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Why is Pluto Not a Planet?

Is Pluto a planet? No, it is no longer a planet of the solar system. It was relegated to a 'dwarf planet' in the year 2006. But why is Pluto not a planet anymore?
Did You Know?

The International Astronomical Union's (IAU) resolution 5A states that: A planet is a celestial body that - (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Discovery of Pluto

Clyde Tombaugh, a junior astronomer at the Lowell Observatory, was allotted the task of photographing a fixed piece of night sky on a daily basis and compare the photographic plates for any unidentified object moving across the sky.
While in the middle of comparing two such photographs, Tombaugh realized that an unidentified object appeared to shift its position, while the stars around it were devoid of any motion. While this object was earlier thought to be an extra-orbital asteroid, the possibility was ruled out since the object moved in an orbit beyond Neptune.
Eventual developments led to this object being given the status of a planet, making it the ninth planet of the solar system. It was named Pluto, after the Roman God of the Underworld.

The First Tremor

The reputation of Pluto as a planet began dwindling after the 1970s. Many objects were discovered around and beyond the orbit of Saturn and Uranus - newer, but relatively smaller than Pluto. In 1978, Pluto's natural satellite Charon was discovered, which was only half the size of Pluto.
So much so, that many experts believed that the two objects should be treated as a binary set instead of being called a planet and its moon.

The Second Shake Up

With the discovery of the Kuiper Belt (where Pluto physically lies) in 1992, Pluto's planetary status took a major dent. Eventually, as the Kuiper Belt became a point of fascination for astronomers and space enthusiasts, many more objects within the belt were discovered.
Many of these objects were only slightly smaller than Pluto, and some of them were even heavier in mass. Many predicted that the day was not far away when an object larger than Pluto could be discovered in the Kuiper Belt. That day arrived in the year 2005.
Astronomer Mike Brown and his team discovered a planet (Eris) outside the orbit of Pluto, which was possibly as large as Pluto. Further research proved that Eris had a mass that was 25% heavier than that of Pluto.

Final Nail in the Coffin

According to resolution 5A of the IAU, Pluto does not qualify to be a planet. While it satisfies the first two conditions of the resolution, it is the third condition where Pluto fails to conciliate. As planets have formed over millions of years, their mass and size have made them the dominant gravitational force in their orbits.
This gravitational force either helps planets consume smaller or equally large bodies around them or push them away with their gravity. Pluto is only 0.07 in mass as compared to most objects around in its orbit.
Since Pluto has a lighter mass, it has managed to leave alone a lot of objects in its orbit which are the same size or possibly larger than itself. This is why Pluto is not considered to be a planet anymore.
To qualify as a planet, Pluto will have to crash into one of these objects around it and gain mass, so that it manages to create a gravitational force within itself, huge enough to take in or push away other objects in its orbit. Until that happens or another object that satisfies all the three conditions mentioned in IAU's resolution 5A is discovered, the solar system's planetary count will stay at eight.