What happened last time when somebody tried to use actual modeled planets?
That would take care of the lighting and shadow - the only problematic area I see is the air coronas. Putting them into the skybox would also solve the scaling issues and make them render after everything else.
It wouldn't take care of shadow as FSO has no shadows (yet). Only lighting. Atmosphere could be a problem, but there's also the fact that unless you make the model really high-poly, it is easy to see the edges and know that it is not round (just look at the Earth model from inferno in the last mission ). There's also the fact that with a skybox, you couldn't scale the planet and you would be stuck with whatever size it was.
There are several issues with using models as planets. Here are what I can think of...
1. Resource intesity. To look good, planets need BIG textures. Practical aesthetic minimum is 4096x2048 sized texture. However, many non-recent cards (as I've come to notice) don't support textures this big, and only go up to 2048 pixels as maximum width and/or height of the texture. With ready planet images, a 2048x2048 sized planet has HUGE amounts of detail and usually 1024x1024 sized planets are more than enough. And look better than models.
2. Need to use anti-aliasing to get non-jagged planets. Sucks for those who get a slideshow with any kind of anti-aliasing... With readily baked background images of the planets, this is not a problem at all.
2. Lack of advanced lighting effects like volumetric/shaded atmosphere layer result in visually less stunning end result than planet background images, in which you can bake the effects in with varying amount of accuracy - depending on whether you use renders or purely photoshopped/GIMPed planets. Renders have more realism, but for some reason I prefer doing my planets in GIMP from the beginning to the end... I've done some experimenting with adding an "atmosphere layer" above the ground, with very limited success... it makes the planet flicker from long distances and doesn't look too stunning from nearby.
3. Same applies to clouds. You have two options with clouds - either use another model layer above the ground level to house the clouds (which ends up problematic from long distances) or bake them onto the terrain texture. Neither method ends up looking spectacular, and usually the clouds look flat no matter what you do... normal/height maps might help in this, though, but I haven't tried that.
4. Shadows are not really that big of an issue, since the model itself is most likely just a sphere, if you don't want to go absolutely nuts with polycount and planetary detail. If you want to have mountains, normal maps can take care of them... but the level of detail you can achieve is rather limited anyway. Talking about polycount, the planet doesn't need to be ultra-high-poly model. Properly done smoothing is way more important. About the only way you can notice the polygon seams is when you go looking at the speculars from small distance.
5. Size. Correctly sized planets just don't seem to work in FS2_Open, or at least with my model conversion skills... also, realistic distances are problematic and end up with flickering planets and stuff. And miniature planets look a bit stupid IMHO, and can't be considered any more realistic than background images.
The modeled planets do have some benefits, though mostly only with objects that have no visible atmospheric glow. Gas giants and their rings work spectacularly well (if you ignore their ridiculously small size), speculars and lighting are somewhat more accurate than with bacground planets - or rather, with model planets the mission designer does not need to think of the planet lighting issues like having the lit side of the background pointing at the sun with roundabout correct angle - and since you can move around the planet, you can see it in more phases than just the one offered by background images.
Mostly, though, the cons win the pro's in this case. At least for the time being.
Here are some screenies... most from Cardinal Spear, although the atmosphere/normalmap test doesnt' belong there.
This one has edited brightness/contrast to bring the Orion forth better...
This is the atmosphere/normalmap test... doesn't look too great IMHO. Normal maps have potential, however they are a bit exaggerated in this version IMHO.