Threshold

Summary
Threshold is a large orange gas giant with one moon, called Basis. over 100,000 years ago, it was a part of the Forerunner empire. They built at least one automated Gas Mine complex in the upper atmosphere of the planet, likely harvesting the Hydrogen gas of the planet. 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. The 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 later activated, destroying all sentient life in the galaxy, thereby killing the Flood through starvation.

In 2552, the Covenant used an artifact from Sigma Octanus IV to uncover this planet as the location of the Installation. They sent the Fleet of Particular Justice here. However, the UNSC Pillar of Autumn arrived as well, and in a series of events, Instalation 04 was destroyed.

The Heretics then took refuge on the Gas Mine before the Arbiter and his Special Ops troops dropped it into Threshold's depths. High Charity and its fleet briefly vivited Threshold as well, until they jumped to Installation 05.

Trivia
In Halo 1, the Assault Rifle has a small digital display that shows how much ammunition is left in the clip as well as a indicator that points towards Threshold, acting like a compass.