Why was Pluto discredited as a planet?
After many dwarf planets like Sedna were found in Kuiper belt,number of planet like objects were going up.
Ancient people called wandering objects among stars as planets.
So in 2006, International astronomical union made a definition for a planet.
1 It should be going round s star.
2 It should have enough mass to form a sphere by gravity.
3 It must clear objects in neighborhood
The third condition pluto was not obeying as it crosses Neptune orbit.So,it was graded as dwarf planet...