Planets are big things made of stuff that orbit stars because a force called gravity. The star, together with its planets (and some other stuff) comprises a Solar System.

Most big planets are made of gas, but some other planets are made of rocks. Depending on how far from their home star they are, they could be really cold, or really hot.

So far, The Earth is the most well-documented planet - and the only one known to be capable of sustaining life - that humans are aware of, what with us living there and all that, but there's bound to be other similar habitable planets orbiting different stars, in other Solar Systems. But we don't know much about them yet.

The other planets within our own Solar System (The Solar System), in order of distance from the Sun are as follows:

- Mercury

- Venus

- (the Earth)

- Mars

- Jupiter

- Saturn

- Neptune

- Uranus

- Planet X

They're the main ones, anyway.  There's another sizeable-ish thing in between Uranus and Planet X, which was called Pluto but then some guys decided it wasn't really a planet after all so it doesn't count.  Apparently.