Cosmic Pack 3 for photoshop
CP3 is a bunch of actions and textures used to build space scenes: Starfields, nebulae, planets, moons, rings, gas giants, suns and galaxies (the last are only meant to be viewed as small additions to the picture rather than the main subject). very comprehensive set of tutorials and, if you like drawing planets the price is good. Basically the workflow to make a planet is in two parts: The first to make the surface texture and the second to wrap it into a planet and give it atmosphere lighting etc.
Absolutely superb bit of kit as you can adapt it to your workflow since it is a bunch of little actions rather than one big action (eg make your own planet textures etc) and you can take some actions and leave others it organises everything in layers so you can tweak afterwards and you can work other photoshop plug-ins like Glitterato into your workflow as well.
Here is a scene I made following the tutorials.