Planet 5

The fifth planet of the Sol system, most often called Planet 5 but possibly named Thyrop-Minerva (PROSE: The Taking of Planet 5) Hestia (PROSE: Oblivion) or Asteris, (AUDIO: Origins of the Cybermen) was the home planet of the "Star People", (PROSE: White Darkness) the Fendahl, and the Memeovore. (PROSE: The Taking of Planet 5)

History
At some point in the distant past, when the Great Old Ones were still present across the universe but after the Earth reptiles had gone into hibernation, the Star People created the Fendahl as genetically engineered servitors. However, they were overpowered by their creations, which went on to hunt the Great Old Ones throughout the universe. (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. (TV: Image of the Fendahl) Prior to this, the planet had been linked to the goddess Hestia. (PROSE: The Rise and Fall of Señor 105)

The time loop enabled the last species left on Planet 5 to evolve into a lifeform that could feed on Fendahl, (PROSE: The Taking of Planet 5) the Memeovore. (PROSE: The Book of the War)

During the War against the Enemy, Time Lords from the Eighth Doctor's subjective future planned to use a fleet of War TARDISes to undo the time barriers around Planet 5 in order to harness the Fendahl as a weapon against the Enemy; however, this instead resulted in the release of the Memeovore. (PROSE: The Taking of Planet 5)

By the 20th century, many Earth cosmologists believed that the asteroid belt was the remains of an ancient planet. The ArcHivists corroborated this with evidence from Image of the Fendahl. The ancient planet was named "Asteris" by ArcHivist Farazea. (AUDIO: Origins of the Cybermen)

Alternate timeline
In an alternate reality, the fifth planet Hestia existed in place of the asteroid belt. (PROSE: Oblivion)

Behind the scenes
Preceeding the explanation given in Image of the Fendahl, the missing planet of the solar system is covered in The Enigma of the Missing Planet, a feature in Terry Nation's Dalek Annual 1978. It speculated that some "super intelligent life form" found a way to motivate the planet out of its orbit, or that it was destroyed in one of the great Dalek Wars.