Calufrax

Calufrax seemed to be a cold oblate spheroid planet with a diameter of about 14,000 kilometres, but in fact was one of the disguised six segments of the Key to Time.

As a planet it was rich in voolium and madranite one-five, and the only place where they were known to naturally occur. This drew it to the attention of the crew of the pirate planet Zanak who needed the crystals to subdue the psychic Mentiads. However, Fibuli noted that it was "in an unexpected sector", and was hesitant in recommending the planet to the Captain, saying there was something "rather curious" about it.

It was the last world captured by Zanak and reduced to a trophy the size of a football. Its metricized structure, having a variable atomic weight, betrayed its origin as not a natural planet, but the second segment of the Key to Time. It was recovered by the Fourth Doctor and Romana I.

Prior to this, the Doctor had visited Calufrax, which he found to be completely uninhabited, cold, wet and ice-coated. He considered the prospect of returning to Calufrax to be "paralysingly dull, boring, and tedious", and was immediately disappointed that the tracer indicated that one of the segments of the Key to Time was there. He described it further as a "horrible place" and a "mean little planet". (TV: The Pirate Planet)

Behind the scenes
Calufrax and the other carefully balanced planets inspired the Daleks' use of missing planets to transport the Earth in The Stolen Earth and Journey's End, one of which was named Callufrax Minor seemingly in reference to the planet.

Calufrax