In other words, you can't do what you want. The alternative ifs what MittiKahn said, when a ship is completed, you launch it away from the enemy and have it run away. After you have a few run to the same place you create a fleet that can hopefully beat back the invasion. With this tactic you have to hope that your ships are fast enough to get away and that the enemy doent have so many ships that there is no safe place.

One more thing that might work, if you have good diplomacy and a lot to trade, you can possibly trade techs/money/trade gfoods for ships from an AI player. They will likely be away from the plants that are surrounded. Go visit the Dengin, that is if they arnt the ones attacking you.

Also if you feel your going to loose a planet, you can build transports and evacuate the population ready to invade again after they take it. It is just better that your population is used as invaders rather than defenders.

Other tricks to avoid loosing planets... leaving several planets ungarrisoned seems to trick the AI into not fleeting it's transports which leaves them vunerable to fast cargo ships (all engine with minimul weapon).

nice trick! Your ships doing a Farscape slingshot move! hehehe lol