Substance

Substance, also referred to as Coelest IV, is a blue-white gas giant in the Coelest system. It is the fourth planet in the system and is the planet which Installation 05 orbits. Though it has a higher mass, it is similar to the planet Neptune in color and composition, being classified as an ice giant. Substance is orbited by eight satellites, only one of which, Opusculus, is a natural moon. The rest include Installation 05, and six dormant Forerunner facilities on predetermined orbital paths. An asteroid field was located close to Substance at 2.4 AU from Coelest.

Unlike on Installation 04, where its parent planet Threshold could be seen quite clearly, especially at night, on Installation 05, Substance is very difficult to see in the sky of Installation 05. In fact, in none of the locations on this ring seen by UNSC forces (namely the Control Room, the Library, and various ruined structures) can the planet be seen. This is due to the Delta Halo's stormy weather conditions caused by a Flood outbreak that occurred some two centuries before the Battle of Installation 05. During the Great Schism that began on Installation 05, Sangheili and Jiralhanae vessels battled each other over Substance.

Weeks after the battle and the subsequent Fall of High Charity, the picked up a dying San'Shyuum aboard a skiff drifting in a debris field irregularly orbiting Substance.

Trivia

 * While not named during Halo 2, 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. The planet was first officially identified in Halo: The Essential Visual Guide.
 * The Halo Encyclopedia (2009 edition) incorrectly states that Installation 04 orbits Substance. This error was not corrected in the 2011 edition.

List of appearances

 * Halo 2
 * Halo: Ghosts of Onyx
 * Halo: Broken Circle
 * Halo 2: Anniversary
 * Halo Mythos