From Halopedia, the Halo wiki
Unified Earth Government
Hephestes is a gas giant located within the Tisiphone system.
The magnetospheres of Hephestes and its moon, Nephis, interact to form the Hephestes-Nephis flux tube, a torus of superconducting plasma. The Nephis Coalition extracts zenostium plasma, a critical component in the manufacture of antigravity plates, from the flux tube.