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 recognized five bodies as dwarf planets since 2008: Ceres, Pluto, Eris, Haumea, and Makemake. Ceres and Pluto are known to be dwarf planets through direct observation. Eris is recognized as a dwarf planet because it is more massive than Pluto (measurements by "New Horizons" indicate that Pluto's diameter is larger than that of Eris), whereas Haumea and Makemake qualify based on their absolute magnitudes. In relative distance from the Sun, the five are: Mike Brown considers an additional six trans-Neptunian objects to be "nearly certainly" dwarf planets with diameters at or above 900 kilometers. These objects are: Tancredi et al.