Pluto is no longer classified as a planet by the International Astronomical Union (IAU), the organization that officially defines what constitutes a planet in our solar system. The decision to redefine the term planet came after extensive debate among astronomers in the late 20th and early 21st centuries, particularly after the discovery of numerous objects of similar size and composition in the Kuiper Belt, a region beyond Neptune that is filled with icy bodies.The primary reasons for Pluto’s reclassification are as follows:

1. Lack of Clear Orbit: Pluto’s orbit is significantly eccentric and inclined relative to the orbits of the other planets. This means that its path around the Sun is not as stable or consistent as the eight “classical” planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune).
2. Small Size and Composition: Pluto is much smaller than the other planets and is composed mostly of rock and frozen volatile substances, similar to many other objects in the Kuiper Belt. Its size is comparable to some of the largest objects in the belt, such as Eris, which is slightly larger than Pluto.
3. Overlap with Kuiper Belt Objects: The discovery of many other objects in the Kuiper Belt that are similar in size and composition to Pluto raised questions about what distinguishes a planet from other celestial bodies. This led to a debate over the definition of a planet and whether Pluto’s unique characteristics were sufficient to maintain its planetary status.

In 2006, the IAU passed a resolution that defined a planet as a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces, and has cleared the neighborhood around its orbit. Since Pluto has not cleared its orbit and there are numerous other objects of similar size in the Kuiper Belt, it was classified as a “dwarf planet.”

The IAU’s decision was not universally accepted by the public or astronomers, and there has been ongoing discussion about the definition of a planet. Some argue that the IAU’s criteria are too strict and that Pluto should be recognized as a planet based on its historical classification and its unique characteristics. However, the official scientific consensus, as defined by the IAU, is that Pluto is a dwarf planet.

Thus, Pluto was demoted from being the ninth planet to the first dwarf planet, reflecting a growing understanding of the diversity of bodies in our solar system.