The universe consists of different solar systems and our planet Earth belongs to the solar system situated in the Milky Way Galaxy. A solar system consists of different types of objects called as celestial bodies. These can be classified as planets, stars, asteroids, meteors, dwarf planets, satellites etc. on the basis of their shape, size, light, type, axis of rotation etc.

But who manages the names of these celestial bodies?

Everything related to the universe is managed by the International Astronomical Union (IAU). It consists of astronomers from different parts of the world. These prescribe definitions and thus different celestial bodies can be segregated into different types in accordance with these definitions.

So what’s the definition of the word planet?

As prescribed by the IAU, “a planet is a celestial body which: is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and has cleared the neighborhood around its orbit.”

With this definition, Pluto was removed from the family of planets of our Solar System. Pluto did not fulfill all the three points. It orbits around the sun and has sufficient mass to assume a prescribed shape but it do not have a cleared neighborhood.

A cleared neighborhood, now what does that mean?

Well, this means that the celestial body should be gravitationally dominant in its region. No other celestial body should be more dominant than it. It should be the most powerful in and around its vicinity but Pluto shares its orbital belt with its neighborhood particles or celestial bodies. Thus it does not have a clear neighborhood.

Since Pluto failed to fulfill the three major requirements for a celestial body to be known as a planet as prescribed by the IAU, it is no longer considered as the part of the Planet family of our solar system. However, it is now known as a dwarf planet.