The problem with using real textures is of course that they quickly become recognizable and repeating.
Quite a few variations inside planet classes can be achieved by randomly combining layers of of real object textures. Currently I use 2 to 3 layers (surface, air, clouds), with various types of transparency. The combination of textures and their transparency degree is modified randomly.
Apart from technique the results depend of the textures used, so what has to be done is finding the best ones available.
Since the difference are currently only visual, the classification is done entirely according to appearance:
atmosphere: surface is completely visible like on most
moons and planetoids
(Click on the pictures for the full resolution images)