Just practice mainly on depth. I'll be doing more of this.
The planet are just satellite photos I got off google and just manipulated in PS and made into planets.
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You got the depth down good. There is just something not right about the fore-most planet. I would like to say it is the shading, but I don't know for sure. There is something off about it. Stand back really far, and look at it, and you should be able to see what I am talking about. The shading seems a little unnatural. Other than that, this turned out really really well. Keep up the great work.