I have been wondering this and the teachers at my high school never really gave me an answer. Please help, why was Pluto kicked out of the solar system?
Clyde William Tombaugh discovered Pluto in 1930. He saw that it is pretty big, so he gave him the classification of planet. But then the scientists discovered more objects, sometimes even bigger than Pluto. If they would classify all of them for the planets, then we would have around 20 or 30 of them. But that wouldn't be so great. So they just decided, that Pluto is too small. In fact, Ganymede, Titan, Callisto, Io, Moon, Europa, Triton are all larger than Pluto (they are moons). Eris is just some kilometers smaller than Pluto.
But the main thing here regards the IAU definition of the planet. Planet must clear its neighbourhood, but Pluto clearly hasn't done it.