A scorching, toxic Class Y planet in the Delta Quadrant where Voyager seeks deuterium and discovers a mimetic fluid that copies the crew, as seen in the episodes Demon and Course: Oblivion.
A Demon planet is Starfleet slang for a Class Y world, with a poisonous atmosphere, surface temperatures of hundreds of degrees and radiation that endangers even a ship in orbit. In the Voyager episode Demon, set in 2374, a desperate fuel shortage forces the ship to land on one to collect deuterium.
Tom Paris and Harry Kim go missing on the surface and are later found able to breathe the atmosphere. They are duplicates, created by a silvery mimetic fluid that gains awareness by copying the life it touches. Janeway agrees to let it sample the whole crew, leaving a colony of copies behind.
Course: Oblivion reveals what became of them. The duplicates built their own Voyager, forgot their origin and set out for Earth, and they begin to break down far from the planet that sustains them.