Trapezium Seven

At some point during their travels together prior to the Flux debacle, Thirteenth Doctor and Yasmin Khan participated in a "high-gravity circus workshop" on Trapezium Seven, finishing as top of the class. (TV: The Halloween Apocalypse)

Behind the scenes

 * Half a decade prior to The Halloween Apocalypse, the short story The Baker Street Dozen (in Tales of the Great Detectives) mentioned a planet Trapezium. It is unknown if this was an intentional reference and, at any rate, whether the two planets can be understood to be precisely the same; this is however unlikely, as "high gravity" would imply inhabitants with short, stout body plans, whereas the posthumans of Trapezium in the short story were known for their elongated limbs. It is probable, however, that both Trapeziums are part of a single solar system, as per usual planetary naming conventions, and that the lower-gravity Trapezium has a number which went unmentioned in The Baker Street Dozen.