Substance

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 was 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 it's color and mass.

Unlike on Installation 04 where Threshold can be seen quite clearly (especially on the Halo: Combat Evolved level The Truth and Reconciliation), on Installation 05, Substance is very difficult to see in the sky. In fact, in 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.

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 Shield 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 orbited by their own Halos. Substance is only featured in Halo 2 and Halo: Ghosts of Onyx.

Trivia

 * Substance may in fact be HD 108147 b.