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 storms [Titan] shouldn't be put into the same category, whatever you call it. However, he goes on to say that, "For most people, considering round satellites (including our Moon) "planets" violates the idea of what a planet is." Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term "satellite planet" for a planet-sized satellite. In 2017, Stern, K. D. Runyon et al. proposed a new geophysical definition of planet that defined a planet simply as an object large enough to be