This was one of the planets where all humanoid life went extinct; four others were Cheron, Exo III, Kataan, and Scalos. (TOS: "What Are Little Girls Made Of?", "Wink of an Eye", "Let That Be Your Last Battlefield"; TNG: "The Inner Light")
The planet was subject to a massive cataclysmic event which destroyed all corporeal life there. When all life died, only a hundred or so of the resident civilization lived on as a pattern of lights and non-corporeal energy. (TOS: "The Lights of Zetar").