Planet 5, sometimes referred to only as the fifth planet and possibly named Hestia (PROSE: Oblivion) or Thyrop-Minerva (PROSE: The Taking of Planet 5) was the home planet of the "Star People" and more notably the Fendahl, the perfect predator organism.
At some point in the distant past when the Great Old Ones were present across the universe and the Earth reptiles had gone into hibernation, the Star People, having created the Fendahl as genetically engineered servitors, were the first to be attacked by the creatures. The Old Ones fled from the Fendahl, which may have been weapons in a war being fought by the Old Ones. (PROSE: White Darkness)
Consequently, circa 12,000,000 BC, the Time Lords removed Planet 5 from the Sol system by placing it in a time loop. As a result, the fifth planet and all records of it were made permanently invisible. 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 Planet 5 in order to release the Fendahl. (PROSE: The Taking of Planet 5)