Editing Trans-Neptunian object (section)

== History ==

=== Discovery of Pluto ===
[[File:Pluto in True Color - High-Res.jpg|thumb|[[Pluto]], the first known TNO, imaged by ''[[New Horizons]]'' in 2015.]]

The orbit of each of the planets is slightly affected by the [[gravitational]] influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional [[planets beyond Neptune]]. The search for these led to the [[discovery of Pluto]] in February&nbsp;1930, which was progressively determined to be too small to explain the discrepancies. Revised estimates of Neptune's mass from the ''[[Voyager 2]]'' flyby in 1989 showed that there is no real discrepancy: The problem was an error in the expectations for the orbits.<ref>{{cite press release |first1=Chris |last1=Gebhardt |first2=Jeff |last2=Goldader |date=20 August 2011 |title=Thirty-four years after launch, Voyager&nbsp;2 continues to explore |website=NASA Spaceflight (nasaspaceflight.com) |url=http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/ }}</ref> Pluto was easiest to find because it is the [[apparent magnitude|brightest]] of all known trans-Neptunian objects. It also has a lower inclination to the [[ecliptic]] than most other large TNOs, so its position in the sky is typically closer to the search zone in the disc of the Solar System.

=== Subsequent discoveries ===
After Pluto's discovery, American astronomer [[Tombaugh, C. W.|Clyde Tombaugh]] continued searching for some years for similar objects but found none. For a long time, no one searched for other TNOs as it was generally believed that Pluto, which up to August 2006 was classified as a planet, was the only major object beyond Neptune. Only after the 1992 discovery of a second TNO, [[15760 Albion]], did systematic searches for further such objects begin. A broad strip of the sky around the [[ecliptic]] was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers. [[Eris (dwarf planet)|Eris]], the most massive TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, and whether objects like Pluto can be considered planets. In 2006, Pluto and Eris were classified as [[dwarf planet]]s by the [[International Astronomical Union]].