I think this is a perfect background for people who want a fantasy sky without having to make one themselves. The colours are beautiful, the 'space dust' is great and very atmospheric.
My only negative critique is that the planets look identical. I think rotating and flipping one of them so it's not immediately obvious would be great (hopefully easy) and fix the problem easily. The little blob of purple dust under the blue planet is a bit too thick as well, maybe a bit more opacity there would help? Another minor and rather unnecessary change would be to the composition... I think moving the blue planet and the dust around it a little lower, to make it a bit more of a "rule of thirds" composition, would help. The two planets seem a little bit too close together and I think it would have more impact if the blue planet were just a tad lower.
I'm not sure if there are layers in this or if it's all flat, which would make it harder to edit, but if there are layers those little changes should be fairly easy and would make the piece perfect, in my opinion <img src="e.deviantart.com/emoticons/b/b…" width="15" height="15" alt="" title=" (Big Grin)" /> It's already exactly what seems to be most popular among the manip crowd and I'm sure you're going to get many many uses from this!
Of course you can use these! If you're posting them somewhere outside of deviantART though, I ask for a link or URL back to my page either in the comments section/post where the image is (if possible) or printed right on the image. That way other people can find my stock and use it in their artwork as well! The URL is http://ayelie-stock.deviantart.com
Good luck with figuring out Photoshop, it's a great program and I'm sure you'll get the hang of it quickly