Why Pluto is not a Planet?
Pluto was considered the ninth planet in our solar system until 2006 when the International Astronomical Union redefined the criteria for what constitutes a planet. Here are the reasons why Pluto is no longer classified as a planet:

Size and Mass
- Pluto is much smaller and less massive than the other planets in our solar system. It is only about two-thirds the size of Earth's moon.
- Its small size and low mass mean that Pluto does not have enough gravitational force to clear its orbit of other debris, which is one of the criteria for being classified as a planet.

Orbit
- Pluto's orbit is more elliptical and tilted compared to the eight planets in our solar system. It crosses the orbit of Neptune at times, which is another reason why it does not meet the criteria for a planet.

Composition
- Pluto is composed primarily of ice and rock, unlike the gas giants and terrestrial planets that make up the rest of our solar system. Its composition is more similar to that of objects in the Kuiper Belt, a region beyond Neptune where many icy bodies are found.

Conclusion
- Due to its small size, unusual orbit, and composition, Pluto was reclassified as a "dwarf planet" in 2006. This new classification reflects its unique characteristics that set it apart from the other planets in our solar system.