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What makes a planet?

3 October 2014 Last updated at 17:07 BST

Pluto was a planet wasn't.

But it might, one day, become one again as some scientists think it should be.

There could also be up to another 100 in the next 10 years.

So what does make a planet and what will happen to Pluto?

Facts about the Solar System

Martin's being finding out.