19. When you'll finish to work with stars, merge your background and stars' layer, but not planet. Copy Background layer, and change Blend mode of "background copy" on Overlay. Play with opacity.

When merge down and apply some lighting effects again to make our space deeper! I've applied this additional lighting effect behind my largest planet.

20. My final picture is here.

Thanks for this my tutorial you can create many different and unic planets.

Tips: your result depends on lighting; details on the 2d step; gradient map (applied or not, if "yes": which gradient do you use).

Good luck!
Nathalie.