Dwarf planet

Dwarf planets were celestial bodies similar to actual planets, which were nevertheless not considered to fall under the definition of a planet. They were generally smaller than planets, but larger than planetoids and asteroids.

Dwarf planets in the Sol system included Ceres, Eris and Pluto. (AUDIO: The Bounty of Ceres)

While Pluto was considered a planet in the 20th century, it was classified as a dwarf planet Steven Taylor's times. (AUDIO: The Anachronauts) However, some still called it a planet in later eras. (TV: The Sun Makers)

A Cyberman Cyber-Citadel was located on a dwarf planet in the Kuiper Belt. (AUDIO: Last of the Cybermen)

Wupatki was a gas dwarf planet colonised by humans. It had a habitable, oxygenated atmosphere. (COMIC: The Singer Not the Song)

Behind the scenes
In the real world, Sedna is also considered to be a dwarf planet, but has not been referred to as such in-universe.