Calufrax

Calufrax was one of the planets 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, and was recovered by the Doctor and Romana. (DW: The Pirate Planet).

However, it was later stolen by Davros along with 26 other planets and moons.(DW: The Stolen Earth).