Threshold

Threshold is a planet in the Soell star system. It is an immense gas giant with a radius of 107,302 km (66,674.5 miles), and one natural satellite, Basis which has a radius of 11,924 kilometers. The atmosphere is made up of white Ammonia clouds, amber-colored Ammonium Hydrosulfide clouds, Phosphorus compounds, ice crystals and other gases. Threshold is known to have storms in its atmosphere similar to the storms that occur on the planet Jupiter, since there was a large storm present at the Battle of the Gas Mine.

Threshold's Earth Survey Catalog number is B1008-AG.

Forerunner Activity
Over 100,000 years ago, Threshold was a part of the sizable and extensive Forerunner empire. They built at least one automated, viable Gas Mine complex in the upper atmosphere of the planet, likely harvesting the hydrogen gas of the planet. The hydrogen was probably used as transmutation material for construction, though this is not confirmed. Threshold was a neutral location.

Several hundred or thousand years later, upon the discovery of the Flood, the Forerunners refitted the Gas Mines to serve as Flood Research Facilities. These facilities proved only one thing: more extreme measures were needed to combat the Flood and to keep them from spreading.

The Forerunners, then chose the planet as a staging point to construct one of their massive fortress worlds, known as Halo and numbered as Installation 04. The Halo was constructed on the Lagrange point between Threshold and Basis and was designed to destroy all sentient life in the galaxy, thereby halting the spread of the Flood, due to starvation.

In 2552, the Covenant were looking for an artifact from Sigma Octanus IV to uncover this planet as the location of the "Sacred Ring", in order to start the Great Journey. The Fleet of Particular Justice followed the UNSC Pillar of Autumn there, and, in a series of events, Installation 04 was destroyed.

The Heretics then took refuge on the Gas Mine (they had been based at a makeshift camp on Basis until this point) before the Arbiter, Thel 'Vadamee, and his Special Ops troops dropped it into Threshold's depths. High Charity and its Fleet briefly visited Threshold as well, until they jumped to Installation 05. The UNSC prison ship the Mona Lisa ended up picking up Covenant survivors in the system including one infected by the Flood and ONI began experiments with the Flood on the ship before the Flood escaped and the Mona Lisa was destroyed by a nuclear missile fired by the UNSC Red Horse. Also in the system at the time was a single Covenant ship apparently left behind to monitor or guard the Halo's remains, which are gradually being pulled into Threshold and destroyed.

Size
Threshold is an immense gas giant, far larger than Jupiter from the Sol System. Its moon, Basis, is in fact the size of a huge rock planet. Basis' diameter is nearly double that of the Earth. A completely-to-scale accurate planetary comparison sheet authored by Stephen Loftus is available here.

Trivia

 * The inner core of Threshold is liquid nitrogen, as told by 343 Guilty Spark, during the battle with the Heretic Leader, Sesa 'Refumee.
 * Threshold is described as purple in the book, Halo: The Flood, though throughout Halo: Combat Evolved and Halo 2, it is only shown as orange. This may be caused by the position of the system's sun at different times or the proximity to the gas giant.
 * In Halo: Combat Evolved, the Assault Rifle's compass arrow points at the gas giant.
 * In the book Halo: First Strike, Cortana is able to jump into slipspace with the Covenant flag ship Ascendant Justice through Threshold's atmosphere.
 * Although Threshold has been charted by the UNSC, there is no explanation for how humanity failed to notice the Alpha Halo in its orbit. Humanity had charted Threshold and at least one satellite, its moon, Basis.