Creating a realistic planet in the GIMP

Go to File>New. Set size to 3000×3000, and Foreground (black) under advanced options. Open 3DW_QnA_Planet_Undertexture.jpg and 3DW_QnA_Planet_Overtexture.jpg. Select all and copy each, and paste them into the file you just created via Edit>Paste As>New Layer. Set the Over_texture to Grain Extract, and merge it down via Layer>Merge Down.

Click the merged layer in the layer-dock, and go to Filters>Map>Map Object. Select Sphere in the dropdown, check Transparent Background, and, under the Light-tab, select No Light in Lightsource type. Click OK. For the shadow, create a new layer, and set it to white. Go to Filters>Re-show Map to Object. In the Light tab, Set Lightsource type to Point light. and lightsource colour to black. Set the X and Y Directional vectors to -1,2 and the Z to 2. Under Material, set all values but both Diffuses to 0. Set Diffuse to 1 under Intensity levels, and to 0,3 under Reflectivity. Click OK Set the layer to Hard Light. This will create a realistic planetary shadow

Use Map to Object instead of using a black sphere with Gaussian blur, as it gives more control, like increasing anti-aliasing for softer planet-shadows.

Add a planetary shadow hue by rightclicking the Planet-layer in the layer dock, and clicking Alpha to Selection.

Create a new layer, set it to soft light, and fill it with R45, G140, B200. Rightclick the layer and click Add Layer mask. Leave it at White, and click OK. Click the blend tool in the toolbox, and reset the fore and background colours to black and white, and set the gradient to FG to BG. Drag blend direction from the upper middle of the planet shadow diagonally to the right, just outside the selection With the selection still active, create a new, transparent layer under the planet shadow. Fill it with the same colour as the shadow hue, R45, G140, B200.

Using Stroke Selection is very handy when faking layer effects, as you can define stroke settings in detail before finalising with filters and layer modes

Create a new layer. With the selection still active, Select>Shrink and set it between 40-80. Fill the selection with black. Unselect, and run Filter>Gaussian Blur at 300. Merge the layer down, and set it to screen. Set opacity somewhere between 40 and 60. Generate a new selection again, and create a new layer. Set colour to R45, G140, B200 again. Click the paintbrush in the toolbox. Select Circle Fuzzy 11 and set scale to 2,3 Uncheck Pressure/Hardness under Brush Dynamics. In the select menu, Select >Select to Path. Then go to Edit>Stroke Selection. Check Stroke with a Paint Tool and emulate brush dynamics. Click Ok. Sset the layer to overlay, Give it a a gaussian blur between 40 and 80. Set opacity to 80, and you have a basic planet ready for detailing

You should have a result similar to this

Tip: For a more visible outer glow, blur, tweak and set the texture to screen and then increase the size of your planet shadow a little to make the glow line up properly.
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Grab the Planet Tutorial for the GIMP files here (~85 Mb) – Remember – downloading means you agree to this