Brown Dwarfs

Failed stars: too heavy to be planets, too light to burn hydrogen.

Past about 13 Jupiter masses a ball of hydrogen gets hot enough in its core to fuse deuterium, and it stops being a planet at all. It is not a star either: that needs roughly 75 Jupiters, the mass at which ordinary hydrogen ignites. In between sits the brown dwarf, glowing on deuterium and the heat of its own contraction, cooling slowly over billions of years from dull red through magenta to barely visible. Strangely, piling on mass barely changes its size, because degeneracy pressure holds it near Jupiter's radius no matter how heavy it gets. Build one by pushing the mass slider past 13 Jupiters on a hydrogen-helium recipe.