Class Y planet

In the Federation standard system of planetary classification, a class Y planet was characterized by a toxic atmosphere, sulfuric deserts, surface temperatures exceeding five hundred Kelvin, and thermionic radiation discharges. Due to the inhospitable and dangerous nature, and often hellish appearance, of such planets they were also nicknamed "Demon class" by Starfleet. Even entering a standard orbit of a class Y planet could be hazardous to starships. (VOY: "Demon", "Flesh and Blood")