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The Planet Killer: Star Trek's Biggest Unsolved Mystery Explained

Throughout "Star Trek: The Original Series," Captain James T. Kirk (William Shatner) and the rest of the USS Enterprise faced off against a vast assortment of intergalactic villains –- ranging from sentient space clouds to invading alien commanders. 

Although the Enterprise usually meets these adversaries face-to-face, one "Star Trek" villain remains perhaps the most mysterious character in the franchise: the creator of the so-called "Planet Killer." The Planet Killer is the focus of the Season 2 episode "The Doomsday Machine," wherein the Enterprise investigates a solar system with seven planets that have been reduced to rubble. The crew quickly discovers this planetary massacre to be the work of an automated doomsday device, which is nearly invulnerable and consumes any planet it comes across, floating through the Milky Way.

Decades after this episode premiered in 1967, it's still unknown who constructed this device or why, though Spock (Leonard Nimoy) and Hikaru Sulu (George Takei) confirm that the Planet Killer came from somewhere outside our galaxy. Several theories are put forward in the episode, with Kirk guessing that the Planet Killer was built for some ancient extraplanetary war before somehow getting loose and Matt Decker (William Windom) suggesting that the Planet Killer is part living organism. Though the crew chooses to run with Kirk's theory, we're never given a concrete answer about the device's origins. The mystery behind the Planet Killer has haunted "Star Trek" fans for years.

A tie-in novel suggests an ancient civilization built this weapon to fight the Borg

The most popular prevailing theory for the Planet Killer's origin is that it was a weapon created by an ancient alien civilization to defeat the Borg — a hive mind of cybernetic invaders who have threatened the galaxy across a variety of "Star Trek" media.

This theory primarily comes from the non-canon "Star Trek: The Next Generation" tie-in novel, "Vendetta," by Peter David, which centers around an alien named Delcara, who is piloting a second Planet Killer in an attack against the Borg. It's revealed the first Planet Killer was a prototype intended to fly into Borg space and consume any planets the Borg previously assimilated. The much larger one Delcara is piloting is the completed version of the super-weapon. Perhaps the novel's most interesting aspect is that the Planet Killer is "haunted" by the psychic spirits of its mysterious creators, whose own race was destroyed by the Borg. They support Delcara on her mission for vengeance.

The book also features a brief appearance from Captain Jean Luc-Picard (Patrick Stewart), who reviews the Planet Killer story from "The Original Series" and suggests the Planet Killer was hidden just outside our Milky Way rather than arriving from a new galaxy. This is later confirmed by the psychic "ghosts" aboard the weapon itself. Though we again don't receive any confirmation about who these haunting creators are, some have suggested they are related to an extinct super race known as the Preservers, who went around the galaxy helping up-and-coming civilizations from going extinct.

What are some other theories for the Planet Killer's origins?

Because this popular explanation comes from a non-canon novel, "Star Trek" fans online have spent years creating a plethora of theories about the Planet Killer's origins.

Reddit users agree the Planet Killer probably came from within the galaxy but were less bullish about the Borg connection posited by Peter David. Many suggested the Planet Killer simply came from one of the countless unknown species that live in the Milky Way. "The galaxy is full of thousands, if not millions of civilizations," argued u/roto_disc. "The odds that the Doomsday Machine was somehow related to the Borg are infinitesimal." "It was older then the borg," added u/ceweked. "The borg has only been around 1000 years or so."

U/Daystrominstitude posted an intriguing theory that claims the Planet Killer was intended to collect materials for the Dark Matter Anomaly from "Star Trek: Discovery," harvesting planets before returning to fuel the DMA. A more fun theory from u/Thaliur suggests that the machine originated in the "Star Wars” galaxy — citing the famous tagline "A long time ago in a galaxy far, far away" in reference to the Planet Killer's ancient origin. The fact is, "Star Trek" canon has yet to provide a concrete answer, and until we get an explanation, fans will continue to speculate.