Planet Nine: A Science Fiction Series

In real life, Planet Nine is a hypothetical planet about ten times the mass of Earth that could explain the oddly-shaped orbits of certain dwarf planets in the outer Solar System.  If Planet Nine exists, it would orbit the sun far outside the orbits of the other planets, which is why it has never been detected.

In the story, Planet Nine is a rocky world about eight times as massive as Earth.  It orbits the sun every 1,500 years and is inhabited by several intelligent species and lots of fungi.  Instead of receiving heat from the sun, Planet Nine is warmed by volcanic eruptions that release heat into the air, where it is trapped by a thick cloud cover.