Planet (An Unearthly Child)

The original home planet of the Doctor and Susan in the 49th century was one of many potential origins among many due to shifting timelines and alterations to biodata.

Other universes
In a parallel universe, Martin Bannister was uncertain whether or not to make it explicit that Doctor Who and Susan Who came from Venus in the 49th century. (AUDIO: Deadline)

Behind the scenes
The unnamed planet in the 49th century, the home of Dr. Who and Susan, was first mentioned in The Pilot Episode, where it was explicitly mentioned as the origin of the characters; in the the televised story, this line is removed and made more ambiguous, although it should be noted that some of the dialogue still reflects the origin, even though it was never spoken about in so many words. Vague allusions to the Doctor's home, typically inferring that he and Susan were simply humans from another planet, were made over the course of the First and Second Doctor's eras, until the Time Lords were established in TV: The War Games with their development being initiated in the Third Doctor's era to evolve into the generally accepted "Time Lord" backstory of the Doctor.

Since then, despite the Doctor having numerous origins being an accepted and oft-mentioned part of the character's backstory, this origin has been very rarely referenced, with the only two references to it outside of the First and Second Doctor's eras being PROSE: Unnatural History and PROSE: Escape Velocity (though the planet was referenced in AUDIO: Deadline, which isn't set in the "main" Doctor Who universe), even then only entertaining it as a possibility as opposed to directly confirming it as a backstory. Other stories, such as PROSE: Doctor Who in an Exciting Adventure with the Daleks usually play with the general idea, albeit a different take upon it.

It was originally intended for Susan to be a princess from her home planet, which interestingly would've been established as a separate planet from the Doctor's. Thereafter, she was envisioned as a fugitive from the Doctor's home planet. It was Anthony Coburn who altered the character so that Susan became the Doctor's granddaughter, instead of being a biologically unrelated female teenager travelling with an old man.

In TV: The Sensorites, Susan's description of her home planet was reminiscent of Venus, albeit with a science-fiction spin. Venus being the homeworld Susan and the Doctor was later referenced in AUDIO: Deadline. However, this description was later retroactively applied to Gallifrey, becoming part of the well known design as seen in the post-2005 of Doctor Who. Due to how the concept of this planet was changed to become Gallifrey, it is quite reminiscent of Jewel.

In the unaired 1960s audio drama Journey into Time starring Peter Cushing, that version of the Doctor mentions that his civilisation was actually Earth, but three thousand years in Mike's future (who came from the mid-twentieth century).

A behind the scenes note on the Second Doctor had this to say about his origin: "[Dr. Who] is the eternal fugitive with a horrifying fear of the past horrors he has endured. (These horrors were experienced during the galactic war and account for his flight from his own planet.)"

- 49th century on the Fringes of War, Nate Bumber's Tumblr