Objects that fail the criterion – those that have not cleared other similar-sized bodies from their orbit – are considered dwarf planets.
Pluto falls into this category, as do many other objects in the Kuiper Belt, a vast area beyond Neptune.
Pluto is very far away; on average it is about forty times farther away from the Sun than Earth is. The planet is exceptionally cold, and the sun is very dim from that distance. Even our best telescopes cannot see it very clearly.