At the end of the mission to destroy the heart of the black garden your ghost says "We're back..on Mars" and in the recently added crucible map of The Black Garden, Pantheon, the garden is located on Mars. The garden itself was isolated from space and time, and it may be impossible to determine where it was originally from, but since it returned to Mars I just assumed it was originally apart of that planet. You are right about the ruins though, however, the Vex are prolific time travelers...