Rosie, I hope this isn't too late a reply to your message.

Rendered on black backdrop with sun turned on but colored a very dark blue.

Large planet is two spheres nested in each other, same mat on each.  Sphere two is tweaked just a little larger than second and mat made "fuzzy" in mat lab to give atmosphere. (probably not the most efficient way but it was fast)

The moon sphere is set back quite a ways.

There is three square parallel lights.

Light one is near the camera facing the planet to give me some better control of front light.  It is blue also. 

There are two huge square parallel, blue lights to the rear of the planets.  The intensity it turned up rather high on them and they have "no fall off" in lighting attributes. I used two lights here to get a better wrap around effect on the rim glow by setting one a little lower on the y axis.

If I were to continue I would render the ground and trees separately. Make a mask of the two renders for photoshop, and stack them all in post work against the backdrop galaxy image.