Hope you're still up for a little criticism. First of all, pretty nice colors chosen, they work really well together.
On the other hand, one thing that could get some more rework is the planet in the top-left corner. There is absolutely no curvature visible. I don't know if you intended it to look like that but I'd appreciate a slightly rounded surface. Please note that this is just a personal preference.
Nevertheless, a really great piece and quite minimal compared with other images - and I do like it
Great image! In a somewhat obscure related topic, I saw an article on Yahoo talking about the massive ring discovered around Saturn. It's huge! Completely dwarfing and engulfing the planet! Check it out if you get a chance.