One civilisation’s space trash may be another’s space treasure. Technologically advanced aliens on distant worlds could be revealed by the satellites and other space junk orbiting their planets.
When we think of signs of civilisation that may be visible around other worlds, many focus on those that require extreme technological advancement, like giant lasers or Dyson spheres – huge artificial structures that harvest power from a star.
There aren’t nearly as many ways to look for civilisations that have advanced to about our own technological maturity. But at the Canary Islands Institute of Astrophysics in Spain calculated that it may be possible to find these civilised – but not extraordinarily advanced – worlds by looking for the satellites that orbit them.
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Ring around the world
Many types of satellites work best in geosynchronous orbits, which match the planet’s rotation so that the satellites remain over the same general location on the surface. This can be key for telecommunications, Earth observation, and surveillance satellites.
These orbits are all at about the same altitude above the surface – on Earth, about 35,786 kilometres up. Because of this, geosynchronous satellites form a ring around the planet known as the Clarke belt.
Socas-Navarro calculated that the opacity of Earth’s Clarke belt has increased exponentially over the last 15 years. He found that if this trend continues, it will be observable from nearby alien worlds around the year 2200.
If we know a planet’s mass and rotation period, we can determine the radius of the Clarke belt. Then, when the planet passes between its star and us, we can look for an additional dip in the starlight as the strip of satellite blocks it out.
For that dip to be observable from tens of light years away, Socas-Navarro determined that it would need between 10 billion and 1 trillion satellites, each with an average radius of 1 metre and mass of 100 kilograms. For reference, there are only a few thousand satellites orbiting Earth now.
Winning lottery ticket
Launching a trillion satellites into similar orbits is very different from launching a million: at that density, it would be extraordinarily difficult to keep them from forming clumps and crashing together. “It’s like building the pyramids,” says at Harvard University. “Each building block is easy, but putting it together is the hard engineering task.”
Socas-Navarro says this may actually be a plus: a civilisation could build a Dyson sphere and then die off, but a Clarke belt requires active management – which means their operators would be alive, not dead or gone.
If nearby planets like Proxima b had enough satellites, Socas-Navarro calculated that we would have already been able to detect them with existing telescopes. Rings or moons around other planets should cause similar signals, though, and we haven’t found any of those yet despite exoplanet surveys.
“The nice thing is that looking for Clarke exobelts comes for free with the search for moons and rings,” says Socas-Navarro. “It’s as if someone gives you a free lottery ticket. You know it’s utterly unlikely that you’ll win the prize but wouldn’t you check, just in case?”
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