Soell system

The Soell system is the planetary system in which Threshold, Basis, and Installation 04 are located. It is located in relative proximity to the Epsilon Eridani system within the Orion Arm of the Milky Way. Soell is orbited by nine planets: four outer planets and five inner planets separated by an asteroid belt.

Planetary system
The five inner planets of the Soell system all appear to be terrestrial. The innermost planet is small with one small natural satellite. The second planet is small and moonless. The third orbiting body is a tiny, cratered planet with two small natural satellites. The fourth planet is orbited by three moons. The fifth planet orbiting Soell is a terrestrial planet with a slight ovoid shape and a single natural satellite.

Beyond the relatively sparse asteroid belt, the sixth orbiting planet is a large, ringed planet. Threshold is the seventh and largest planet in Soell's orbit. Threshold is orbited by 12 satellites, including Basis, its largest satellite, and Installation 04. The eighth planet is a shattered, rocky world. The ninth planet orbiting Soell is a large, cratered terrestrial planet. Soell's outermost orbiting planet has an elliptical orbit, thus occasionally bringing it closer to Soell than the eighth closest planet.

Ancient history
During the waning years of the Forerunner Ecumene in the midst of the Forerunner-Flood war, the Soell system was chosen as the location for Installation 04, with the gas giant Threshold serving as its anchor. Facing utter annihilation at the hands of the Flood, the IsoDidact activated the installation, triggering the Halo Array and eradicating the Flood and all sentient life in the galaxy. For nearly a hundred thousand years the Soell system and its strategic significance went unnoticed even as the galaxy steadily repopulated.

Human-Covenant war
In 2552, the penultimate year of the Human-Covenant War, the humans of the UNSC Pillar of Autumn made a supposedly blind slipspace jump into the Soell system, having narrowly fled the Covenant who had completely destroyed the colony world of Reach. As the Autumn approached the ring, 343 Guilty Spark sent a signal to the human vessel threatening to activate the ring's defensive countermeasures if they did not retreat from its vicinity. However, upon recognizing the humans, the apparent Reclaimers of the Forerunner race, Guilty Spark disabled the ring's defenses and welcomed the humans to approach. During this, the Covenant Fleet of Particular Justice emerged, having beaten the humans to Soell and initiating an attack on the cruiser, thus triggering the climactic Battle of Installation 04.

In the days that followed, the humans and Covenant battled for control of the installation, both unaware of its true value as a weapon of mass destruction. During the prolonged engagement, the Covenant unknowingly unleashed the Flood, which had laid in stasis as the installation's vast army of Sentinels studied its nature. The parasite spread quickly and soon overtook both sides, threatening to use the Covenant's remaining ships to flee the Soell system and infect all life in the galaxy. 343 Guilty Spark sought the aid of Spartan-II supersoldier John-117 in activating the ring's defenses and with it the entire Halo Array. Instead, with the aid of Cortana the Spartan evaded both Spark and the Covenant invaders and detonated the Pillar of Autumn's fusion reactors, causing a chain reaction that destroyed Installation 04 and prevented the Flood from escaping the system.

In the days that followed, the was sent to investigate the system, encountering the human prison ship Mona Lisa in the vicinity of Installation 04's debris field. A team of Marines was sent to investigate, only to find the vessel overrun by Flood who had escaped containment. Recognizing the threat, the Red Horse was forced to destroy the Mona Lisa, destroying the surviving Flood once again.

The Covenant holy city of High Charity occupied the Soell system as the High Council publicly prosecuted Shipmaster Thel 'Vadamee for his failure to safeguard the ringworld. However, the Council opted to promote 'Vadamee to Arbiter, and tasked him with the elimination of a group of Heretics that had assembled on the Threshold gas mine, unaware of the Flood specimens also alive on board. The engagement saw the destruction of this facility, the Heretics, and all Flood encased within, as well as the capture of 343 Guilty Spark.

Following this battle, the Covenant abandoned the Soell system and relocated to the Coelest system to the newly-discovered Installation 05.

Post-war
After the conclusion of the Human-Covenant War that saw the final collapse of the Covenant, the Soell system fell into the newly-established Joint Occupation Zone and is under the strict supervision of the Sangheili-led Swords of Sanghelios and the United Nations Space Command.

The Soell system was chosen to host the replacement installations constructed by Installation 00 immediately after Halo's destruction. The first of these was destroyed in 2552 and the second was seized by the Created while in slipspace in 2559. The second replacement was launched to the Soell system by the crew of the specifically due to the system's proximity to human space: it was believed that the signal from a distress beacon placed on the new installation would be able to reach human space and alert them to the Spirit of Fire's predicament on Installation 00.

Trivia
The system is named after Matt Soell, a former Bungie employee.

List of appearances

 * Halo: The Fall of Reach
 * Halo: Combat Evolved
 * Halo: The Flood
 * Halo: First Strike
 * Halo 2
 * Halo Graphic Novel
 * Breaking Quarantine
 * The Last Voyage of the Infinite Succor
 * Halo Legends
 * Origins
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Mona Lisa
 * Halo: Reach
 * Halo: Combat Evolved Anniversary
 * Terminals
 * Halo: Fall of Reach
 * Invasion
 * Halo 2: Anniversary
 * Terminals
 * Halo 5: Guardians
 * ''Halo Mythos
 * Halo: Tales from Slipspace
 * Dominion Splinter
 * Halo Wars 2
 * Halo: Fireteam Raven
 * Halo: Official Spartan Field Manual