Eris, Sedna, Quaoar, and Orcus are KBOs and therefore don't meet the definition of a planet.

Question

Quizplus · Accepted Answer

KBOs (Kuiper Belt Objects) are bodies in our solar system that orbit the sun beyond Neptune's orbit. They are mostly composed of ice and rock and are smaller than planets. While Eris, Sedna, Quaoar, and Orcus are all interesting bodies in the outer solar system, they do not meet the criteria to be classified as planets.

Eris, Sedna, Quaoar, and Orcus Are KBOs and Therefore Don't