Simulating simple 2D mist

I trying to write a 2D shooter game for each of the planets in the solar system. The next one is for Uranus - I’ll probably call it Planet 7 to stop the snigering. The game itself is reasonably well thought out but my problem is how to portray the background scenery of the planet. The whole screen will just look grey which just looks a bit too dull. I thought swirling mists with the aliens flying through the mists might be acceptable but I’m a bit stuck on how to do this. Any thoughts?

I wondered about having a plain grey background and then drawing lots of irregular shaped grey shapes and changing their transparency on the fly while moving them about slowly on screen. Is there a better way?

Thanks for any thoughts

http://nineplanets.org/images/uranus.jpg