The planet in halo 4... what can it be?

I have been wondering that after E3 2011 and that concept art vid.

I’ve been looking up in the section “Universe” and checked out “Locations”.
I found out this planet is similar to a Shield world (Planets shiny opening)

Then i saw many comments about this planet being called “Sigma 7”

What do you think it can be?

Here’s a picture of it:

It’s called Sigma 7 (at least by the fans), it’s Forerunner, it’s not Onyx and there are some funky landscapes inside.