I have a ship in my system and want to use it to defend the system, not just a planet. I have tried putting it in Guard or Sentry Mode and have had several instances where enemy troops ships fly by unhindered to invade my planet, I didnt even know they were there until they invaded. Why didnt my ship intercept it?
Also, what is the point of having your ships defend a planet when any fleets they were in will be disbanded when you fly them to a planet. If you dont have a fleet manager, your ships will be destroyed one by one. If this is the case you will want your ships in fleets in the system in Guard or Sentry mode (which hasnt worked for me), and therefore other planetary defences will be useless since you dont have ships over the planets.