Eris

Astronomers at Palomar Observatory discover a body beyond Neptune’s orbit that initial observations show is larger than Pluto. Eris is quickly dubbed the tenth planet by the media and the scientific community, and it is later found to have a small moon of its own in a close orbit, which is later named Dysnomia. But events overtake Eris and Dysnomia before the science textbooks have a chance to be rewritten to include a tenth planet: Eris becomes a case study in an ongoing debate within the International Astronomical Union about the definition of a planet. In 2006, the IAU will establish a set of parameters which determine that Eris isn’t a planet – and then rewrites the history books by deciding that Pluto isn’t either.