The main point of interest on Planet X; Doctor Chaotica's Fortress of Doom
Planet X was a fictional planet featured in chapter 18 of Tom Paris' holodeck program The Adventures of Captain Proton, home to Doctor Chaotica's Fortress of Doom. As a nod to the practice of re-using sets in the 1930s serials Captain Proton was emulating, Planet X looked almost identical to the mines of Mercury, though it lacked an erupting volcano. (VOY: "Bride of Chaotica!")
The Star Trek Encyclopedia, 4th ed., vol. 2, p. 148 classified Planet X as a Class M world.