Why did Pluto lose its status as a planet in 2006? Its old news, but I hope it helps!
There are 3 criteria that determines a planet.
1: The object must have sufficient mass for its self-gravity to overcome its 'rigid body forces' and assume a hydrostatic equilibrium shape. Hence becoming round.
2: It must orbit around a star, hence our moon cannot be considered as one.
3: It must have 'cleared its neighbourhood around its orbit' of everything except much smaller objects. This rules out Pluto as a planet as its orbit which has Neptune, which is much larger, being in close proximity due to their unique orbits that seem to overlap each other on a 2 dimensional plane.
Source: David A Rothery
