nice, yes add an atmosphere, for example a simple rim on planet material is nice.for for the background maybe the stars are too "clean" or the space is too black, need a bit noise, maybe a perlin as layer, a bit like this : http://www.listofimages.com/wp-content/ ... -other.jpg meteors are greats a lens flare and it will perfect :p
For atmosphere you can use the SphereFog shader or a static Glowrect sprite behind the planet. For nebulae you can use sprite particle clouds and 'imprint' them into the skycube as I did in the desert scene.