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Substance is the name of the greenish-blue gas giant around which Installation 05 orbits. 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 unknown if there is a gas mine on Substance like Threshold, though it is probable. It may be similar to the planet Uranus due to it's color.[1]
Unlike on Installation 04 where you can see Threshold quite clearly (especially on the Halo: Combat Evolved level The Truth and Reconciliation), on Installation 05, Substance is very hard to see in the sky, indeed, in none of the levels in Halo 2 except for the cutscenes can you see the planet.
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 honored with the presence of their own Halos. Substance is only featured in Halo 2 and Halo: Ghosts of Onyx.