You can also mask the sky out and render your scene and your planets in different images. Then you go to your favourite 2D-app open the planet-image. Add a layer with only the sky (wich you can do with your mask) on top of it. Then you can fiddle with the layer-options, like putting it to overlay or screen mode. This way you can accurately create the exact effect you are looking for and you don't have to make lot's of test-renders in bryce. When you've got the planets and the sky right you add another layer with the whole (planetless) scene with only the sky masked away and put this layer all the way on top. And you have your image with the planets behind the sky.

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