Cinnabar was the capital planet of the Republic of Cinnabar, located in the star system of the same name.
It was settled before the Hiatus, and managed to stay out of the horrors of the Population Wars. When human interstellar travel resumed, it soon became a center of trade. (RCN2)
The planet was the location of the capital city of Xenos, seat of the Senate and the Navy School. (RCN1)