New computer simulations performed by researchers at the University of Edinburgh suggest that space probes constructed by aliens are likely to have visited Earth on multiple occasions throughout our planet's history. Visitations are not just possible; they're probable.

A fairly simple network of self-replicating space probes were shown to be capable of surveying the entire Milky Way galaxy within just 10 million years, according to the simulations. Given that the Earth is around 4.5 billion years old, that leaves plenty of time for alien probes to have investigated our planet many times over.

The results expand upon a previous study which showed that a single Voyager-like probe could explore the galaxy 100 times faster by utilizing 'slingshot' maneuvers around the stars it visits, rather than by relying on powered flight. Such maneuvers make use of a star's immense gravity to build up speed.

For the new simulations, researchers investigated how quickly a fleet of probes, programmed with a simple decision-making algorithm, could explore the galaxy using this method. They found that exploration timescales could be further shortened if the probes could self-replicate, i.e., if they were capable of independently making copies of themselves. This would allow the fleet to grow in size as the probes explore, allowing it to cover more ground.

“From the scaling of the probes’ performance with star number, we conclude that a fleet of self-replicating probes can indeed explore the galaxy in a sufficiently short time to warrant the existence of the Fermi Paradox,” concluded the research team.

The Fermi Paradox, originally proposed by physicists Enrico Fermi and Michael H. Hart, suggests that Earth should have been contacted by aliens by now, given the high probability that the Milky Way has contained numerous extraterrestrial civilizations throughout its history. That high probability is estimated by the famed Drake equation, which calculates that our galaxy, given its sheer size (100-400 billion stars), ought to be teeming with intelligent E.T.'s. The Fermi paradox arises because evidence for all of these supposed alien civilizations remains non-existent.

The new calculations by the University of Edinburgh researchers only heighten the paradox. They suggest that it would only have taken a single alien civilization, intelligent enough to invent and implement the proposed fleet of space probes, to make contact with Earth innumerable times throughout history. Since it has been estimated that our galaxy may have contained thousands of such civilizations, the odds of contact only increase.

So the question remains: Where are they? One possibility is that the probes' alien architects made them stealthy, so as not to be easily detected. It's also conceivable that we've just been unlucky. Modern astronomers aren't anywhere close to cataloging every single small object passing through the solar system. It's therefore possible that alien probes are out there, hiding in 'plain sight,' and we just haven't come across them yet.

Or maybe we really are all alone.

Related on MNN: