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
Dwarf planet 6a of 2006 recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as "plutons" and "plutonian objects" but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their "pluton". On June 11, 2008, the IAU Executive Committee announced a name, "plutoid", and a definition: all trans-Neptunian dwarf planets are plutoids.