I've actually done a lot of work with planets and atmospheres with EvilJedi over at the relic boards. After a few days of stubling around with .lua files and the fx tool I figured out how to make an atmosphere effect and play it at the location of the planet in a level (this needs to be done in the "insertgametype".lua file unfortunately)
However once we were able to get decent atmospheres ingame other problems started to present themselves, and are unfortunately hardware and software limitations, the biggest of which is the horrid clipping on the atmosphere effect. After a few more days of experimenting, EJ and I decided that having planet models inside game space was just not practical, and using them as background objects only seemed the logical choice as you could then use little tricks to kill the clipping.
Oh yeah before I foget, Heres the final form of my earth and atmosphere effects before finaly deciding not to continue (also shows off some nebula, and star distribution scripts I was working on with EJ)
Sorry 56k-er's, this includes me, I'm just too lazy to recompress and upload the shot.