Trans-Neptunian object

Euler diagram showing the types of bodies in the Solar System.

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[nb 1] 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).[1]

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 Albion. As of February 2017 over 2,300 trans-Neptunian objects appear on the Minor Planet Center's List of Transneptunian Objects.[2][3] Of these TNOs, 2,000 have a perihelion farther out than Neptune (30.1 AU).[4] As of November 2016, 242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation.[2][5]

The most massive known trans-Neptunian object is Eris, followed by Pluto, 2007 OR10, Makemake and Haumea. The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space,[6] though treatments vary and a few objects such as Sedna do not fit easily into any division.[nb 2]


Discovery of Pluto

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 1930, which was too small to explain the discrepancies. Revised estimates of Neptune's mass from the Voyager 2 flyby in 1989 showed that the problem was spurious.[8]

Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has a lower inclination to the ecliptic than most other large TNOs.

Discovery of other trans-Neptunian objects

After Pluto's discovery, American astronomer 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 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, 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. Pluto and Eris were eventually classified as dwarf planets by the International Astronomical Union.

Other Languages
беларуская (тарашкевіца)‎: Транснэптунавы аб’ект
Bahasa Indonesia: Objek trans-Neptunus
Lëtzebuergesch: Transneptuneschen Objet
македонски: Заднептунец
Bahasa Melayu: Objek trans-Neptun
norsk nynorsk: Transneptunsk objekt
Simple English: Trans-Neptunian object
slovenščina: Čezneptunsko telo
srpskohrvatski / српскохрватски: Trans-neptunski objekt