Its the planet Onyx.
Wrong. Onyx was destroyed as the trillions of unique sentinels that make up the surface broke their formation and went in to space, subsequently destroying the Covenant Seperatist fleet above the planet. The only part of the planet left intact was the Dyson Sphere, a Slipspace bubble created by the Forerunners for the very purpose of shielding the planet's intelligent life from the effect of the Halo Array. Doctor Catherine Halsey and a team of Spartans was trapped inside the Sphere during the Battle of Onyx, before the planet literally fell to pieces. I don't think the Sphere would be visible after Onyx was destroyed, as it exists in another dimension. Thus, the planet was not Onyx. Also, it couldn't be the Forerunner homeworld, Ghibalb, as the planet was "dosed" with a massive level of radiological energy from nearby stars going supernova as a result of an astro-engineering accident. The planet would not look as it did in the Legendary ending if it were that heavily irradiated.