The last incarnation of the game (FFE) had Ceres as a normal small rocky planet, as well as "asteroid-moons" like Phobos, Deimos, Amalthea around Jupiter, Miranda around Uranus and a couple other asteroid-sized moons; see the pic on
this website. In proc-genned systems these little asteroids never spawned as "planets", only as moons of other planets.Sol didn't have anything past Pluto, because in the early 1990s they hadn't been discovered yet.
Another possible lore reason for it's disappearance:
You know all those generation ships that went missing, as well as all the ones that didn't go missing but arrived safely at their destination? They had to make those things out of something. Giant starships capable of carrying 100,000 people don't just grow on trees.