Esselven Minor was a planetoid orbiting a white dwarf.
It was one hundred and seven kilometres in diameter, less than 350 kilometres in circumference with the distance from equator to pole just over 80 kilometres. Esselven Minor had a core composed of degenerate or collapsed matter, giving it the mass of a small planet. The Sixth Doctor speculated that the world may have been artificial.
The planetoid was used by the royal family of Esselven for private retreats. When Glavis Judd invaded Esselven, King Hathold and some others escaped there. Whilst trying to shore up the planetary shield, there was an accident which caused time to run many times faster inside the shield than outside. The Sixth Doctor was able to deactivate the shield, bringing the planetoid back into temporal synch with the rest of the universe. (PROSE: Palace of the Red Sun)