The planet itself looks a little bit glossy, maybe you should try to use a
different kind of texture? Take a quick look at any of those sites for more info.

¤ Rabieshund's Pro Planet tutorial
¤ Alyn's Advanced Planet tutorial

These tutorials also shows you how to add a bit of glow on your planet,
which makes the planet itself look a lot better.

The stars is important to, if you use brushsets, don't overdo or use the
same brush over and over again, that would only make it look bad.

The stars should be visible on the black areas to.

Good work though, I'd like to see more spacescenes from you.