planetry Gears, whats the advantage?

What are the advantages of planetary gears and what are they used for?

thanks

Planetary gears take up less space than other gear types of the same size and tooth ratio.
With planetary gears 2 or more teeth are meshed at any one time to carry the torque load.
They make for more efficient and ease of gear shifting.
They are used in automobile transmissions.

www.howstuffworks.com has a good explanation of transmissions, take a look there at it and it'll explain a use of planetary gears

nice gearbox for projects.