If an extraterrestrial civilization were to turn its scientific instruments toward Earth, what would it see? If its radio telescopes were on par with the kind we use today, it might notice large amounts of carbon dioxide in our atmosphere, a biomarker indicative of an advanced industrial civilization. If its radio telescopes were far more powerful than our own, it might be able to detect the faint electromagnetic radiation given off by our television broadcasts or radar defense networks scanning for launched nuclear warheads.
Other than that, there’d be little indication to an extraterrestrial astronomers that Earth hosted intelligent life. Yet according to new research by Hector Socas-Navarro, a physicist at the Instituto de Astrofisica de Canarias, in about 150 years Earth will probably have so many geostationary satellites in orbit that they could be detected by extraterrestrials. Moreover, by studying the patterns produced by dense collections of geostationary satellites, astronomers on Earth can begin looking for this type of signature among exoplanet data as a sign of intelligent life on other planets.
In 1945, the science fiction author Arthur C. Clarke wrote a paper for Wireless World that imagined a network of three crewed space stations in orbit around the Earth that would be capable of providing radio coverage to the entire world. These satellites would be in a geosynchronous orbit, meaning that they are far enough away and moving at a velocity such that they are always above the same fixed point on the Earth’s equator. This orbital region is 22,236 miles above the Earth and is often referred to as the Clarke belt.
At the time, Clarke’s idea may as well have been just another one of his science fiction stories, but in 1963, NASA launched the first satellite into geosynchronous orbit. This satellite, Syncom 2, was used to facilitate the first satellite phone call in history between US president John F. Kennedy and Nigerian prime minister Abubakar Tafawa Balewa. Among other things, the two leaders shot the shit about boxing. 50 years later, there are now well over 500 satellites in geosynchronous orbit providing everything from the GPS used on your cellphone to the data needed for weather forecasting, and boxing matches are broadcast live on satellite TV.
One of the most successful methods used by astronomers to discover new exoplanets is transmission spectroscopy. This basically looks for dips in the amount of light emitted by a star caused by a planet passing in front of it. The same thing technically happens when a satellite passes in front of a star, but the effect is far less noticeable because satellites are minuscule compared to an entire planet. Yet as Socas-Navarro found in his research, if you had a dense cloud of satellites in orbiting in the Clarke belt around an exoplanet, the light blocked by this satellite cloud could be detected by astronomers on Earth.
The distance of the Clarke belt from a host planet differs depending on factors like the mass of the planet and its rotational period. Astronomers are pretty good at determining these characteristics for some exoplanets (particularly those orbiting red dwarf stars), which means they’d be able to look for “Clarke exobelts” in a pretty precise region around a given exoplanet.
Geostationary satellites around Earth, for example, have basically circular orbits and only differ in their altitudes by 150 meters, which is almost negligible in the grand scheme of things. Moreover, the industry standards for geostationary satellites limit their spread to a 15 degree orbital inclination. In short, the distribution of geostationary satellites around our planet is pretty regular and within well defined bounds.
Using the distribution of geostationary satellites around Earth as a starting point, Socas-Navarro made a model that predicted what the signature of a dense belt of geostationary satellites would look like around an exoplanet. To do this, he calculated how much light is blocked by the Clarke belt as it moves in front of the Sun based on the density of satellites in that region and the size of the individual satellites.
So when would an extraterrestrial be able to spot Earth’s Clarke belt? Based on the “exponential” growth of geostationary satellites around Earth in the last 15 years, Socas-Navarro predicted that the signature of our geostationary satellite belt should be visible to extraterrestrial astronomers 10 light years away by about 2200, assuming we keep sending satellites up.
Looking outwards, our ability to detect Clarke exobelts will basically depend on the capabilities of the next generation of exoplanet hunting telescopes. In the paper, Socas-Navarro calculates the minimum opacity of a ring of satellites necessary to be detectable around some of the closest exoplanets to Earth, such as Proxima-b and the TRAPPIST-1 system, which has seven rocky planets in or around the habitable zone. The density of the satellite clouds required for them to be detectable mostly depends on the brightness of their host star. For example, the satellite ring around a planet in the Trappist system would have to be more dense than around Proxima-b because the Trappist star is significantly less bright.
Given that most other schemes for detecting technological civilizations around other planets involve imagining technologies that are far more advanced than our own (Dyson spheres, anyone?), Socas-Navarro’s work on detecting technological signatures of extraterrestrial civilizations with currently existing technology is super promising. As increasingly sophisticated exoplanet hunting telescopes like TESS are coming online, approaches like these may very well end up being the tell-tale sign that we’re not alone.