Empyreal worlds only occur in bright nebulae. Empyreal worlds generally have moderate deposites of trillum and hexacarbide and major deposits of aetherium and chtholon, but no chronimium. They require no special life support or medical equipment. They tend to have large base populations but do not support habitation upgrades. Empyreal worlds get a significant bonus to organic food production.
Empyreal is an uncommon synonym for empyrean, "relating to the heavens; celestial". The word "empyreal" is of ancient Greek origin, rooted in the word pûr, "fire"; it is unrelated to the word "imperial", which comes from the Latin root parō, "to prepare, obtain, or resolve".