The planet textures are generated by the game in 96x9 pixel slices from the full-sized maps.
The same model is used for both the planet itself and the cloud layer, but not the shadow.
I included an edit of the planet model that allows the full-sized texture to be used.
The "index" texture indicates which pixel to pull from (RGB value = X coordinate).
The "offset" texture is my edit of the index data for use in a standard distortion shader.
The "mask" texture is used for the alpha and also turns the hidden pixels black.
You can find pre-generated planet textures in a separate sheet on the Spriters Resource.

Scale values for each planet:
Corneria = 5
Fortuna = 2.5
Titania = 2.5
Katina = 2.5
Macbeth = 2.5
Aquas = 2.5
Zoness = 2.5
Venom = 9 