A89: Pluto is not a planet because it is way too small, and it doesn't meet the necessary requirement needed to be a planet.
The requirements are:
It needs to be in orbit around the sun--Yes, Pluto does orbit the sun.
It needs to have enough gravity to pull itself into a spherical shape -- Pluto has sufficient gravity to have become spherical. (This is called hydrostatic equilibrium, by the way.)
It needs to have "cleared the neighborhood" of its orbit -- Uh oh. Here's the "problem" with Pluto. According to this IAU rule, Pluto is not a planet.
We see the "rule violation" that has arisen, but what does "cleared its neighborhood" mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects along their orbital path, they either consume them or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.
Any object that doesn't meet this 3rd criteria is considered a dwarf planet. And that makes Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.
It's not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could qualify for planet-hood status. Then our Solar System would have 9 planets again.
Even though Pluto is a dwarf planet, and no longer officially a planet, it'll still be a fascinating target for study. And that's why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and it will capture the first closeup images of the (dwarf) planet's surface.
Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We'll just be able to appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.
Pluto is not considered a planet because it shares its orbit with many other objects, so it fails one of the criteria for a planet established in 2006:
The object must orbit a star such as the sun
Must be rounded by its own gravity and be in hydrostatic equilibrium
Must have cleared its orbital path of other objects .