A tip, use bigger resolution on your planet (I used 2500x2500 to start with, then I moved up to 8000x8000) and get textures from www.mayang.com or any other site and manipulate them. (clone tool is very effective, takes long time to get it nice though) Lastly you just drag your planet into your project and resize it.

The starfield looks good, but I would like them smaller, I suppose you have used a brushset. Try creating own starfields. (http://gallery.artofgregmartin.com)

Good luck, looking forward of seeing more from you.