Pluto in size of Charon and Pluto has prompted some astronomers to call it a double dwarf planet. The system is also unusual among planetary systems in that each is tidally locked to the other, which means that Pluto and Charon always have the same hemisphere facing each other. From any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter. In 2007, observations by the Gemini Observatory of
Definition of planet considered planets. On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their Minor Planet Catalogue, giving them the official minor planet designations (134340) Pluto, (136199) Eris, and (136199) Eris I Dysnomia. Other possible dwarf planets, such as 2003 EL, 2005 FY, Sedna and Quaoar, were left in temporary limbo until a formal decision could be reached regarding their status. On June 11, 2008, the IAU executive committee announced the establishment of a subclass of dwarf planets comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new class of objects,