Planetary Protection Shield

From Wikitroid, the Metroid wiki

The Planetary Protection Shield was created by the Chozo on Zebes to protect them from attack. It is known that they bolstered their defenses from the first Space Pirate attack, and this may be one of the results of that. However, the second Pirate assault on Zebes (which won them the planet) broke through the shield. The Space Pirate Flagship used the gravitational field of a micro black hole to ultimately break the shield.

Mother Brain later remarks that her own "shield is made by molecule transference of Zebetite. It uses the same system that governs the planet's shield."