Love your work...it really conveys the infinite depth of space that so much other space work lacks, like a Hubble photo of the early universe that has seemingly been exposed for hours to wring out every last detail. Fabulous.
This sort of thing is my specialty, so get ready to be nitpicked.
First of all from seeing your works, theres no varriance of color. Its always one layer colored with an orange or blue, and then set to color. This really is a bad move, considering you lose so much opportunity to color things yourself, and give it a variety and life that a full colored layer can do.
Second the atmospheres on the planet are a bit subpar, they dont seem really realistic, and could definately be improved upon. The top planet doesnt have a atmosphere, yet there is a brush of orange on the bottom of it, which doesnt make much sense technically. When adding light to your planets, do it by soft light layers and overlay layers, so that the true colors of the texture come through. Try to hide any mere "brushing of light" upon the planet. Make it invisible.
Also, planets should glow, and have a corona, which you have in the small middle planet, but dont have for the top.
More nitpicking is the shape of the middle planet. Its circular, but it doesnt look like a perfect circle. It stands out like a sore thumb if your planets are not perfectly cirular, or at least, rounded in such a way that no flaw can be seen.
Also, instead of using a lens flare as a sun, then putting a motion blurred rectangle over it, try to make your own stars and lighting effects. If its the main focus of the painting (which it is) it should be made with the upmost care and precision.
Lastly, be watchful of your image splicing. I can see 3 seams in the image where you didnt blend very well.