Pluto

The outermost known planet in our solar system, Pluto is a small, frozen world circling the sun every 248 Earth years. Its orbit is more elliptical than that of any other planet in the solar system, sometimes taking it closer to the sun than Neptune.
In 2006, after a review of bodies in the Kuiper Belt — a disk-shaped region of icy bodies beyond Neptune –the International Astronomical Union (IAU) stripped Pluto of its planetary status and reclassified it as a dwarf planet. The IAU’s decision met with initial resistance from some astronomers and many in the public who felt that Pluto deserved better.
The debate was similar to one that took place in 1802 when Dr. William Herschel suggested that “a Comet being a heated Body must be formed … of materials much less dense than those which compose Saturn or Jupiter.” His idea was opposed by other astronomers such as Johann Bode and Heinrich Olbers, who maintained that every object in the Solar System had been created at the same time and out of the same materials. They believed that there were no planets beyond Saturn and that comets were meteors produced when passing stars disturbed the primordial material within the Solar System.

For most of the twentieth century Pluto was the ninth planet. In 1999, however, astronomers discovered another object in the far outer solar system that was larger than Pluto. At first they called it a planet, but then they realized their mistake: Pluto was smaller than this new object, and so by definition could not be a planet.