When it was first discovered, in 1930, Pluto was considered to be a planet, like the other 8 planets we knew of. But as our telescopes became more accurate, astronomers began to find other large objects – in the asteroid belt between Mars and Jupiter and in the Kuiper Belt beyond the orbit of Neptune that were similar in size to Pluto. As long as these objects were smaller than Pluto then its position wasn’t threatened. But when objects were discovered that were larger than Pluto it became obvious that there needed to be an agreement about which objects were classed as planets, and which were not.

Amazingly, until this time, we didn’t have an actual definition of a planet. The word Planet came from the greek for Wandering Star and originally was given to bright objects in the sky that didn’t behave in the same way as the other stars. Back then they just knew about Mercury, Venus, Mars, Jupiter and Saturn, the planets you could see with the naked eye. Uranus was discovered in 1781 and Neptune in 1846. We didn’t need an actual definition until we got bigger and better telescopes and started spotting lots of other objects moving around the sun.

In 2006, after much deliberation, the International Astronomical Union decided on a definition of a planet. To be a “proper” planet any object had to meet three criteria.

• It had to orbit the Sun. If it orbited a planet, then it could only ever be a moon.
• It had to have enough mass for gravity to have shaped it into a round ball
• It needed to to have cleared its orbit of all other objects. Either by crashing into them and absorbing them, or flinging them out of the way with their gravity.

Dwarf planets meet two of the three criteria – they orbit the sun and have a round (or almost round) shape. But what they haven’t done is clear their orbit of other objects.

Objects that large enough to have their shape formed by gravity but which share their orbit with other objects are called Dwarf Planets. At the time of writing, there are currently 5 dwarf planets; these are called Ceres, Pluto, Haumea, Makemake and Eris. Some, such as Ceres were previously classed as asteroids. But other large rocks are still being found, so in the future more dwarf planets might be added to the list.

This video from Nasa JPL might help:

Simple answer is that Pluto is too small to have muscled its way around the sun and so there are other asteroids and rocks that share its orbit.

For more information, this article from the IAU is worth a read.