Are your planets created in a 3D program or just drawn in something like Photoshop?
It would be neat to have the plants slowly rotating, or the clouds/storms on the planets moving about.
Having the odd shooting star in the background would be cool as well - or if spaceships are a thing you could have the casual ship fly past. Apart from that, like you said - some twinkling stars or a galaxy that's in motion would be cool additions as well. The hardest part is not knowing enough about your game, or how your existing graphics have been made. If they're 3D objects in a 3D graphics program, rotating the planets would be pretty easy... but if they're static images made in a 2D program then you can probably throw that idea out of the window because, too hard.
~Sol