Planetary Shields are, effectively, an improvement upon Deflection Shields. They are 50% cheaper on Moons and 25% cheaper on Asteroids than on Planets. They're often used in pairs with Planetary Rings.
Like Deflector's, these inflict minimal damage on attacking forces. Rather, they are intended to absorb the power of the attacking fleet and lower the damage inflicted to the defending turrets' total armour. This makes it possible for more defending turrets to be regenerated after the first wave of the attack, when the defences are rebalanced according to the total percentage of armour left. To counter the effect of Planetary Shields, the attacker must bring more fleet to the fight in order to overwhelm the defences in a single attack wave.