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 list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets."" As of Feb 2018, Brown's list includes 952 objects, identifying ten known trans-Neptunian objects—the four accepted by the IAU plus the six bodies: , , , , and —as "near certain", with another 26 "highly likely". Notably, (1535 km) has a larger diameter than the dwarf planet Makemake (1430 km). Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The