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As a "space artist" myself, i can hopefully give you some useful tips.

1) think of background before foreground normally works best for a space scene, first put where you want the light source, then handling the stars, then add the nebula if any, then the planets. Keep in mind concept is good, because realism does add to it.

For example, stars are not always stuck together like freckles on a face, normally distant. Some shining brighter then others, some larger then others. Playing with Noise, layer masking and levels can give you good effects with little effort.

2) Scale, scale in a space scene is very important, and the best way to show scale is with planet texture and planet effects, such as glows, shadows and where you place the objects. Such as clouds, cities and other effects.

3) Lighting, space is a dark place. However a lot of your scenes come out very bright. By adjust stars, planet shadows and glows. you can make massive impacts on the scenes and i recommend you try play around with lighting a little more and see what you come up with.

Some Resources that may help:
http://geeks.pirillo.com/profiles/blogs/
meduim-hd-digitial-art-planet - planet tutorial i wrote a while ago

http://the-danzor.deviantart.com/gallery /#Stocks-Tutorials - Space Art Tutorials and resources i have made over the years.

Hope this has been helpful to you.

The only advice i can think of, if you can't think of ways to improve your current style. Is try and experiment with other styles to create similar works. Normally this gives you a different prospective and you can try and combined what you have tried. Into a new or expsanding the style that best suits you.

As well as the advice above, i would recommend getting a sense of scale. Things should normally get smaller as they go more into the distance of the background, but in this piece it seems the scale of things seems slightly off. Those are just my thoughts and hope they help.