Xenon was the native planet of the metamorphic Whifferdills, including the Sixth Doctor’s companion Frobisher. It was located in the Mutter’s Spiral (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. (Milena)
BEHIND THE SCENES
There is also a planet called Xenon in the computer game series “Space Quest”.
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.