SETI enthusiasts have devised all sorts of complicated ways for us to find signs of alien life, but a new paper suggests we may be overthinking it. Instead of looking for megastructures and spaceships, we should consider something a bit more obvious: Alien satellites and space junk in orbit around distant exoplanets.
Artist's conception of an exoplanet and its satellite-laden Clarke Exobelt moving in front of its host star. GIF: Instituto de Astrofísica de Canarias/Gizmodo
Sufficiently dense fleets of satellites in geosynchronous orbit around exoplanets should be detectable from Earth using current technologies, according to new research published in The Astrophysical Journal.
Hector Socas-Navarro, an astronomer at the Instituto de Astrofísica de Canarias and the sole author of the new study, says we could do it using the transit method of detection, which is the same technique used to sniff out exoplanets. He argues that a ring of satellites and accumulating space junk should produce a characteristic light curve signature when an exoplanet passes in front of its host star from our perspective on Earth.
Intriguingly, he says this strategy could help us find alien civilisations at a similar level of technological development to our own.
The search for extraterrestrial intelligence (SETI) began in earnest in the 1960s when scientists started scanning for alien radio signals. This approach has yielding nothing, highlighting the need for alternative strategies. In recent years, scientists have proposed that we look for alien megastructures, such as Dyson spheres, and other technosignatures, such as signs of radically advanced propulsion systems, antimatter power plants, and traces of asteroid mining.
A fundamental limitation with these strategies, however, is that they presuppose the existence of super-advanced extraterrestrial intelligences (ETIs) - a completely hypothetical stage of development. Advanced aliens may not exist (which would be sad), but we do know from our own experience that moderately advanced civilisations, if we can call ourselves that, do exist.
Socas-Navarro's proposal that we search for alien satellites is exciting because, he argues, it's possible to find these so-called moderately advanced ETIs, and we already have, or soon will have, the tools and techniques to do it.
Using the transit method, astronomers have detected hundreds of exoplanets over the past three decades. We're also entering into a new era in which scientists can discern the chemical elements found in the atmospheres of these distant worlds. This capability will only get better in the future with the addition of the James Webb Space Telescope, the Giant Magellan Telescope, the European Extremely Large Telescope and Hawaii's Thirty-Meter Telescope (TMT).
Socas-Navarro's new paper suggests these tools could be used to search for artificial satellites and space junk in orbit around exoplanets. More specifically, he says we should be able to detect objects within a region of space around planets called the "Clarke Belt", named in honour of novelist Arthur C. Clarke, who published a paper in 1945 proposing the use of geostationary orbits for telecommunication satellites.
Socas-Navarro says we should be on the lookout for Clarke Exobelts (CEBs), which "is formed by all objects, including functioning devices and space junk, in geostationary and geosynchronous orbits around a planet," he writes in the new paper.
"A CEB does not require any technology that we do not have, only a more extensive use of orbital space. Perhaps their civilisation 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."
Artist's impression of the exoplanet Proxima Centauri b. Image: ESO
Indeed, for the CEB to be detectable from Earth it would have to be sufficiently thick, containing vast fleets of satellites and space junk. To that end, Socas-Navarro ran some simulations to determine just how thick, or opaque, these bands would need to be to produce a detectable light curve signature, or imprint, as an exoplanet moves across a star's disk.
Different stars produce different amounts of light, so the detectability of each CEB will be different. His calculations showed that it should be possible to detect CEBs around Proxima B and around several planets in the TRAPPIST-1 system. From Earth, we should see dips in luminosity at the right distance as the planet and its CEB moves across the host star.
Socas-Navarro says the signature produced by a CEB will be qualitatively different from a natural ring, like the one around Saturn.
A diagram of space junk in orbit around Earth. Note the difference between objects in Low Earth Orbit (nearest to Earth) and those in Geosynchronous orbit (the outer ring). Objects are not to scale. Image: NASA
Our own Clarke Belt, which consists of geostationary and geosynchronous satellites, isn't dense enough to be detectable at interstellar distances, he argues. As it stands, about two-thirds of our satellites are in Low Earth Orbit (LEO), between 160 to 2000km above the surface. This distance makes it practically impossible for an alien civilisation to detect our satellites. By contrast, Earth's Clarke Belt is located 36,000km above Earth, but it's far less populated by satellites compared to LEO.
But as Socas-Navarro points out, the density of satellites in this orbit is growing at an exponential rate, and given its current rate of growth, our Clarke Belt should be detectable in about 180 to 200 years. This prediction comes with some caveats, as Socas-Navarro himself points out:
Obviously, this extrapolation should not be viewed as a prediction. There is no reason to assume that the current exponential growth will be sustained for another 200 years. It might slow down if the demand for orbital devices were to decline, or it could accelerate if new technologies were developed that either require or facilitate the addition of more devices. In this respect it is worth pointing out another Clarke invention: A "space elevator" system would tremendously facilitate access to a geostationary orbit, which is a natural place to stop, and would likely speed up the rate of [satellite] growth. In summary, the 2200 date is not even a rough guess of when humanity will reach the detectability threshold but rather an indication that this outcome is a reasonable expectation for the near future, given current trends.
Interestingly, and perhaps disturbingly, this means our civilisation will eventually be detectable whether we like it or not. With each satellite we add to GTO, we are getting closer to being discovered by an ETI. That may or may not be a good thing, and it's something we should probably think about.
Sure, we're also leaking radio signals, but they degrade terribly over vast distances, so the claim that we're already broadcasting ourselves across the Milky Way is grossly overstated.
A cool aspect of Socas-Navarro's proposed strategy is that it would cost us virtually nothing, as it can be done during routine searches for exoplanets. All it requires are astute astronomers who can discern the CEB signature in the observed light curves.
As a final note, while this approach appears to hold some promise, we don't know how long each ETI's CEB window will be open. It could be conceivably very short (only a few hundred years), as the transition from a moderate to an advanced state of technological development would see an extinguished CEB signature. For example, a civilisation wrapped within a Dyson shell would not have a detectable CEB.
Consequently, we'd have to be exceptionally lucky to detect ETIs using this method. But that doesn't mean we shouldn't try.