Creating spheres is very easy with Photoshop. Finding good textures that give good results is another matter. My daughter-in-law occasionally bakes bread in a cast iron pot, and the loaves have incredible texture. I’ve made most of my planets from just a few textures. With a little manipulation, such as using inverse, saturation, and hue, you can have planets of most any color. Add a few clouds and a shadow, and perhaps an outer glow for an atmosphere, and for the most part they are very believable.
Earth (above) gains some realism by using an actual shot of the moon. With the sun in the distance and stars, it’s actually quite hard to distinguish it from the real thing. It would certainly be a sight to behold.
Planet Sunrise shows a rather large gas planet and a moon that is clearly inhabited with cities and sparkling lights. In such shots is doesn’t hurt to add a spacecraft, even if they aren’t highly sophisticated.
On to New Pandora, which might be just around the large gas planet, or perhaps in another solar system altogether. Once you’ve built your library of components, then the combinations are endless. The amount of complexity is just a matter of duplication and adding variations.
Viridian is my favorite planet, Earthlike in many ways, and the home of my characters from Silent Trek. The book cover has a less complex version, but I think the halo stars and colorful nebula can certainly make a captivating image.