Question

Why Pluto is not a planet? What rules does it not follow?

Answer 1

1. American astronomer Clyde Tombaugh discovered Pluto in 1930. From 1930 until 2006, Pluto sat comfortably beyond Neptune as the ninth planet in our solar system.
2. In 2006, however, changes were made. The truth is, nothing about Pluto changed, but the definition of a planet did. Once these changes became official, Pluto no longer fit the definition of a planet.

According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:

1. A planet must be round.
2. A planet must orbit the sun.
3. A planet must have “cleared the neighborhood" of its orbit. This means that as a planet travels, its gravity sweeps and clears the space around it of other objects. Some of the objects may crash into the planet, others may become moons.

Pluto follows the first two rules: It is round, and it orbits the sun. It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this rule, Pluto is no longer considered a planet.