I don't believe planets have shields in Sins. As a side-note antimatter is also seperate from shields. It allows your ships to use special attacks, and depletes when these attacks are used or your ships jump to phase space.

the planetary shield generator does really have a need fro antimatter. but what is does is reduce the damge done by planet bombarment if fully upgrade it can reduce 75% of damge dont by siege frigets and cap ships

Ya, you pretty much have to destroy the generator first, since it can take forever to try and bombard a planet with one on it.  With full Emergency Facilities, and researching that tech that rebuilds faster after/during bombardment means it's even more resistant.

But I've not had a planet with a shield on it bombed yet since 1.1, so I'm not sure. . .

Correction, Planetary Shields can mitigate up to 80% of planet bombing damage.

Initially, they begin with 30% bombing mitigation, but a Tier 7 upgrade can increase this to 50%, and then 80%.

Planetary Shields do NOT stack, so two of them at max will not have an overall mitigation of 96%.