Well, it's definitely not my tutorial, but it's certainly worth mentioning. It's one of the absolutely best planetary tutorials I've ever found, and if you have a bit of spare time, feel free to complete it and post your results on Zelaron.
And here's my result after completing the tutorial (which didn't take especially long):
"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram
Tutorial: Photorealistic planet creation in Photoshop
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