Hi, Redmaw.
You can easily do this with a 'default shader' and a 'simple shape shader'.
Make the simple shape at the same size and coord's of the heightfield('apply colour' checked to white and no 'edge profile'), pop a default shader into the 'shaders' tab before the planet node and use the simple shape shader as the 'opacity function' of the default shader.
See attached image. In the background, you can see the atmosphere only, no planet surface.
Is this what you mean?
[attachimg=#]
