From Lexicanum

A δ-class (delta class) or Dead World is an airless and completely sterile world, totally devoid of an atmosphere and native natural life, and unsuitable for supporting any. With the possible exception of Imperial facilities such as Research Stations, based on the planet, its population is otherwise nonexistent.

Some planets have always been dead worlds; others were originally living worlds, reduced to dead world status through apocalyptic events, such as exterminatus, Tyranid consumption and destructive internecine war.

A dead world's tithe grade is aptus non ("not applicable").¹

Notable Dead Worlds

• Adrantis Five
• Belis Corona
• El'Phanor
• Gryphonne IV
• Isstvan III
• Naogeddon
• Olympia
• Prandium
• Truan IX
• Tyran
• Valos
• Zhoros

Sources

• ¹: Warhammer 40,000 3rd Edition Rulebook p.115

Worlds of the Galaxy
Imperium	Adeptus Astartes Homeworld • Agri World • Cardinal World • Civilised World • Dead World • Death World • Desert World • Feral World • Feudal World • Forge World • Hive World • Ice World • Industrial World • Jungle World • Mining World • Ocean World • Paradise World • Penal World • Research Station • Shrine World
Xeno	Xenos World • Daemon World • Exodite World • Tomb World • Maiden World • Crone World • Ork World • Tau Empire • Craftworld