Space scientists may have missed alien probes because they're just too small.

That's the bold claim from an astrophysicist who reckons we've been looking for extraterrestrial life the wrong way.

The argument is an attempt to explain the Fermi Paradox, a decades-old thought experiment.

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It says that the universe is so unimaginably vast that the chance of alien life existing is quite high – and then asks, where are they?

No one really knows that answer, except perhaps astrophysicist Zaza Osmanov of Georgia's University of Tbilisi.

He thinks that aliens might be exploring the universe using tiny robots that measure just a single nanometre.

It's based on the idea of Von Neumann probes.

This is the theory that aliens could send self-replicating spacecraft out into the universe to explore in a risk-free way.

But these self-replicating probes could easily run out of new materials to function properly.

In a paper, Osmanov argues that these probes could instead be built on a nanoscale.

The robots could then use hydrogen atoms from interstellar dust to power themselves.

These nano-droids could already be floating around, but we may not have spotted them because we're looking for larger evidence of alien life.

"The conventional approach to the search for extraterrestrial intelligence implies the search for artificial radio signals, or interstellar beacons targeted at the Earth," Osmanov argues.

"But...the mentioned method strongly restricts the search."

In his paper, Osmanov reveals detailed calculations about how it may be possible to spot these nano-robots.

He says that they could easily self-replicate into a swarm numbering trillions of billions.

This swarm of probes would give off some light as they hoovered up photons along their journey.

And Osmanov says that they might look just like a comet several kilometers across, appearing as infrared light.

"All the...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," he wrote.

"We have considered the scenario when the Type-II civilization needs to 'invade' the interstellar clouds by means of the self-reproducing robots.

"And it has been shown that this process will inevitably lead to the observational consequences."

However, they would likely only be visible if they remained as a swarm – and would almost certainly be too small to spot if traveling on their own.

This story originally appeared in The Sun.