Why is Pluto no longer a planet?
Since Pluto was first discovered in 1930, there has always been some uncertainty about its properties and how they compare to those of the other celestial bodies defined as planets in our solar system. In large part, it is Pluto’s small size that led to it being removed from the list of bodies classified as planets.
According to the International Astronomical Union (IAU), a planet is defined in the following way:
Pluto meets criteria (a) and (b) mentioned above, but it regularly encounters the orbit of the larger planet, Neptune, which is the technical reason used to remove Pluto’s status as one of the planets of our solar system. It is probably worth noting that this resolution was met with some criticism, as even our own planet Earth encounters asteroids in its own orbit on a fairly regular basis.