Lunar Homeworld
This was the fifth planet of the solar system, once the home of the race of people who would become the Lunarians. In both the original Final Fantasy II and Final Fantasy IV Pixel Remaster scripts, FuSoYa indicates that this planet was once located between Mars and Jupiter. It’s thought that the asteroid belt is the remains of this world.
FuSoYa telling of the Lunar Homeworld
Though the planet existed and was destroyed in both the Bluestar and Darkstar dimensions, they were conceptualized differently by each Author.
The Bluestar world was called The Fifth Planet. It had a culture and design that leaned much more into technology and the science fiction elements of the Lunar race.
The Darkstar world was called Runne. While the Runnians did invest in technology, their origins were from Nefol, and leaned more into the fantasy and magical elements of the Lunar race.