Discovered 75 years ago by Clyde Tombaugh - but still a mystery. That's the story of Pluto, the 9th planet in our solar system. After the death of its founder scientists began to argue whether Pluto should really be considered as a planet or not.
Pluto, with its elongated orbit and odd orbital plane, seems to behave more like other Kuiper Belt objects than other planets, some astronomers say. They also point out Pluto is very small, smaller than Earth's moon.