Planet X was a planet where cities had taken over most of the landscape, except for the agrarian Forbidden Zone. It was inhabited by X-ians. Pebbles Flintstone and Bamm-Bamm Rubble, two Earthling babies, were taken here for study after the Imperial Council deemed planet Earth suitable for invasion. After passing all the tests with flying colors, the two babies were returned to Earth unharmed.
Several millennia later, the planet was invaded by Queen Octa-Loona and her warriors, who wiped out all the X-ians except for those hidden safely in the Forbidden Zone. Still no one knows what happened to the survivors.
Later still, in the twenty-fourth and a half century, Duck Dodgers and his “eager junior young space cadet” Porky Pig traveled to Planet X to find the last known source of illudium phosdex, the shaving cream atom. Though the planet by now was completely abandoned, it was very precious to Dodgers and when Marvin the Martian claimed the planet for Mars, they immediately declared a war that reduced the entire planet to almost nothing.