If you export the alpha-channel together with the colour render of the foreground scene/image, you can clip out the background atmosphere (using the alpha-channel as mask), just be sure  not to render infinite planes in the alpha-channel (uncheck in the render options).
Also, the z-channel can be used to fake mist, using it as a mask (maybe enhancing the contrast (autolevels or manually)), and dragging a gradient over it on a separate layer (then adjusting the opacity etc.) .

• Lars "bigbraader"