That grunge texture you used has given the piece an interesting "hand-made" look. I'll keep this in mind the next time I want to make something like this.
If I had to work over it, I would address the following points:
- Add some
atmospheric perspective by slightly tinting the back clouds with the sky colour.
- Make that radial pattern on the sky (the rays coming from the centre) slightly stronger, as it may be too much subtle right now.
- Frame the content closer to the edges of the canvas as to zoom in on it, as I think the sky is occupying too much on the picture (a subjective choice).
- See if the Levels tool (Colours -> Levels) can
improve the overall contrast. The Levels tool gives more control than the Brightness-Contrast tool because of the middle handle.
- Make sure that the lighting is
consistent. The clouds and part of the planet seem to indicate that the light-source is located at the top-left corner of the picture, so the bottom-right side of planet should be in a subtle shade to convey volume.