Time required: 30 minutes
Gimp 2.0, a free version of photoshop found here:
Next, download and install the script found here:
It goes in the folder where you installed gimp, then /share/*gimpversion/scripts/
Step 1: Creating the File
Go to File>Create>Misc> Planet Render...
Step 2: Choosing the Settings
Planet color is the color of your planet.
Outer atmosphere doesn't have to be white, but it should be brighter than your planet coor.
Sun orientation changes where the light is cast from.
Sun tilt changes how much sun shows.
Click OK to create your planet.
Step 3: Adding a Background Pt. 1
First, add a new layer from the layer window.
Next make it the very bottom layer in the list, this sets it to the back.
Step 4: Adding a Background Pt.2
Step 5: Adding a Background Pt.3
Hue changes the color
Saturation changes how many stars there are
Value changes the random seed used to generate the stars.
When the preview window matches what you want, click ok.
Step 6: Final Result.
Feel free to save in any file format you decide is best.