Gallifrey's moon

According to the work of one Gallifreyan author, the First Doctor took Gallifrey's moon with him when he escaped Gallifrey. (PROSE: A Brief History of Time Lords)

Across its history, the planet Gallifrey in fact had at least three moons. An unnamed satellite, the heavily industrialised Pazithi Gallifreya (COMIC: Agent Provocateur) and the lost moon, Botoya. Built into the latter was a device that could rewrite history on a grand scale, which was why the Doctor suspected the Ancients of Gallifrey had hidden the moon. Botoya subsequently became a legend in Gallifreyan culture, one not widely believed in. (AUDIO: The End of the Beginning)

Missy claimed to Clara Oswald that she had cared for the Doctor "since the night he stole the moon and the President's wife" amongst others. (TV: The Magician's Apprentice)