Question

pluto is a planet but a drawf planet.

Answer 1

it's right it is dark planet the planets which is similarly like Pluto in our galaxy some many there so it removed from 9th position

Answer 2

Pluto become a dwarf planet because its size is comparatively too small and sometimes it violates the rule by getting into the orbit of neptune and its distance from the sun is more and pluto 

and there were many bodies similar to the size of pluto and some of them were bigger than pluto and they had certain different characteristics therefore the scientists decided to consider pluto as dwarf planet 

there would be chances of collision of pluto and neptune but  stable orbital resonance with Neptune prevents them from colliding