Hi There, I'll take a shot at this quetion! Pluto was taken off the official list in terms of being a planet on account of its size and orbit, in relation to other solar system bodies, for instance asteroids, or outer solar system bodies beyond the orbit of Pluto recently discovered! Though, I must admit, I feel, personally, it should have been left with the status as a "Planet"! You can see, and learn more about this debate here; http://spaceweather.com On the homepage (right hand) side, you'll see a section called the Pluto Petition! It should answer all your questions, perhaps leave you with more! Thanks for visiting Solar Week!