Why isn't Pluto a planet any more?
Short answer: Recently we have found many other things in the Solar System that are like Pluto. If we call Pluto a planet we'll have to call all the other things planets too. It's easier to make a new class of objects called dwarf planets for all the Pluto-like objects.
Long answer: Even when Pluto was discovered in 1930 its status as a planet wasn't necessarily assured. Some astronomers pointed out that Pluto is quite different to the other planets and perhaps should be considered an asteroid, minor planet or some new kind of object. However it survived this initial controversy and became recognized as a planet.
By the early 1990s, however, it was time to revisit this question again. More objects were being discovered beyond Pluto in an area called the Kuiper belt, and as many astronomers had predicted, these objects were quite similar to Pluto.
In 2005 a dwarf planet was discovered that is 27% more massive than Pluto. Named Eris, this was the final straw that forced the issue, because if Pluto is a planet then Eris is a planet too. A decision had to be made about exactly what the word "planet" means.
These things are decided by an organization called the International Astronomical Union (IAU). Before the 21st Century there had never actually been a formal definition of a planet but it was now up to the IAU to make one and decide if Pluto measures up. In 2006, the IAU voted on a new definition that includes three criteria. In order to be called a planet, a celestial object must do three things:
- Be big enough to be round
- Orbit the Sun
- Dominate its orbit
Pluto meets the first two criteria but fails the third, so it's not a planet.
Not everyone is happy with the IAU definition and the debate continues to this day. Some astronomers are trying to get the decision overturned or at least modified. For now, though, our Solar System has eight planets and Pluto isn't one of them. Deal with it.
- Pluto takes 240 years to orbit the Sun. Therefore it was known as a planet for less than one of its own years