Nice idea!
There are som things that I'd correct though. First, the lens flare is cut off at the top, like Deadloader said. And now for a little lesson about lens flares! :P Do not use the lens flare you did in this pic. Use the second one (I can't remember what it's called, don's have PS on this computer). The first one creates much more ugly spots everywhere that are just hard to get rid of. Anyways, when you use it, do the following:

1. New layyaa!!
2. Select the eliptical (or wtf it's called.. the circle thing) marquee tool.
3. Make a selection, as big as you want it.
4. Fill it with black.
5. Add a lens flare (not the one you used for this pic).
And now for the important stuff!
6. Keep the selection and go to Filter > Blur > Radial Blur and blur it quite alot with the "spin" blur (I hope you get the idea :P).
7. If you think it's needed, use the zoom blur too.
8. Deselect! Set the layer's blending mode to Screen or Linear dodge. Note that the two blending modes will have different effects on the layer.
9. Open up levels and pull the gray arrow to the right till you get rid of the ugly edge of the lens flare.
10. Done! :P Now you can play with the color of it with Hue/Saturation of Colour Balance or whatever.

If you look at my space scenes, you see that I have different sizes on stars. There are the background stars and there are bigger stars. All the bigger stars are made this way!

Anyways, back to the critisicm! >;D
The second planet is too big. It's so close, so it should be a moon, but it's way too big to be a moon since it would just crash into the bigger planet. I would keep the rings, but scale it down pretty much.

The last thing I want to complain about (:P) is that I think the lens flare is positioned too low. It looks like the planet is trying to swallow it or something.

Though it's not the most original of ideas, I like it. The planets are looking good, especially the small one which is looking VERY good! Maybe you should redo the starfield and make a better one. But anyways, good job. Keep 'em coming!