Why don’t we count Pluto as the ninth planet anymore?

Pluto has never changed, but our definition of “planet” has changed several times in history. The most recent change happened in 2006, when scientists decided to call Pluto a “dwarf planet.” To understand why, you have to go back to Pluto’s discovery in 1930. At the time, scientists assumed that Pluto was quite large, perhaps like Uranus or Neptune, so it seemed natural to consider Pluto as the “ninth planet.” However, we later learned that Pluto is far smaller than any of the other eight planets, and that there are many other objects in the outer solar system (including Eris) that are similar in size to Pluto. Although the decision was controversial, in 2006 astronomers decided that Pluto, Eris, and other similar-size objects were too small to count as “official” planets and instead decided to call them “dwarf planets.”