NPC took over home planet

My first run of the game and after an NPC civ declared war against me I set all of my combat craft on sentry around my home planet and suddenly - without any combat or even any invasion  - the NPC civ owned my home planet.  

My home planet had 80%+ approval so it was not a rebellion - there was no notification in the game - no battle, no enemy ships or transports, my home planet just changed ownership.  

If this is an intentional part of the game please let me know as I will no longer be playing it.