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In 1950, Dutch astronomer Jan Oort proposed that certain comets came from a vast spherical shell of icy bodies near the edge of the Solar System. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.) 

The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.

The Kuiper Belt, extending out to about 50 AU around the Sun, is populated with thousands of small icy bodies.  
  
]In 1992, astronomers detected a reddish speck about 42 AU from the Sun-- the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,000 KBOs have been identified since 1992. (They are sometimes called Edgeworth Kuiper Belt objects, acknowledging another astronomer who also is credited with the idea, or they are simply called Trans-Neptunian Objects (TNOs.) 

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.  
  
The boundary between (major) planet and minor planet has never been defined and the recent discovery of other "Trans-Neptunian Objects" (TNOs), including some larger than Pluto, triggered the IAU to form a Working Group on "Definition of a Planet" from its Division III members.