Peter Capaldi Auditioned for Star Trek: Deep Space Nine

Departing Star Trek: Deep Space Nine.

Capaldi has portrayed the mercurial Time Lord for three seasons, after taking over for Matt Smith in 2013. A lifelong fan of the series, Capaldi decided to bow out after this year's Christmas special, 'Twice Upon A Time,' that will also see the departure of longtime Who showrunner Steven Moffat and the debut of Jodie Whittaker, the first woman to portray the Doctor.

Related: Doctor Who Christmas Special Trailer

But Capaldi could have had an entirely different sci-fi legacy. A documentary about Star Trek: Deep Space Nine is currently in production to commemorate the series' 25th anniversary in 2018, and the producers of the film have responded to fan trekdocs's discovery of documents showing Capaldi was up to be the series' lead with visual evidence of his audition on Twitter.

We can do you one better, @trekdocs. Here's a sneak peek at some original audition tapes from 1992. #DS9 #AuditionTapes #WhatWeLeftBehind pic.twitter.com/zT56qpkxtB— The DS9 Documentary (@DS9Doc) September 7, 2017

The tweet also confirms that Anthony Stewart Head auditioned for the part. Head would go on to TV legend status for his role as Rupert Giles on Buffy The Vampire Slayer.

Aside from the obvious novelty of a future Doctor almost becoming a Starfleet captain, this suggests the producers of Deep Space Nine were at one point considering a massively different version of Benjamin Sisko. The role would eventually be played by the fantastic Avery Brooks, the first non-white lead in Star Trek's history. Sisko was framed as something of a reaction against Star Trek: The Next Generation's Captain Picard. Whereas Patrick Stewart embodied that character with his signature British gravitas and an air of intellectual purity, Sisko was a much more volatile character. Haunted by the death of his wife and attempting to raise his young son on what was essentially the wild west of space, the character shared much more DNA with William Shatner's swashbuckling Captain Kirk than with Picard.

It's easy to suspect Capaldi and Head would have been similar takes on Stewart's staid, brainy Starfleet captain, and while there's nothing wrong with that take - especially when you can get Stewart to play it - Brooks' emotionally raw Sisko was a defining tenet of Deep Space Nine, and helped set it apart from its Star Trek contemporaries.

It worked out for everyone in the end, as Capaldi got to live out his childhood aspirations to board the TARDIS and Deep Space Nine cast the right man for the job, but it's hard not to wonder what the Twelfth Doctor would have looked like in a Starfleet uniform.