The planetary class for gas giants, enormous worlds of dense, turbulent atmosphere with no solid surface to land on, one of which becomes the battleground between the Defiant and the Jem'Hadar in Starship Down.
Class J is the Federation classification for a gas giant, a huge planet made up mostly of hydrogen and other gases with crushing pressures and violent weather in its depths. Such worlds cannot be landed on and are hazardous even for starships, whose sensors and shields are badly affected by the atmosphere.
The class has its largest role in the Deep Space Nine episode Starship Down in 2372. While Sisko holds trade talks with the Karemma in the Gamma Quadrant, Jem'Hadar ships attack and the fight moves into the atmosphere of a Class J gas giant. With sensors all but useless, the Defiant hunts and is hunted like a submarine, using probes as improvised sonar while damage traps the crew in separate sections of the ship.
Other series describe gas giants on a numbered scale instead, such as the Class 9 giant in Enterprise's Sleeping Dogs.