Threshold viewed from space
Threshold was a Gas Giant that provided a foothold for Installation 04 to orbit around. It was designed as a failsafe protocol for Installation 04 in case the ring was ever destroyed, which it was thanks to Johnny and Cortana's little fireworks display. When the ring was destroyed, the debris was pulled into Threshold to be destroyed and what little remnants of the ring remained were stabilised so they could not drift away.
Threshold has only one moon, designated Basis by the Humans.
Threshold was also the site of a Forerunner Gas Mine on which the Heretics set up their base. This was also the sight of the Threshold Gas Mine Incident.