step 1
Find and save images, including the background image. Make sure they are of good quality.
You should cover up the entire surface of the planet with textures, plants, buildings, etc...
step 2
Create a new file.
Image size: 8.5"x11", Resolution 200
Double-click on the Background Layer in the Layers Palette to make it a regular layer.
Use an Elliptical Marquee Tool and draw a circular selection;
step 3
Save the selection: Select > Save Selection
step 4
Choose a Foreground and a Background colors.
Background color should be a dark version of the Foreground color.
Apply a Radial Gradient, using Foreground to Background option.
Deselect.
step 5
Place a texture layer on the top of the planet.
This can be ice, grass, sand, cardboard, fabric, rocks, fire, water, cotton ball, etc....
Load the Circle Selection.
Add a Layer Mask
In the Layers Palette select the texture thumbnail:
Load the circle selection again.
Apply Spherize Filter:
Filter > Distort, Spherize...
step 6
Move up the bottom layer (with your initial planet)
Change its blending mode to add shadows to the object.
You blending mode might be different from mine - it depends on the texture layer that you are using.