A trans-Neptunian object (TNO) is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune.
The first trans-Neptunian object to be discovered was Pluto in 1930, and in 1978 Pluto's moon Charon. Due to the vast distance, it took more than 60 years to discover the second trans-Neptunian object, (15760) 1992 QB1, in 1992.
Now over 1200 trans-Neptunian objects appear on the Minor Planet Center's List Of Transneptunian Objects. As of November 2009, two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.
Distribution and classification
The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division.
According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:
1. 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:
a. Resonant objects are locked in an orbital resonance with Neptune
Objects with a 1:2 resonance are also called twotinos, and objects with a 2:3 resonance are called plutinos, after their most prominent member, Pluto.
b. Classical Kuiper belt objects (also called cubewanos)
These have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1, 50000 Quaoar and Makemake.
2. The scattered disk -
Contains objects further 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.
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