Planet 5

Planet 5 (sometimes referred to only as "the fifth planet") was the home planet of the Fendahl, the perfect predator organism. Consequently, circa 12,000,000 BC, the Time Lords removed Planet 5 from the Sol system by placing it in a time loop. The Sol system's asteroid belt subsequently occupied the orbit of the planet, 107 million miles out from Earth, between Mars and Jupiter. (At least one Fendahl Core, "Eustace", nevertheless made its way to Earth.) (TV: Image of the Fendahl)

During the War with the Enemy, Time Lords from the Eighth Doctor's subjective future had planned to use a fleet of War TARDISes to undo the time barriers around the Planet 5 in order to release the Fendahl. (PROSE: The Taking of Planet 5)