Step 1
First search google for satellite images. I prefer images of the desert or anything that looks like it has good texture.
Step 2
Use the elliptical marquee tool and make a circular selection within the image.
Step 3
Press Ctrl+C to copy the selection. Now make a new layer and press Ctrl+V. Hide the original image.
Step 4
Duplicate this layer. Now on the duplicated layer Ctrl+click the layer to get a selection around the circle and then goto Filter>Distort>Spherize at 100%. Do this twice.
Step 5
Now press Ctrl+U and bring the saturation down to -100 so it is black and white. Do this to the original circle as well.
Step 6
Now move the original circle above the duplicated circle and set it to vivid light. The combined layers give the soon to be planet more texture.
Step 7
Goto your duplicated layer and double click it to get to its Layer Style. Set the outer glow to the color you want your planet to be. I chose blue. Change the glow size to 30 as well.
Step 8
Now make a new layer above these two circles. Ctrl+click one of the circles to get the selection. Make sure you are on your new layer and fill it with the color you set the outer glow to. Change the blending mode to Soft Light.
Step 9
Now to make the shadow. Make a new layer and Ctrl+click one of the circles again to get the selection. Now with the selection, select the elliptical marquee tool. Hold Alt and click and drag from the top left to the bottom. You should end up with a selection on the right side. Fill it in with black on the new layer.
Step 10
You should be left with a black moon-shaped object. On this layer goto Filter>Blur>Gaussian Blur and set it to 16. Duplicate this layer and stretch it so it covers most of the right side of the planet.
Final Result:







More Photoshop: