Gimp is your best friend if you want a simple planet done I just drew all kinds of noise and blurs with different colors, then finally apply "Make seamless" effect. This makes the texture wrap from sides so that you can use it as texture, without visible edges when coords 0 and 1 clash. Different planet types ended up looking like this:
https://docs.google.com/open?id=0B7F...HhhbVRnam9rTVU
Each have 2 cloud layers which rotate the planet at different pace.