Mercury Meets the Definition of a Planet
Mercury is classified as a planet because it orbits the Sun, has enough mass for a nearly round shape, and has cleared its orbital neighborhood of most debris. These criteria are set by the International Astronomical Union (IAU).
Pluto Fails the Clearing Requirement
Pluto orbits the Sun and is nearly round, but it has not cleared its orbital zone of other objects. It shares its path with many bodies in the Kuiper Belt, which means it does not meet the third requirement for planet status.
Key Differences in Orbit and Size
Mercury is the closest planet to the Sun, with a stable, dominant orbit and no comparable neighbors. Pluto has a highly elliptical orbit that crosses Neptune's path and exists among many similar-sized icy bodies. This difference in orbital dominance is central to why Mercury is a planet but Pluto is not.
For more details on the IAU definition, see What is a Planet?.
