Dwarf planet
From Tardis Wiki, the free Doctor Who reference
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. Some of them were located in the Kuiper Belt.
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)
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.
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