Trans-Neptunian object

A trans-Neptunian object (TNO, also written transneptunian object) is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune, 30 astronomical units (AU). Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs).

The first trans-Neptunian object to be discovered was Pluto in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, (15760) 1992 QB1. As of January 2016^[update] over 1,750 trans-Neptunian objects appear on the Minor Planet Center's List of Transneptunian Objects. Of these TNOs, 1,563 have a perihelion farther out than Neptune (30.1 AU). As of November 2016^[update], 242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation.

The largest known trans-Neptunian object is Pluto, followed by Eris, 2007 OR₁₀, Makemake and Haumea. 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.

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