The first one didn't really jump out at me, I think because of the black background, but the second one? Oh, my god. Its so awesome. The background certainly helps, but I think the planet makes it stand out even more. I would maybe just change the background in the first one. The second one I see no flaws with. Awesome job!
I agree with what Star_Angel said about the first one, but for the second one, what you should do is make everything darker, since there isn't enough light in space for it to look that bright, unless it's right next to the sun, which I doubt, since it looks more like Mars then Mercury. So, I'd just darken up the colors and add more shadows if you can. Also, even though it's supposed to pop out at you, the objects pop out so much that they seem to not fit with the background, but that might be fixed if you darken them up more.
