Xenon

Xenon was the native planet of the metamorphic Whifferdills, including the Sixth Doctor's companion Frobisher. It was located in the Mutter's Spiral (COMIC: Voyager) and had an atmosphere similar to Earth. The Fourth Doctor's companion Milena was the daughter of Xenon's ruler, an old friend of the Doctor's. (PROSE: Milena)

Behind the scenes

 * There is also a planet called Xenon in the computer game series "".
 * Milena is called a Zenonian, with a 'Z' instead of an 'X.' Although it is never shown, as a native of the planet it's possible she is also a Wifferdill.