We are now done with the texture, so we must go and get started on the planet.
Make a new document. Because my texture was at 4000x4000, i made a new document at 2000x2000 (1/4th of the original). Do the perfect circle move again with the marquee tool.
Once you're done with that, duplicate the layer and make the duplicates black (you can do this by turning down the brightness under the image/adjustments menu. Call the blue layer "base,"the next one "shadow," and the next one "atmosphere".
If you applied the settings properly, you should have something similar to what you see to your bottom left.
Make a new layer, link it with the atmosphere layer, and then merge the 2 together (layer menu - merge linked). Then set this new layer to screen, and rename it "atmosphere".
Once you're done with that, take the shadow layer and move it over the atmosphere and apply a gaussian blur of 100. The layer should not be selected (dotted line around) This should leave you with something that looks like what you see to your lower-right.
Hit ctrl+T to transform the shadow layer. Now we need to decide upon a light point. To keep it simple, my light will be coming from the upper left, so we hold down the shift key, and stretch the lower right corner of the layer. Resize it until you are satisfied. You should end up with something that looks similar to this.
That looks rather nice, however, there is a problem. If we add a background, like a star field, then the shadow will get in the way. We only want the shadow to obscure the planet.
Ctrl+click or apple+click the layer base. This should add selection marks (marquee). Now go to the shadow layer, copy (ctrl+C for pc, apple+c for macs)) and then paste (ctrl+v for pc, apple+v for macs). Then you can go ahead and delete the bigger shadow layer. Rename the new layer to "shadow." You should have something that looks like so.
Now we have a nice texture that we can use, you may go ahead and save it, but don't close the document yet.