Threshold

Threshold is a planet in the Soell star system. It is an immense gas giant, Diameter: 214,604km (133,349 miles), with one satellite, its moon Basis. 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, it 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 is 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.

The Forerunners then chose the planet as a staging point to construct one of their massive fortress worlds, called Halo and numbered as Installation 04. Installation 04 was constructed on the Lagrange point between Threshold and Basis. It was designed to destroy all sentient life in the galaxy, thereby killing the Flood through 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 formally been on Basis) before the Arbiter 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.

Size
Threshold is an immense gas giant. Far larger than Jupiter, the largest planet in our solar 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.
 * Threshold is described as purple in the book, Halo: The Flood, though throughout Halo: CE and Halo 2, it is only shown as orange.
 * Threshold could possibly have a Portal located on the planet due to the raging storm that forms under the Gas Mine that resembles the storm over the Ruin/Crater of New Mombasa.
 * It is speculated that Portals could be used to transport replacement Halo Installations from the Ark to the location of the Halo it replaced, but that would not explain why the portal would already be activated, unless it was automatically activated when building of the replacement Halo started.