How i did it?
1) photoshop brushes (you can find free steam, vapor & clouds easily)
2) create a texture from brush, adjusting alpha, transparency, fading to zero the bottom part of the cloud
3) repeat for 4/5 different clouds (merging various parts)
4) using planes, position your clouds on the planet surface according to the curvature
5) (THIS is the REAL point) become CRAZY tryng to find the proper TRANSPARENCY mode to fit your clouds w/o loose particle effect, get glitches or strange transparency adding problem.
if oyu are good enough with graphics and coding the points from 1 to 4 are easy..
the 5th is a PAIN!
Lucka - gawteam coder - www.gawgames.com