Sub-Moon

Inspired by Galileo's Planet Pack, a mod for Kerbal Space Program, I decided to make this simulation with four celestial bodies. There's the star at the center (Alpha Centauri A renamed), the planet orbiting the star (Jupiter renamed), the moon of the planet (Venus renamed), and the moon of the moon (Pluto renamed to "Sub-Moon"). I don't know if such a thing could naturally form, and the game just calls it a sattelite of the moon here. Why did I make Venus a moon? Its counterpart in the mod's stellar system has traits of both a super-Earth and a gas giant while orbiting a gas giant. Its sub-moon has an "overblown atmosphere" and enough liquid for spacecraft to splash down.