To create their Machine worlds, the Vex begin by digging deep into the planet's core where a world-eating machine converts the planet into another link in their intergalactic chain.  The process typically only takes a matter of days as Mercury was transformed unnervingly fast during the Collapse but for reasons unknown, the transformation of Venus was slowed until hundreds of years later.  Some believe that the Vex have structures in every celestial object. In the process, most of the Planet's matter, down to it's molecular structure is convered into vex material.
Mercury and Nessus are the only known Machine Worlds in the Sol System. Io's conversion is believed to have been stopped, or at least delayed, and the same could be said for Mars. Venus's conversion status is unknown
List of known machine worlds
List of appearances