Vadia IX was the uninhabited ninth planet in the Vadia star system. It had a single moon. This was the long-forgotten homeworld of the Q, before their evolution into non-corporeal beings. Ancient remnants of their advanced civilization remained on the surface into the 23rd century, at which time the planet was controlled by the M'Kroon.

Mount Caleb was a famous mountain on this planet.

In 2261, Doctor Roger Korby conducted an archaeological dig on Vadia IX, assisted by Nurse Christine Chapel. They were noticed by Trelane, who proceeded to follow them to Starbase 1 and disrupt their relationship. (SNW: "Wedding Bell Blues")

Later that year, crew from the USS Enterprise assisted Korby in exploring an ancient site on the planet, which they would discover to be a temporally-shifted prison for the Vezda. (SNW: "Through the Lens of Time")