In order to be defined as a planet, an object needs to meet 3 criteria: - Orbits a star (Pluto does and I assume that's what you were asking about) - The obje

In order to be defined as a planet, an object needs to meet 3 criteria: - Orbits a star (Pluto does and I assume that's what you were asking about) - The object has enough mass to pull itself into a sphere (meets this criteria) - The object has enough gravity to have cleared its nearby orbit of debris and other objects Neither Pluto nor Eris qualify for a planet because of that last one and, like I mentioned, the Moon has more mass than Pluto and Eris. The Kuiper belt objects all have this issue, but my point was that dwarf planets are pretty small - especially compared to other objects around Sol. If you walked non-stop around the Moon it would take 91 days compared to Pluto's 62 days and Eris's 61 days. The designation of dwarf planet is not that big of a deal imo.