On August 24, 2006, the International Astronomical Union decided that Pluto was no longer defined as a big planet by the empire, but was classified as a dwarf planet. Do you know why?

1. Pluto is too small. It is not a big planet.

2. The orbit of Pluto is too strange. Unlike other big planets, it is basically an ellipse close to a circle. Its orbit is like a flat ellipse.

I think another reason may be that it is too similar to many discovered Kuiper Belt objects.