Why is Pluto no longer a planet?

Pluto has never really fitted quite right in the series of planets in our solar system. Pluto is too small, Pluto has an extremely elliptical orbit running around the sun and that even at a too unconventional angle. That is why, the experts have never agree, whether Pluto is a planet or not.

Just as often, the definition of a planet was discussed or the question of "When is a planet a planet". One could not agree in this point, until, some day, one decided to open a new category of dwarf planets.

On the 24th August of 2006, the time had come. By the International Astronomical Union (IAU), it was decided with a vote among 2500 scientists from 75 countries, that Pluto belongs to the category of dwarf planets, so Pluto officially became the planet status removed. This was achieved through the adoption of a specific definition for a planet, by which a planet, among other things, has to have a certain mass and has to dominate its surrounding by gravity and magnetism, which is not applying to Pluto.

Of course, even today, many people (also in the scientific world) are grieving for their planet Pluto and often, Pluto is also erroneously still called a Planet - knowingly or unknowingly.
2011-12-26 at 18:13

Incidentally, there are many more "planets" like Pluto in our solar system, that we only have not discovered at the time, we called Pluto a planet, because of their black color due to the not reflecting sun.

Since Pluto has already been discovered so long ago, it has been called a planet and with this, he has gotten a sort of special status. 

Theoretically, if we keep Pluto as a planet, we would now have to call all the other Pluto-like planets also a planet. So, it is all quite rightly so, to deny Pluto the planet status.
2011-12-27 at 21:18