Is "Kuiper Belt Object" the term in use rather than "Trans-Neptunian object"? See:
http://europe.cnn.com/2001/TECH/space/08/24/minor.planet/index.html
according to http://web.mit.edu/newsoffice/tt/2001/jul18/kuiper.html the newest object is in the Kuiper belt,(between 30 AU and 50 AU form our sun). The long peroid Comets are thought to originate further out (between 50 and 100 AU) in what has been called the Oort Cloud.
Current speculation in the astronomical community due to the gravitational disturbances of the planets suggest that there may be a larger planet...
How mainstream are these speculations? Last I heard, the perturbations of the outer planets were essentially accounted for, and the only speculated extra body was Nemesis, taken more seriously by biologists than by astronomers.
Main text has had the offending verbage removed See Planet X information. -- Mike Dill
Added "most" to description of orbit beyond Neptune's, since plutinos like Pluto's orbits can be inside Neptune's.