I think you may be better off creating your own planet definitely, it'll give greater control over the lighting.
Gas Planet tutorial Fr_Eng by *Lord--K on deviantART
Planet Creation Guide by ~Kaioshen on deviantART
Adding a nebula in the background could also create a more interesting backdrop, wether you use a stock or create your own, it doesn't really matter. It's only a backdrop :P
Nebula Tutorial by ~Superiorgamer on deviantART
Paint Space: Nebula by ~ColdFlame1987 on deviantART
Aurea Mediocritas by `taenaron on deviantART
You were probably already thinking this, but you should use stock images for the gears rather than painting or brushing. Brush looks plain and boring, and painting a geometric structure would be too complicated. Better off manipulating a stock.
Gears Transparent PNGs by *redheadstock on deviantART
Also consider rendering images that were taken at different angles, and then add them as smaller detailing, like some gears floating away to join meteor cluster or whatnot, whilst the front on gears will serve as the structure of the planet.
Browsing Stock Images on deviantART
As for the break off, make it violent and warm, add some particle effects and floating chunks of rock near it.
FISSURE by ~Nameless-Designer on deviantART
Loca Infernorum by `taenaron on deviantART
Not much of a CnC, more of my opinion on the direction you should take.