Phostris was a planet in the RE 461 galaxy. It was a rocky, desolate planet that was uninhabited until the Hyperspace Pioneers arrived there. At some point after the launch of the Pioneer 1 space capsule containing a Terran cat, Mitzi, a mysterious intelligence took control of the cat's body and gave her intelligence similar to a human. She landed the capsule on the planet and used it to create lush forests and gardens, allowing her to live on the planet. When the pioneers arrived, they were enslaved by Mitzi, who wanted to wage war on the human race as revenge for domesticating cats on Earth. Eventually, she was defeated by the Tenth Doctor and Rose and the planet was abandoned. (PROSE: The Cat Came Back)