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
IAU definition of planet the 26th General Assembly, is: The IAU further resolves: The IAU also resolved that ""planets" and "dwarf planets" are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets. Alan Stern, the lead scientist on NASA's robotic mission to Pluto, contended that Earth, Mars, Jupiter, and Neptune have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. Stern has asserted: "If Neptune had cleared its zone, Pluto wouldn't be there." Some astronomers counter this opinion