Galorndon Core

Galorndon Core is a planet located a half light year from the Romulan Neutral Zone. Ranging from class M to class L conditions, its atmosphere is ionized. Making most starship and other technological systems inoperable. Including transporters. Humanoids exposed over long periods of time to this planetary energy field are prone to (treatable) neurological damage.

(TNG episodes : The Enemy, The Defector, Unification, Part II,, Star Trek: Generations (game))

Miscellaneous
It is not known if the planet Galor IV TNG episode The Offspring is near or part of this system. Many have speculated this sector of Neutral Zone space is along the Romulan/Cardassian borders. For the Galor-class Cardassian warship.