I have no idea how you could accomplish good looking clouds with sprites. The clouds need to be pretty uniformly close to the edge of the planet to look natural, and 2D sprites won't give that effect. Have you seen it done before? I'd be curious.

NASA has some really great, freely available composites of clouds and the earth on their website (wish I would have known sooner). That's where this texture started from, though it required adjustments to get it 3D and suitably transparent.