • Classical Kuiper belt objects (also called cubewanos) have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB₁, 50000 Quaoar and Makemake.

• The scattered disk contains objects farther from the Sun, usually with very irregular orbits (i.e. very elliptical and having a strong inclination from the ecliptic). A typical example is the most massive known TNO, Eris.

The diagram to the right illustrates the distribution of known trans-Neptunian objects (up to 70 AU) in relation to the orbits of the planets and the centaurs for reference. Different classes are represented in different colours. Resonant objects (including Neptune trojans) are plotted in red, cubewanos in blue. The scattered disk extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU (Sedna) and aphelia beyond 1000 AU ((87269) 2000 OO₆₇).