As it turns out, the aliens' homeworld was slowly dying out, leading them to develop an anti-gravitational system to move the entire planet out of its native star system and into the Sol System. This same technology is what allowed the planet to safely approach Earth without disturbing its orbit, moving it to a distance suitable for the teletransportation of an invasion army. A single alien agent was sent first, piloting a conventional spacecraft that landed in a small village in Scotland.