A trans-Neptunian object (TNO; also written transneptunian object) is any object 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. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, (15760) 1992 QB1, with only the discovery of Pluto's moon Charon in 1978 before that. 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.
The largest known trans-Neptunian objects are Eris and Pluto, followed by 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.
Read more about Trans-Neptunian Object: Distribution and Classification, Notable Trans-Neptunian Objects, Physical Characteristics
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