Planet killer

The Planet Killer, sometimes (incorrectly) 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.

Note - in the original TOS Episode is was always called "The Planet Killer" only in a casual conversation is it compared to the "Doomsday" Weapons of the late 20th Century.