I have had combats Last more than a year (10 game turns), and eventually won.
What happens there, is the planet will remain under the control of the original owner, and both forces remain on the surface. The planet will be marked with a "Forces of the XYZ Empire are attempting to capture the planet"
Ground Combat resumes at the end of every subsequent turn until the conflict is won by one side or the other.
That makes sense then. Your troops can't stay on his planets if you sign a peace treaty with him. The game engine has no idea when a troop transport of yours will be around to pick them up (as the transport could easily have left, or been blown up by other enemy ships). So, it just gives them to the defender.
I guess it's a good enough decission on the computer's part... sure beats the whole game crashing!
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Jimbob
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