It will stay there doing nothing.
The current setup is such that a DS is only destroyed as a result of a battle. That means you can have a DS in a system where you don't exist, but it won't do anything as you can't be attacked in said system. Similarly, if you re-enter the system you can then have an active DS while only having a single planet.
The DS would then be destroyed appropriately upon your next planet loss.
Interestingly, I *think* this also means a system can have multiple families' defense systems in it. I've never seen this but so far I'm not seeing anything that prevents such a situation.
It's not clear whether this behavior is intended, a design error, or a bug.
Got a few bucks? The Imperial Tip Jar is accepting contributions!