 Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be reclassified.Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly.They were also concerned about the classification of planets in other planetary systems.