In the radio show Area 53, Marvin Trill talks about a recently discovered planet, but doesn't mention any details with regard to the planet. However, on his website, Marvin Trill discusses migrating to a planet in his spaceship due to apocalyptic and Alien threats. Steve Henderson, in his blog, mentions that the secret planet is known as "Planet X." These strange talks have led players to ponder over the existence of such a planet in the game.
Some players have theorized that the planet is located somewhere in the Rockstar constellation. However, no such planet has ever been found in the game or its internal files. Coincidentally, Dwarf Planet Eris was also discovered right after the game's release. Although the official discovery was announced in January 2005, it was a debated subject since October 2003, when it was first observed. This may be possible that the concept of Planet X is based off Eris.