The Advent already possess a form of this - increased shield mitigation when fighting under own culture.
It is also known that, in sports (soccer for example), when a team play in its home field, the odds of victory are increased, because they feel more obliged to protect their turf.
Similarly, this effect could be translated to the game, Having the planet owner having a upper hand (a small bonus initially) by virtue of being the defendant, and having this effect increasing depending on the amount of culture.
