Difficulty level: Beginner
Time required: 30 minutes

Pre-Start Requirements:
Gimp 2.0, a free version of photoshop found here:
http://www.gimp.org/

Next, download and install the script found here:
http://mygimptutorial.com/images/planet/planet-render-gtuts-edition.scm

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

Here you can set your planet size, 500 pixels ought to be a good start.
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

Our planet looks cool and all, but let's give it a background, after all what's a planet without stars?
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

Fill in that layer with black using the bucket tool then go to filters>noise>hsv noise, this will let us add stars.

Step 5: Adding a Background Pt.3

Holdness changes the start pattern
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.