In 2006, Pluto was officially declared not to be a planet anymore. It was, instead, announced a dwarf planet. What caused this decision?

Pluto is tiny - scientists think it's about 2,300 kilometers in diameter. There are several moons in our solar system larger than the planet. Another feature that caused it to be demoted is its orbit. The planet's orbit cuts through that of Neptune's.

Scientists began to find objects beyond Pluto. They were mostly smaller than Pluto, and have been named Kuiper Belt Objects (KBOS). There is estimated to be at least 70,000 that are over 100 kilometers across. In 2000, astronomers started to find objects closer to Pluto's size. Then they found Eris. Eris was larger than Pluto. It cuts inside Pluto's orbit when closest to the sun and, when furthest, is twice as far as Pluto's orbit. It takes more than 550 years to circle the sun.

All these discoveries meant that, if Pluto was to be called a planet, then Eris should be called one too, as well as many KBOS. Scientists, in 2006, voted on the definition of a planet. A planet had to be a sphere. A planet had to be in orbit around a star. And a planet couldn't be with a group of other objects, orbiting peacefully. That was the final blow for Pluto. It is now classified as a dwarf planet.