Ember (planet)

The planet Ember was a human colony dating back to the 38th century. (AUDIO: The Top of the Tree) It was, by the 44th century, a foggy Victorian era-esque planet that came to be controlled by the Sardick family in the 44th century. (TV: A Christmas Carol) It eventually became the refuge of the last humans in the universe. (AUDIO: Singularity) Using a psi-gate, they sent their minds back to 21st century Earth and took over the Somnus Foundation. They wanted to survive the end of the universe by turning the human race into a group mind. They were defeated by the Fifth Doctor and Turlough. Afterwards, the Doctor visited Ember to say goodbye to the dying Xen, the last surviving human in the universe. (AUDIO: Singularity)

Astronomical data
The planet was characterised by the cloud belt which covered the planet. These clouds were made of ice crystals with tiny electrical charges. The sky fish evolved to fly through these clouds, but they could only live outside them for a little while. (TV: A Christmas Carol)

Culture
The early colonists modelled Sardicktown on a Victorian urban environment, seeking to recreate the ambience and atmosphere of an earlier time period. Compromises had to be made and not everything could be brought in line with historical reality. (PROSE: The Whoniverse) They used Gideons as a currency. They originally called Christmas "the Crystal Feast", but it was later just called Christmas. There was no lottery.

Despite the planet having a president, he could not order/force the Sardick family to open the cloud belts despite the life-threatening nature the closed clouds caused spaceships. (TV: A Christmas Carol)

History
Ember was colonised by humans in the 38th century with help from the Eleventh Doctor and a young Kazran Sardick, a native of Ember from several centuries down the line, whom the Doctor had made his companion. (AUDIO: The Top of the Tree)

The Eleventh Doctor visited the planet when the ship Thrasymachus containing his companions, Amy Pond and Rory Williams, began to plummet through its atmosphere. The Doctor visited Sardicktown and convinced Kazran Sardick to allow the ship to land, but (after rewriting the man's history) he could no longer control the skies. However, they had Abigail Pettigrew sing for the fish instead, which calmed the atmosphere, enabling the ship to land. (TV: A Christmas Carol)

At a later point in universal history, the people of Ember were the last remaining humans in the cosmos. Using a psi-gate, they sent their minds back to 21st century Earth and took over the Somnus Foundation. They wanted to survive the end of the universe by turning the human race into a group mind. They were defeated by the Fifth Doctor and Turlough. Afterwards, the Doctor visited Ember to say goodbye to the dying Xen, the last surviving human in the universe. (AUDIO: Singularity)

Behind the scenes
The name of the planet was not mentioned onscreen in A Christmas Carol, and it is unlikely Steven Moffat intentionally meant to connect his planet to the one in AUDIO: Singularity. The link was instead drawn in the video game Worlds in Time, during which the planet was attacked by clockwork robots, and in non-narrative sections of The Brilliant Book 2012. Although neither of these is a valid source, the planet in A Christmas Carol would definitively be identified as Ember in AUDIO: The Top of the Tree.