They're called 'lurkers', and they may have been covertly surveilling us from space for millions of years – since before we even existed, perhaps.
That's the bold proposal being made in a new scientific paper by American physicist James Benford. But even though Benford's ideas sound radical, they draw upon a long history of conjecture in the SETI (Search for Extraterrestrial Intelligence) community.
In 1960, Stanford radiophysicist Ronald Bracewell first suggested the idea that "superior galactic communities" could disperse autonomous interstellar probes as "hypothetical feelers" throughout space in order to observe, monitor, and maybe even communicate with other life-forms, including those on Earth.
"A probe located nearby could bide its time while our civilisation developed technology that could find it, and, once contacted, could undertake a conversation in real time," Benford explains in his new paper.
"Meanwhile, it could have been routinely reporting back on our biosphere and civilisation for long eras."
But while this decades-old concept of Bracewell probes has been explored in subsequent research and embraced by science fiction – most notably as the eerie monolith in 2001: A Space Odyssey – there's never been any evidence for the existence of such robotic sentinels.
Now, Benford has proposed the ideal place where alien-made 'lurkers' could be present in our Solar System, stationed to observe in ever-watchful silence.
Like their name suggests, these quasi-satellites of Earth perform orbital loops around the Sun that are similar to Earth's own orbital pattern, and they do it in close proximity to Earth, being gravitationally bound to our own planet in addition to the Sun.
Only a small number of such objects has ever been found by astronomers. The closest known one to Earth, called 2016 HO3, is a small asteroid described by NASA as "Earth's constant companion".
"2016 HO3 loops around our planet, but never ventures very far away as we both go around the Sun," NASA NEO researcher Paul Chodas explained in 2016.
"In effect, this small asteroid is caught in a little dance with Earth."
But co-orbital objects could turn out to be much more than Earth's dancing partners, Benford suggests. Due to their constant orbital proximity, these nearby space rocks might offer an optimal vantage point for robotic probes seeking to keep tabs on us.
"These near-Earth objects provide an ideal way to watch our world from a secure natural object," his paper explains.
"That provides resources an ETI might need: materials, a firm anchor, and concealment."
Because of this possibility – and the fact that co-orbitals are indeed so close to Earth – the physicist argues investigating them should be a priority for SETI astronomers.
"We should move forthrightly toward observing them, both by observing them in the electromagnetic spectrum and planetary radar, as well as visiting them with probes," Benford writes.
Aside from the prospect of finding alien sentinels, it's a case that could make sense for other scientific reasons too – especially since we know so little about co-orbital objects, with less than 20 ever having been discovered.
As it happens, Benford may actually get his wish sooner rather than later.
China has already announced plans to launch an ambitious 10-year mission that would include visiting and collecting samples from 2016 HO3: a perfect opportunity to see up close if there's anything funny (or alien-y) about Earth's constant companion.
Not that others in the SETI community necessarily expect we'll find evidence of a grand alien technosignature.
"How likely is it that [an] alien probe would be on one of these co-orbitals? Obviously, extremely unlikely," theoretical physicist and astrobiologist Paul Davies from Arizona State University, who wasn't involved with Benford's research, told Live Science.
"But if it costs very little to go take a look, why not? Even if we don't find E.T., we might find something of interest."
The findings are reported in The Astronomical Journal.