Most of this has been copied from my thread at Bungie.net.
The Legendary Planet or the Unknown Planet is the planet, presumed to be Forerunner, which is seen glowing in the mystical light after the credits when you complete the last level on Legendary. More information can be found on the link above.
People have gone over the cutscene, found tiny details which you'd miss at first and basically made the whole thing a lot more confusing.
Allow me to either do the same or do the opposite.
There are theories that Chief got teleported somewhere when the portal collapsed, theories that he's travelled through time and finally, the most believable, he didn't teleport. He's drifting in space.
In the cutscene we can see the blue 'smoke' which is said to be the left over bits of the portal, so Chief hasn't gone too far, which means that planet is in roughly the same place as where Ark was.
Now lets return to the stuff we know. In fact, lets take a few steps back, lets go back to the level The Ark.
Towards the end of the level, Spark shows us a map of the Ark, how nice of him. Its very detailed, you've probably all gone back to this level, looked to the centre of the holo-map, seen Installation 04 being built and gone "Ooooh, it was there all the time!"
Was there anything else that caught your eye? Possibly something that wasn't Halo, the Core or the Ark? Something highlighted by pink box? How about a picture.
Want another? Under Guilty Spark
I'll be honest, I was playing on a mod when I saw this, not the campaign. I'm not going to say which mod its on or where to get it but I'll say that it has this holo-map on it.
I then went in to campaign, watched the cutscene and saw it! (View cutscenes here)
I've thought about this, at first I thought it could be 1 of 3 things.
1) The portal - The portal was big, but not that big.
If a Halo is slightly small than Earth then that sphere would be about the size of a large chunk of Europe.
We saw ships going through the portal on Earth. It wasn't that big. Unless for some reason the portal at the Ark is much larger. I think that's unlikely.
2) Artificial light source - I'm not sure about where the light comes from, I thought it came from a large light bulb, or a dish with a similar shape to the Ark itself (the 'Sun' on Valhalla has odd prongs coming out of it).
But it can't be a 'Sun' because you can see its either buried in the Ark or just below it, which would make it night time over the whole Ark and we know that its not because the level is set during the day.
3) The Legendary Planet - or, if it is then it would be a Planetoid, its smaller than our Moon.
it is suggested there were Forerunner installations which could survive the Halo Array. The Ark was one but only because it was outside of the Arrays range.
Guilty Spark even says he thought the Ark was a 'Shield Installation, it appears I [he] was mistaken'. But only partly mistaken, the Ark was not the Shield installation, the mysterious sphere within the Ark is (possibly).
The Forerunner apparently like to shove things out in the open (the Halo Array) or just bury them (the Ark Portal). No change here then. They buried a shield world, in case something went from on the Ark, a bunker.
Responses from the Bungie.net
GS said that that pink box is a superluminal communication array which Truth was at.
- Spark looks towards the other side of the Ark when he says that
- The Superluminal Communication Array is towards the centre of the Ark, at the big hole. We go there to kill him
This is the only suggested alternative I think it possible, I disagree with it.
Looking at your ppicture, and judging from the sky on the ark, I would say that sphere in the map room is where the cartographer is.
As I said before, the marked sphere is the size of a large chunk of Europe and when playing through the level The Ark we see that the Cartographer is in the middle of no where, on the edge of a desert; it is the largest structure and its not spherical.
Also, when Spark points out where the Cartographer is he doesn't look at the sphere.
So, still no certainty on what the planetoid is but I think its possible that it was buried in the Ark all along.