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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〔 is excluded from the count because it has an observation arc of only (76 days and hence its semi-major axis is not securely known. )〕 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 in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, . over 1,650 trans-Neptunian objects appear on the Minor Planet Center's ''List Of Transneptunian Objects''.〔(IAU Minor Planet Center ''List Of Transneptunian Objects'' )〕〔 Of these TNOs, 1471 have a perihelion further out than Neptune (30.1 AU).〔 As of November 2009, two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.〔〔(List of Centaurs and Scattered Disc objects )〕
The largest known trans-Neptunian object is , followed by , , and . 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 do not fit easily into any division.
== History ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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