Sites discussing whether Pluto should be properly considered a planet, a Kuiper Belt object, or some other form of Trans-Neptunian Object (TNO).

In 1999 the International Astronomical Union announced that it was premature to consider removing the "planet" label from Pluto, but as more TNOs are discovered, the debate may be re-opened. For example, a TNO larger than Pluto, 2003 UB313, was announced in 2005.

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Gravity Rules: The Nature of Planethood
Article discussing possible definitions that qualify objects, such as Pluto and Sedna in the Kuiper Belt, as being planets or not.
Is Pluto a Giant Comet?
Discusses related discoveries and excerpts.
Much ado about Pluto
News release from NASA affirming Pluto's planetary status.
Argues that Pluto is the most important Kuiper Belt object, not the least important planet, based on its orbit and other properties. From astronomer Dave Jewitt, who studies comets and other primitive solar system bodies.
What is a Planet
Reviews newly-discovered objects that stretch the definition of the term, including massive superjupiters, free-floating planets, and other exoplanets. Suggests a definition based on the way an object is formed. From astronomer Gibor Basri.
When Is a Planet Not a Planet?
Articles discusses the debate whether Pluto should be included among the planets. [Atlantic Monthly]
IAU Resolutions 5&6
The International Astronomical Union's definition of a planet in the Solar System and ruling on the classification of Pluto. [PDF] (August 24, 2006)
Astronomer Upset at New Planet Proposal
Washington Post article describes Michael Brown's disagreement with a proposal to expand the definition of a planet to include, among others, his own discovery. (August 19, 2006)
CBS News: Pluto: Planet Or Junk Snowball?
Outlines the history of the debate. (February 14, 2005)
National Geographic News: Pluto: Planet or Comet?
Discusses Pluto's similarity to other Kuiper Belt objects, and introduces a 5-way classification scheme for solar system objects to break down the lumping together of dissimilar planets. (February 23, 2001)
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