Doctor Who Series 10: 14 Easter Eggs & References You Might've Missed In 'Smile'

4. United Earth

On the doors of the colonists' ship are emblems bearing the phrase 'United Earth.' While this might fly over the heads of Who fans, Star Trek lovers will recognise this immediately.

In that other long-running sci-fi show, United Earth is the planetary state formed when the world unifies following humanity's first contact with the Vulcans in the 22nd century. Apparently, the Doctor Who universe has its own version.

The planet the episode is set on, by the way, is Gliese 581D. In the real world, the Gliese star system really exists but, while there are definitely three planets in the system, the fourth is only conjectural. If it does exist, 581D is believed to be much larger than Earth.