Tycho IV was the inhabited fourth planet of the Tycho system. The system was a thousand light years from Argus X. This was the homeworld of the sentient dikironium cloud creature. Lieutenant Commander Arlene Fukai, a crewmember aboard the USS Enterprise-D, was born on this non-Federation planet.
Tycho IV was initially visited by the Federation starship USS Farragut in 2257. During the mission, the Farragut crew encountered the planet's native dikironium cloud creature, an encounter that ultimately cost the lives of two hundred men, over half the crew, including the ship's commanding officer Captain Garrovick. Lieutenant James Kirk experienced survivor's guilt at being one of the lucky half of Farragut's crew to leave Tycho alive.
While Kirk hunted the creature years later after encountering it again on Argus X as captain of the USS Enterprise, he followed up on a hunch which led him to Tycho IV. The creature was preparing to reproduce by fission, into thousands and had to be stopped. Assisted by Ensign Garrovick, the son of his former captain, he beamed down to Tycho IV with an antimatter bomb, which destroyed the creature, cratered the planet, and blasted half its atmosphere into space. (TOS: "Obsession")