Keep in mind, I have not tested anything out in SOASE. I've looked through the files before, but I don't know how SOASE defines collision boxes (telling units what to avoid)
Method One (Use this method if SOASE render collision on shape of each individual object)
I'm not sure (and don't count on this being true) but you could easily make a model like this.
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O o
O = big, center planet.
o = 'orbiting planet'
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I don't know if SOASE supports planets having it's own animation (for example: The moon spinning), since SOASE rotates the planets with a variable.
The good thing about the game engine turning the planets, is it turns the ENTIRE model. That means, that the moon is orbited also, in a way that's very realistic to the way the earth's moon resolves around the earth (one side is always facing the earth; always).
The plus side: I THINK SOASE renders clicks based on the objects shape, so clicking the moon MIGHT have the same effect as clicking on the planet.
The bad side: It might be the opposite; SOASE might draw a 'clicking' box around all the objects in the model file, normally used for easier clicking on smaller objects. This would make it very fun selecting units that are between the moon and the planet.
The bad side: I'm not sure how SOASE targets planets. Perhaps it attacks the center of the model file (center in the model editing program) and when the bullet hits a object belonging to the model, it explodes (which would be easier on the engine, since planets vary in shape and size). Or, it might pick the center of all the objects (aka: If you have the planet and moon far apart, it will pick the center to 'attack'), which would look very stupid.
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Method Two (use this if first method doesn't work, obviously)
In SOASE b3, there are sometimes asteroids around the planet.
The astoids themselves are either many, individual asteroids strewn around randomly, or are part of a single file.
If it's individual asteroids, than you can probably modify how many asteroids, rotation speed, ect for each one.
If it's a single file, than this just made it alot easier.
Switch the asteroid file with a 'planet' file. The planet won't rotate, and I don't think it will have collision (since it would be stupid to do so, waste of resources since they're outside of the gravity well) and might 'glitch' with interspace lines. (lines going through the moon, making a bad effect)