X-Planets is a catch-all, to include extrasolar planets (by far the most common), but also Planets X (previously unknown, hypothetical, or essentially fictional planets in orbit around our Sun) and exploding planets (such as the EPH of TVF).
Duck Dodgers in the 24 1/2th Century (1953): Daffy plays Duck Dodgers in this great science fiction spoof, with Porky as his eager (and much smarter) first mate. Daffy must set out to find Planet X and recover the coveted "shaving cream atom." According to Daffy, they will begin their journey by going "30,600 miles due 'up'." Porky suggests they simply follow the planets in alphabetical order. Marvin the Martian makes an appearance in this cartoon, as well, and there's plenty of great pseudo-scientific gadgets, such as Daffy's "Ultimatum Dispatcher " that fires a bullet that unfurls a sign reading, "Surrender, or be blown into 17,670,002 micro cells." Of course, this leaves Marvin no choice but to use his "Ultimatum Answerer" which fires a bullet that simply shoots Daffy in the face.
regarding UB 313, "Lila" or "Xena", the 10th planet: