I was playing a simultaneous game, and the ships in orbit were getting a battle every turn.
They just sat back out of range, so I suppose the contested colony is not considered a valid target except by troop transports.
quote:
One thing that occurs is that the invasion makes the population unhappy quickly, so the defenders can only build new defenders there for a short time, unless there are construction bases to build more.
What about the defender "attacking" the planet with troop transports? Can you drop more defender troops to help out?
If the battle is going to be Lasting a year, you'd have plenty of time to collect some ships from neighbouring colonies/systems, smash the attacking fleet, and drop reinforcements.