The planet with their giant super important simulation was being eaten by it (and might still be eating it, we only decoupled it from the sun)
I'm kind of surprised they haven't telaported onto it and tried shutting it down or converting it into something to nullify the sun like in the future that they attempted to make, and you gotta admit it'd make an interesting strike :p
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The vex don't fully control their simulation engine anymore as stated by osiris the many times I've cleaned up his rampant experiments. The relationship between mercury and the vex is in disarray because of the (underwhelming) events with osiris.