Pluto is officially no longer classed as a planet, and here's why. The International Astronomical Union defined three rules in 2005 that a celestial body (an object observable within space) must follow to be classed as a planet:

1. The planet must orbit the sun.
2. The planet must be large enough that its gravity can smooth it into a nice spherical shape.
3. It must have cleared up the surrounding objects around it.
Pluto fails number three as it simply isn't massive enough to pull the surrounding objects into it's orbit. This is really hard for Pluto as it lives in the Kuiper Belt, a belt of other large rocky objects.
Fortunately for Pluto it matches the first two rules, so it can be classed as a dwarf planet instead!