Pluto is not a planet because it did not full filled 3 needs IAU uses to define a full sized planet. It was eliminated from solar sysytem in August 24 2006

Why Pluto is not a Planet?

Pluto, once considered the ninth planet of our solar system, was reclassified as a "dwarf planet" in 2006. This decision was made by the International Astronomical Union (IAU), a global organization responsible for classifying celestial bodies. The reclassification was based on several factors:

1. Size and Shape: Pluto is smaller than any other planet in our solar system. In fact, it is even smaller than seven moons in our solar system. Its diameter is about 2,370 kilometers, which is less than one-fifth the diameter of Earth.

2. Orbit: Pluto has an eccentric and inclined orbit that takes it closer to the Sun than Neptune for about 20 years of its 248-year orbit. This unusual orbit is unlike the other planets which have more circular and aligned orbits.

3. Composition: Pluto's composition is different from that of the eight classical planets. It is predominantly composed of rock and ice, whereas planets like Earth, Mars, and Venus are mainly made up of rock and metal. This difference in composition further supports its classification as a dwarf planet.

4. Clearing the Orbit: According to the IAU's definition, a planet must "clear its orbit" by being the largest gravitational force in its orbit. However, Pluto shares its orbit with other objects in the Kuiper Belt, a region beyond Neptune containing numerous icy bodies. This means that Pluto has not cleared its orbit, leading to its reclassification.

When was Pluto eliminated from the Solar System?

The decision to reclassify Pluto as a dwarf planet was made on August 24, 2006, during the 26th General Assembly of the IAU held in Prague, Czech Republic. The IAU introduced a new definition of a planet, which led to the reclassification. According to this definition, a planet must meet three criteria:

1. It must orbit the Sun.
2. It must be spherical in shape due to its own gravity.
3. It must have cleared its orbit of other debris.

Since Pluto did not fulfill the third criterion, it was reclassified as a dwarf planet. This decision was met with mixed reactions from the scientific community and the public, as many people had grown up learning that Pluto was the ninth planet. However, the reclassification was done to establish a clear definition of a planet and to differentiate between different types of celestial bodies in our solar system.

Overall, the reclassification of Pluto was based on scientific evidence and a desire to have a consistent classification system for celestial bodies. While Pluto may no longer be considered a planet, it remains a fascinating object of study and exploration in the Kuiper Belt.