Planet X (Cybertron)

There is no such place as Planet X. It exists solely as a codeword, an identifier for an underground army of loosely-affiliated Transformer criminals. The true number of "Planet X's" forces is unknown, and many of their identities are similarly shrouded in mystery.

What is known about Planet X is that the warriors, thieves, assassins, and other dubious characters who claim a link to it are the survivors of a dead planet, their homeworld consumed long ago by the limitless growth of the Giant Planet. The few survivors, driven mad in their quest for vengeance, made a deal with no less a force than Unicron himself, offering their services in exchange for the destruction of the hated Giant Planet.

The Decepticons Sideways and Soundwave are currently the only known Planet X operatives.

Note: If Soundblaster is indeed a new character, he is certainly a Planet X agent as well. It is also theoretically possible that earlier Unicron Trilogy characters with Unicronian affiliations, such as Nemesis Prime and Ravenus could be connected to Planet X.

From Teletraan I: The Transformers Wiki, a Wikia wiki.