How can I blend a moon/planet (textured sphere) into the background (procedural sky texture)? I've been trying to tackle this challenge for a week or two now and I'm still somewhat stumped. I am trying to create the effect of a moon or large planet seen in orbit from the surface during daylight, when the darker values fade behind the blue sky and the brightest values shine through. 



I'm trying to make this effect with RenderLayers compositing nodes, which I still don't really understand compared to shader nodes. I know the solution should involve mix nodes and color ramps, but I haven't  been able to get anything close yet. I've tried mixing the environment socket of the layers node with the image socket in various ways but I'm still stuck. Can someone point me on the right path?
In the compositor, use a Color Mix node to mix the moon and the background. Set it to Add mode, so that only the bright areas affect the background.