What happened to Pluto?

In 2006, the International Astronomical Union redefined the criteria for what qualifies as a planet, causing Pluto to lose its designation as such. As of 2014, Pluto is considered one of five dwarf planets, with the other original eight planets maintaining their position in the solar system.

The new criteria states that a planet must orbit the sun, have enough mass to maintain a round shape and must be the dominant planet in its own orbit. Pluto is a celestial body that orbits the sun and has enough mass to stay round, but it is not dominant in its orbit, as there are four other dwarf planets that share an orbit with it.