Blur some of the stars nearer the bottom of the pic to give a deeper depth of field, especially over the nebula. That area looks flat.
Also, the nebula looks to opaque, and I find myself looking for some form it in because of that. Adding a little bit of haze might help considerably, as well as adding a small concentration of stars at some points in the nebula.
I am a little bothered by the eye movement of the piece, though. The nebula is obviously central emphasis of the image, and it moves the eye around the picture quite nicely. But the planet- it just seems out of place. Like an awkward break in a sentence. I almost wonder if the picture would flow better without it, or how it would look large along the bottom of the image (like the planet is a lot closer to the viewer than the nebula, and you can only see part of the planet... like it goes off the bottom left corner kinda... and I think I'm making less sense with this suggestion the more I try to explain it *sigh*)
I hope something I've said here is of some use to you! Good luck!
<disclaimer> This is all, of course, my opinion, and I might be crazy. Take it or leave it as you wish, but I hope it helps! </disclaimer>