By Nola Taylor Redd
Alien civilizations with technology on a par with humanity’s could be detectable using today’s instruments. A new study suggests that if geostationary satellites are thick enough around an alien world, they could be spotted with telescopes already hunting for undiscovered planets.
Both governments and private corporations on our own world use geostationary satellites — which orbit such that they hover over the same spot on Earth — for science, communications, espionage and military applications.
If advanced alien civilizations loft enough satellites into their own geostationary belts, these spacecraft could create a dense, ring-like structure visible from Earth, according to the study.
“It’s … a small chance, but the point is that it’s free,” study lead author Hector Socas-Navarro, of Astrophysics Institute of the Canary Islands, told Space.com by email.
Socas-Navarro simulated the presence of belts of geostationary satellites around exoplanets, to see whether they could be detected by instruments like NASA’s Kepler space telescope and the agency’s recently launched Transiting Exoplanet Survey Satellite (TESS). He found that the belt would need to be about 0.01 percent full for such spacecraft to detect it, whether populated by many small satellites or a handful of large, city-size objects.
“We just need to look for the right signature in the data,” he said.
Socas-Navarro calls this hypothetical structure the Clarke exobelt (CEB), after famed sci-fi author Arthur C. Clarke.
Hunting alien satellites
Both Kepler and TESS detect planets using what’s known as the transit method. The spacecraft watch a field of stars for an extended amount of time. If a planet has the right orbit, and the timing is right, that world will pass in front of its host star from the telescope’s perspective, causing a small, potentially detectable dip in brightness.
In addition to working as an astrophysicist, Socas-Navarro hosts a weekly radio show and podcast. That work helped him come up with the Clarke exobelt idea, he said. One day, a listener asked about a geostationary satellite for the sun.
“As I was doing the calculations to answer this question, I had this mental image of a satellite transiting across the solar disk,” Socas-Navarro said. “That led me to ask myself the question of whether satellites around distant exoplanets would be observable during transit.”
Sufficient material orbiting an exoplanet causes a small dip in starlight before and after the body of the world makes its transit. Scientists have used this method to discover rings around planets outside the solar system and even around distant solar system bodies.
Socas-Navarro said the putative alien-satellite signal would have a signature similar to that of rings — both an exobelt and rings are made up of a swarm of objects orbiting a planet — but there are subtle technical differences in how that signature would look. The signal would also reveal the altitude of the orbiting objects, which could provide a significant clue as to whether the objects were natural or alien-made.
A ring system can occur at any number of distances from the surface of the planet. But if the objects orbited at a planet’s geostationary height — about 22,200 miles (35,700 kilometers) — they are “almost certainly artificial,” Socas-Navarro said.
Similarly, a massive space city or a large station close to a space elevator could look like an exomoon. Again, Socas-Navarro said, altitude is key. If the object hovers at geostationary height, it’s likely to be artificial. [10 Exoplanets That Could Host Alien Life]
“It doesn’t seem to matter too much if they are many small or [a] few large [objects],” he said. “As long as they are spread out all over the orbit, they will basically produce the same signature.”
He also found that the ideal conditions to spot such a satellite belt would be around dim red dwarf stars located within 100 light-years of Earth.
The new study was published last month in The Astrophysical Journal. You can read it for free at the online preprint server arXiv.org.