Awesome, the only problem is the ring system. I see you used the "twist the hell out of someclouds/difference clouds" method on them, when, IMO, you should be using radial blur (spin, 100%) instead. Or you could do what I do: noise gradient, then polar coords.
Great job on this. The colors are great and the planet is well made. Although the spaceship seems a bit out of place. The lighting on it doesn't really match with the planet and the asteroids's. The asteroids provide a good sense of depth and are well lit. Another thing is that the rings on the planet seem too noisy and could be more opaque. The planet is well done, good atmosphere, texture and shadow. One thing I'd like to point out though is that the city lighting is a nice touch, but it makes the planet look flat. If you could better manipulate the city lights on the planet, it would give it a more 3d look.
Overall great, fantastic first. My first was nowhere near this good and some of my more recent pieces may not be either.