Substance
From Halopedia, the Halo wiki
Substance is the planet which Installation 05 orbits.
Summary
Substance is a greenish-blue gas giant. While not named during the game, the file containing the planet's graphics were found to be named "Substance". Similarly, there was a file called "Threshold and Basis" in Halo: Combat Evolved. It is similar to the planet Uranus, due to its color and mass.[1]
Unlike on Installation 04 where Threshold can be seen quite clearly (especially on the level The Truth and Reconciliation), on Installation 05, Substance is very difficult to see in the sky. In fact, none of the levels in Halo 2 (except for the cutscenes) can the planet be seen. This may suggest that Installation 05 has a denser atmosphere than Installation 04, or simply that the locations featured in Halo 2 have thicker clouds. Additionally, the locations at which the levels take place could be positioned relative to Substance such that Substance is 'beneath' the ring, and thus impossible to see.
In the levels Delta Halo and Regret, the other side of the ring can barely be seen in the sky. There is also a heavy storm front in the direction of the Sentinel Wall, and the levels Sacred Icon, and Quarantine Zone are based inside of a heavy snowstorm. These may be reasons why Substance is not visible from Installation 05.
It is very likely that Substance is host to a Gas Mining Facility similar to the one found on Threshold. The two planets are very similar, and they both are orbited by their own Halos. Substance is only featured in Halo 2 and Halo: Ghosts of Onyx.
