Looks alright, but I feel it's too monotonous on the color. Try giving the planets some land/water colors, and color the nebula more vibrant colors, like blue/purple/pink.
Like mentioned, it needs a starfield. On dA, when you submit, where it says "Fullview Size," make sure it's set to Original.
The clouds filter (nebula?) looks really obvious. If you could soften that, or maybe even brush from scratch, that'd make a big difference.
Where the clouds stop and the texture starts on the planet looks really rough. Try blending that a little better.
Lastly, the Terragen terrain needs a lot of help. It looks really (sorry to say) noobish, and could be a lot better with some work. Take some tips from ~alyn on dA, he's really good with Terraspace. Try making the landscape shorter and less jagged, maybe even a beach scene would do well on this piece.
Give it some work, and I think this piece could be ace.