Trans-Neptunian Objects

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Trans-Neptunian Objects
NASA diagram shows the presumed distance of the Oort Cloud compared to the Solar System planets, the Kuiper Belt, and the orbit of Sedna.
Trans-Neptunian Objects are objects having semi-major axes larger than that of the planet Neptune.[1]

Naming conventions

A significant number of astronomers today insist that all trans-Neptunian objects, regardless of whether Neptune has any gravitational influence upon them, should be called Kuiper belt objects.[2][3]But today, an increasing number of astronomers distinguish the Kuiper belt from other trans-Neptunian regions, primarily using orbital characteristics as the criterion for this distinction.[4]

Primary theoretical considerations

As sources of comets

Since 1951, when Gerard P. Kuiper published his original hypothesis concerning non-accreted icy remnants of the solar nebula, trans-Neptunian objects, by whatever name, have been held to be the source of comets, of periods long or short.


All these objects (except for the hypothetical Nemesis) are supposed to be remnants of the formation of the solar system that could not accrete into planets. Most of these objects, except for the most distant objects in the scattered disk, are supposed to have moved into their present orbits under the gravitational influence of Neptune.

Classes of trans-Neptunian objects

Trans-Neptunian objects today belong to four major classes (not counting the Centaurs, an entirely different class of object):
  • The Kuiper belt, containing those objects that remain between 30 and 50 AU distant from the Sun[4]
  • The scattered disk, containing objects having perihelia of 30 AU or greater and semi-major axes of 50 AU or greater.[4]
  • The Oort Cloud, the hypothetical and probably factitious sphere of icy objects held to be the source of long-period comets[4]
  • Nemesis, the hypothetical and probably factitious red or brown dwarf star that is supposed to enter the Oort cloud and perturb it once every 26 million years.

Discovery and observation

The first-ever trans-Neptunian object to be discovered was the dwarf planet Pluto in 1930. The next was Pluto's largest satellite Charon in 1978.[5] After the discovery of object 1992 QB1, astronomers have discovered more than 800 of these objects, but not nearly as many as some astronomers have predicted. Other notable TNO's that have been discovered include:

  • (15874) 1996 TL66, the first scattered disk object to be recognized
  • (48639) 1995 TL8, the earliest discovered scattered disc object, and a binary
  • 1993 RO, the next plutino discovered after Pluto[6]

The New Horizons rocket probe, now on its way to Pluto, will be the first rocket probe to study a Trans-Neptunian object. Mission planners hope to steer the probe to study at least two other Kuiper belt objects in addition to Pluto.


  1. "Space Topics: Trans-Neptunian Objects." The Planetary Society, n.d. Accessed June 24, 2008.
  2. Jewitt, David. "Kuiper Belt." University of Hawaii, n.d. Accessed June 20, 2008.
  3. Johnston, William Robert. "Trans-Neptunian Objects." October 1, 2007. Accessed June 24, 2008.
  4. 4.0 4.1 4.2 4.3 "Types of Trans-Neptunian Objects." The Planetary Society, n.d. Accessed June 24, 2008.
  5. Whitman, Justine. "Pluto and the Kuiper Belt." <>, Apriil 16, 2006. Accessed June 24, 2008.

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