What if alien spacecraft are already in our midst – and we don't know about them because they're tiny nano-structures? That's the new idea put forward by one scientist seeking to explain the Fermi paradox.

This paradox – named after Italian-American physicist Enrico Fermi – posits that because the Milky Way is so vast and so old, the chances of alien civilisations being out there is actually pretty high… so where are they?

Maybe they're buzzing around the Universe in micro-machines just a nanometre long, suggests astrophysicist Zaza Osmanov from the Free University of Tbilisi in Georgia. What's more, he's provided some pretty detailed calculations to go along with this slightly whacky-sounding idea.

Now it's worth mentioning that Osmanov's paper has yet to be peer-reviewed, so we can't place too much importance on it just yet – but it does open up an intriguing possibility about how other civilisations might one day reach us.

The new study is based on the established idea of von Neumann probes: the thinking goes that aliens could send out self-replicating spacecraft to explore the galaxies, without having to risk leaving their home planet themselves.

Previous attempts to figure out how von Neumann probes could function have hit a common problem – self-replication needs a steady supply of new materials, and it's not clear if deep space has enough suitable planets or rocky bodies to go around.

To get around the issue, Osmanov has shrunk the scale right down. Microscopic probes would need much less in the way of materials, and could rely on hydrogen atoms floating through the interstellar dust for power.

These nanoscale probes would, in theory, be much more efficient than any previously imagined, and could replicate in just a few years – getting around the Universe relatively quickly. They'd also become very numerous (up to the level of trillions of billions) in a few short years of steady reproduction.

And that brings us to the question of how to spot them – according to Osmanov, this swarm of exploring probes would give off luminous emissions as they gathered up protons in their flight.

If we were looking in the right direction, we should be able to see them, he says, and assuming they were moving in formation, they might look like a comet several kilometres across in the infrared part of the spectrum.

Back to star gazing then – Osmanov gives us something else to look for in the ongoing search for life beyond our planet.

The idea that other life forms could be out there but perhaps not yet visible to us – or not yet wanting to be visible – is one that's regularly put forward. It's just a question of our technology getting to the level for us to spot the aliens who potentially have us under observation.

"All the aforementioned results indicate that if one detects a strange object with extremely high values of luminosity increment, that might be a good sign to place the object in the list of extraterrestrial von Neumann probe candidates," writes Osmanov.

The research has been published on the pre-print server arXiv.org.