The planets do look a bit as if they were low res images that have been scaled up. Not sure what your creation method for them is but there are several great planet tutorials for Photoshop over at Deviantart that may offer you tips on how to improve them. In addition, you might want to think about adding a bit more shading and atmosphere to the planets (a discussion on how to do so is included in several of the tutorials at DA). The overall layout is nice though.

Cheers,
-Arsheesh