The planet killer, sometimes called the "doomsday machine", was an automated, self-propelled weapon capable of destroying entire class-M planets. It was encountered in 2267 by the Federation starships USS Enterprise and USS Constellation. Its origins are unknown, but based on its apparent trajectory, it was believed to have come from outside the Milky Way Galaxy.
The device was rendered inert when the Constellation exploded inside its forward opening. The Enterprise placed a warning buoy near the device and proceeded to the nearest starbase for repairs. (TOS: "The Doomsday Machine")
In an alternate timeline, when the Constellation detonated it triggered a reaction in the weapon's chroniton emissions and sent it through time to 2255 where it encountered and destroyed the USS Enterprise commanded by Christopher Pike. It duplicated and plunged the Federation into a conflict known as the Doomsday War. Spock, on the planet Gateway, was protected from the changes in the timeline and sent the USS Farragut, commanded by James Kirk, back in time through the Guardian of Forever, and with the help of the USS Daedalus and the USS Enterprise was able to stop the planet killer and correct the timeline. (Star Trek: Phase II: "In Harm's Way")
In 2295, the USS Justice encountered the reactivated device. After a brief engagement in which most of the senior staff was injured, the device was eventually destroyed by a solar flare in system L-374. ("Return to Doomsday")
- Fan films:
In the original TOS episode it was referred to as "the planet killer". Only in casual conversation was it compared to the "doomsday" weapons of the late 20th century.