Trans-Neptunian object

A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. The Kuiper belt, Scattered disk, and Oort cloud are names for three divisions of this volume of space.The orbit of each of the planets is affected by the gravitational influences of all the other planets. Discrepancies in the early 1900s between the observed and expected orbits of the known planets suggested that there were one or more additional planets beyond Neptune (see Planet X). The search for these led to the discovery of Pluto. Pluto is too small to explain the discrepancies, however, and revised estimates of Neptune's mass showed that the problem was spurious.
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