1.) Insert just a standard plane primitive.
2.) Go to the texture room and apply the texture that I've shown. For those out there who don't know, I've doctored this image to show you more than you can actually view in Carrara (I just combined some of the menu views into one image). So, don't get frustrated if you can't reproduce this view (because I cooked it).
There are some very important points to note about this texture.
A.) It's a flat mapped texture. I'm just projecting this texture onto this plane (very brutish but VERY effective).
B.) I've selected the "No Light Interactions When Fully Transparent" option.
C.) I set the transparency to 100%.
D.) The color is set to none.
E.) I turned off the highlight (even though it makes no difference becuase of my choice in B).
F.) And, I put the image that we rendered in the first part into the glow channel.
I also setup the glow channel with a mixer so that I can mix my moon/planet image with a glow of value zero just to give me a control knob on how bright I want this thing to be.