Default: 69 (Super Star Destroyer), all ships owned by races 4 and 10 (depending on PlanetsAttackKlingons and PlanetsAttackRebels)
Numeric Value: 29
Ships with this ability cannot be attacked by planets with ATT or NUK. They only fight a planet if they choose to attack it.
This ability does not give you any special protection against other ships. Other ships can attack a PlanetImmunity ship as usual.
Relevant Configuration Options: AllowPlanetAttacks, PlanetsAttackKlingons, PlanetsAttackRebels.