Just as Ceres began to make sense only when it was recognized as one of a vast population of asteroids, Pluto fell into place only when astronomers found it was one of a vast population of Kuiper belt objects (KBOs) [see "The Kuiper Belt," by Jane X. Luu and David C. Jewitt; Scientific American, May 1996, and "Migrating Planets," by Renu Malhotra; Scientific American, September 1999]. Astronomers began to reconsider whether it should still be called a planet. Historically, revoking the planetary status of Pluto would not be unprecedented; the ranks of ex-planets include the sun, moon and asteroids. Nevertheless, many people have argued for continuing to call Pluto a planet, because almost everyone has grown quite accustomed to thinking of it as one.
The discovery in 2005 of Eris (formerly known as 2003 UB313 or Xena), a KBO even larger than Pluto, brought the issue to a head. If Pluto is a planet, then Eris must also be one, together with scores of other large KBOs; conversely, if Pluto is not a planet, neither are the other KBOs. On what objective grounds could astronomers decide?