any empire capable of weilding a planatary invasion fleet is going to be capable of defeating even an invulnurable shield if nothing else they can set up satellites to stretch out a mylar film blocking the solar primary from ther planet which in most planatary systems will cause sever diffuculties
(sure theyve got fusion etc but hey fusin doesnt grow dinner and that would cause a huge additional power cost to begin planetary light and planetary heating talk about your seige tactics
)