A number of patches ago the planetary shields showed as a scaled-up Version of your race's ship shields. Before that, there was a buggy "ship sized" shield image on the top corner of the planet
. Since then, they've dissapeared.
Under AI/AI_construction_facilities.txt, Just add a call for "planet - shield generation", or in the case of P&N, use "Phased Shield Generation" for the top tech planetary shields.