Solaris is an advanced society in the skies that would manipulate the balance of power and other key events on the surface more directly utilizing advanced technology and manifests from human experiments as a catalyst for that technology. Created 600-800 years prior to the modern events in Xenogears, it was led by Cain, the Gazel Ministry, and, later, Krelian. Its capital is Etrenank.
In modern timelines of Xenogears, it is separated from surface via dimensional gates that were put in place to prevent its destruction that had almost occurred in the events 500 years prior in the Great War and subsequently events known as the Diabolos Collapse. Knowledge of Solaris and its existence was then erased from all surface dwellers as an added countermeasure.