Scottish mathematicians believe that 'self replicating' robotic space probes sent by alien civilisations could already be in our solar system.
The University of Edinburgh study warns that the probes are probably so hi-tech they appear invisible to us as we don't have technology advanced enough to detect them.
While our Voyager probe has just reached the edge of our solar system, older and more technologically advanced civilisations could have launched similar probes years before us that are already here.
Mathematicians Duncan Forgan and Arwen Nicholson believe that alien races could have sent probes that 'slingshot' round stars using their gravity to cover ground quickly, Yahoo! News reported.
The researchers analysed how a 'fleet' of probes could travel through space in a paper published in the International Journal of Astrobiology.