Planet Immunity is a passive ability granted to a ship or race that prevents that ship or the race from being attacked by enemy planets.
It is noteworthy that the Empire of Birds also carry partial planet immunity, but the scope is limited to ships that carry beams, but have no fuel. This partial immunity is necessary to prevent use of Super Spy Deluxe to force a planet to attack fuel-less Dark Wings (unless they have no beams to shoot down fighters).
There are also a few ship designs that carry Planet Immunity. These ships can never be attacked by planets, no matter who owns them. The most notable design, and the only design that comes standard in Planets Nu, is the feared Super Star Destroyer. However, Campaign mode introduced the Topaz and Ru squadrons, which carry Planet Immunity as one of their abilities.