Pluto was reclassified from a planet to a dwarf planet by the International Astronomical Union (IAU) in 2006. The reclassification was driven by advancements in our understanding of the solar system and the need for a clear and consistent definition of what constitutes a planet. The IAU established three criteria for an object to be considered a planet:
It must orbit the Sun: Pluto meets this criterion as it orbits the Sun.
It must be spherical in shape: Pluto is approximately spherical in shape, so it fulfills this requirement.
It must have cleared its orbit of other debris: This is the criterion where Pluto fell short. Its orbit crosses the path of Neptune, and it shares its orbital region, known as the Kuiper Belt, with numerous other objects. As a result, Pluto has not "cleared its orbit" of other debris, leading to its reclassification as a dwarf planet.
The main difference between Pluto and other objects in our solar system, aside from size, is its location and composition. Pluto is part of the Kuiper Belt, a region beyond the orbit of Neptune that is populated by icy bodies, asteroids, and dwarf planets. It is primarily composed of rock and ice, similar to other objects in the outer regions of the solar system.
Additionally, Pluto has a highly eccentric and inclined orbit compared to the relatively circular orbits of the eight planets. It takes Pluto about 248 Earth years to complete one orbit around the Sun, and its orbit is tilted at an angle relative to the plane of the other planets.
These factors, combined with the IAU's definition of a planet, led to the reclassification of Pluto as a dwarf planet. However, it is worth noting that the reclassification remains a topic of ongoing scientific discussion and debate.