Planets Beyond Neptune - Discovery of Further Trans-Neptunian Objects

Discovery of Further Trans-Neptunian Objects

See also: History of the Kuiper belt

After the discovery of Pluto and Charon, no more trans-Neptunian objects (TNOs) were found until (15760) 1992 QB₁ in 1992. Since then, hundreds of such objects have been observed. Most are now recognised as part of the Kuiper belt, a swarm of icy bodies left over from the Solar System's formation that orbit near the ecliptic plane beyond Neptune. Though none were as large as Pluto, some of these distant trans-Neptunian objects, such as Sedna, were initially described in the media as "new planets".

In 2005, astronomer Mike Brown and his team announced the discovery of 2003 UB₃₁₃ (later named Eris after the Greek goddess of discord and strife), a trans-Neptunian object just barely larger than Pluto. Soon afterwards, a NASA Jet Propulsion Laboratory press release described the object as the "tenth planet".

Eris was never officially classified as a planet, and the 2006 definition of planet defined both Eris and Pluto not as planets but as dwarf planets because they have not cleared their neighbourhoods. They do not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the second largest dwarf planet after Eris.

A number of astronomers, most notably Alan Stern, the head of NASA's New Horizons mission to Pluto, contend that the IAU's definition is flawed, and that Pluto and Eris, and all large trans-Neptunian objects, such as Makemake, Sedna, Quaoar and Varuna, should be considered planets in their own right. However the discovery of Eris did not rehabilitate the Planet X theory as it is far too small to have significant effects on the outer planets' orbits.