Why was Pluto downgraded from being a planet?
Pluto was discovered in 1930 by Clyde Tombaugh, and was considered as the ninth and the farthest planet of our Solar System. For decades it enjoyed the limelight of the elite class until during the 90s when its status of being a Planet fell into question following the discoveries of many other similar sized objects in the Kuiper Belt region orbiting around the sun behind Neptune.
Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit led the International Astronomical Union (IAU) to define the term "planet" formally for the first time in the following year to describe Trans Neptune bodies also known as Kuiper Belt Objects (KBOs).
The definition of planet set in Prague in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, 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 neighbourhood" around its orbit.
Any non-satellite body fulfilling the first two criteria is a Dwarf Planet and any celestial body fulfilling only the first criteria is known as a Smaller Solar System Body (SSSB).
Pluto goes around the Sun, so it fulfils the first criteria. It also has sufficient mass to assume a round shape hence the second criteria is also met. But it's the third criteria that led to the downfall of Pluto as it has been unable to clear all the dust floating around it due to its petite gravitational pull, one fifteenth of that of Earth.
Pluto has a staggering number of 5 moons revolving around it.
Pluto and Charon are supposed to be revolving around a common point which doesn't lie in any of the bodies but somewhere in between them.
With time, scientists are finding more and more Pluto like objects revolving around the Sun at the same distance, in the Kuiper Belt.
This was what led to the downgrading of Pluto from being a Planet but the debate continues and many still believe Pluto is still a Planet but technically it's not.
