Making planets is quite easy, once you know how it's done (and once you have created a photoshop-template, as I have *grins*).
All you need is a texture and an idea what colour theme planet and atmosphere will have. You need to vort the texture two times, making it appear like an orb and then play a bit with the filters. That's mostly all there is to it.
If you want to make realistic planets, with seas and continents, that's another story. I know how to do them but my way is extremely complicated and needs a huge chunk of time... anyways, I limited myself to one try (this one), since I still prefer real shots over photomanipulations.