By the power of truth, I, while living, have conquered the universe.
The Ark is outside the Milky Way, it's a safe haven for all who are on it because the firing of the Halos doesn't reach it. The planet in the center is called the Foundry, it's a giant ball of minerals that Sentinels gather in order to create a new Halo ring (should one be destroyed).
It was creating Installation 04B, but John destroyed the Ring by making it fire prematurely (before it was ready) and it collapsed in on itself. The debris of the Ring shattered the Ark and critically damaged it.
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Posted by: R0undedSnow
That "planet" you're seeing is most likely the ruins of high charity.
You're joking, right? Not only is High Charity not on the Ark when you first see it, we know what it is - as stated above, it's called the Foundry. The Halo Rings don't just appear by magic, the Sentinels harvest minerals and materials from the planet to make a new Ring if one is destroyed.
[Edited on 08.20.2010 3:37 AM PDT]