Intelligent aliens might be detected by looking for their TV and communications satellites, a new study suggests. Its author says such a search would look for moderately advanced civilisations using technology similar to ours.
Efforts to listen for messages from ET, using powerful radio telescopes, have so far drawn a blank, says Hector Socas-Navarro, of Spain’s Institute of Astronomy on Tenerife.
And other ideas have assumed very advanced technology, such as Dyson spheres constructed around stars to harness their energy, or the use of antimatter as a power source.
Dr Socas-Navarro suggests looking instead for alien satellites similar to the TV, communications and weather satellites in orbit around the Earth.
Such satellites are placed in so-called geostationary orbit, at a distance from Earth where their orbital speed would keep them permanently above the same spot on Earth as it rotated.
This geometry means that satellite TV dishes, for example, can remain fixed because the satellite relaying the broadcast stays in the same direction in the sky.
Because this locked position can only be held at a particular altitude, it is a region of space that becomes more and more crowded as more geostationary satellites are launched.
Dr Socas-Navarro says that if this zone was packed densely enough with geostationary satellites, then current telescope technology would be able to detect it around a planet passing in transit across the face of its host star.
To sync with the Earth’s rotation, a geostationary satellite must be at an altitude of approximately 35,786 km (22,236 miles) above sea level, which is nearly a tenth the way to the Moon.
Altitudes above alien planets would vary, depending on their mass and speed of rotation, but the principle would be the same. Dr Socas-Navarro calls this collection of satellites the Clarke exobelt (CEB) after visionary Sir Arthur C Clarke, who first proposed its use for communications.
In his paper, Dr Socas-Navarro says: “The CEB is formed by all objects, including functioning devices and space junk, in geostationary and geosynchronous orbits around a planet.
“A moderately advanced civilization might eventually populate it with a relatively high density of objects.”
He added: “Artificial satellites in geostationary and geosynchronous orbits are useful to us for a number of purposes, including telecommunications, surveillance, wildfire control, geolocation, spionage, wildlife tracking as well as other scientific studies and civil or military applications.”
Dr Socas-Navarro expects the denser outer edges of the belt of satellites, as seen from Earth, would reveal themselves by dimming the light from the star being transited. It is a bit like how we observe planetary nebulae in the sky. These are shells of gas blown off by a star, but they look like rings because we see through a denser stretch of material at the edges.
Our own belt of geostationary satellites would not be dense enough to be detected from another star, says the paper. However, our belt is becoming increasingly populated.
Dr Socas-Navarro examines the possibility that similar dips in brightness during a planet’s transit could be due to a ring system. But he believes planets with rings are more likely to be found beyond the ice line in a star system, caused by fragmentation of icy material, rather than around rocky worlds in the inner habitable zone.
He says: “In our own Solar System we find that all the planets beyond the ice line have rings, in addition to other smaller bodies such as Haumea, Chariklo and, possibly, Chiron. In contrast, none of the inner Solar System bodies have rings.”
Dr Socas-Navarro adds: “A CEB does not require of any technology that we do not have, only a more extensive use of orbital space.
“Perhaps their civilization is older than ours and has had more time to populate it. Or perhaps it has been driven by a stronger push for space devices, for reasons that we could only speculate about.”
Dr Socas-Navarro says the best candidates to check for satellites are tidally-locked planets orbiting red dwarf stars. Alien-hunters say the light from brighter stars would overpower the faint signals from their planets.
He adds: “An initial difficulty would be how to distinguish between a CEB and a ring system. However, once a candidate has been identified, detailed follow-up observations may resolve this ambiguity from the shape of the light curve.
“In any case, the detection of a dense belt of objects at the distance of geostationary orbit would be a very strong evidence for the presence of ETI (extra-terrestrial intelligence), especially considering that rings around habitable rocky planets are probably rather uncommon.”
The research is published in the Astrophysical Journal, and the preprint can be read at Arxiv.org.
★ Keep up with space news and observing tips. Click here to sign up for alerts to our latest reports. No spam ever - we promise!