There are a lot of Planet tutorial in the web, you can play whit them as long as you want to do the planet of your dreams, I’ve got may way to do them:
(In planet’s tutorial the beginning it’s always the same.. lol)
Step 1: You have to create 3 layers with circle selection on hit, I fill the first one with brown colors and the others with black.
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Step 2: In a new document grab the circle selection and copy inside the layer texture using the sphere filter twice.
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Step 3: Back to the first document. For the layer at the top I’ve added a Gaussian blur with 100% radius, CTRL+T and changed the size grabbing it to the right and with the soft brush eraser I’ve just erase the areas outside the planet
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Step 4: For the layer in the middle I’ve added an inner and outer glow choosing the same brown color that I’ve used for the first layer. And I’ve copy the sphere made with the floor texture, duplicate it twice and change blending mode: 1) Overlay – 2) Soft Light with 36% opacity – 3) Screen with 36% opacity.
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Step 5: I’ve added 2 layers with light/contrast control. One just under the “Atmosphere” Layer and the other above the “Base” layer. Light -70, Contrast +10 for both.
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Step 6: Created a new layer with those settings: 1500px X 750px, fill it with black and added a Noise (20% and Gaussian settings). Added a level with Clouds filter (black for the foreground and white for the background colors) and masked it with blending mode Overlay and 31% opacity.
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Step 7: Copied the planet on the star field, and started to do the nebula: first I have added a lens flare and erased all the areas behind the planet, leaving the one just in front off. After I have added 3 gradient layers, one red, one blue and green one and make the same thing with the eraser tool and changed the opacity on 30% for all of them.
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Step8: Used the clouds brushes (black color) in the nebula and 50% Opacity. Added clouds to the planet too, and use the smudge tool to change them as I want.
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Step 9: To have more deep in the space I’ve added more stars of different sizes with (for each size a different layer) a white soft round brush, and added a Gaussian blur at 1%.
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Step 10: Added the second planet (Step 1, 2, 3, 4) and just change the texture color of the floor in yellow, but using same steps as show. Duplicated the clouds nebula layer and flip it to horizontal. And more, more stars..!