I agree, it is lacking in realism in the sky. It would look a lot better without the sky manipulation. Also, the text needs better more realistic lighting.
The top right planets look decent as they have been properly merged with the clouds/current scenery. The top left one looks rather out of place.
This may just be the screenshot quality, but some of the sparks look really blurred or are simply lines, whereas real sparks would be rather crisp and thin.