An alien civilization could shift six or seven planets into a series of carefully-arranged orbits as a signpost advertising its presence. To spot it, we just have to think mathematically.
Several bodies exist in what astronomers call resonant orbits around their parent body. For instance, for every orbit Jupiter’s moon Europa completes, another nearby moon called Io completes two orbits. For every two orbits completed by Europa, Ganymede — the next moon out — completes two, giving them a 4:2:1 resonance. There’s even a planetary system, K2-138, which has an entire fleet of planets in resonance.
In a recent paper in the journal Monthly Notices of the Royal Astronomical Society, planetary scientist Matthew Clement, of the Carnegie Institution for Science Earth and Planets Laboratory, and his colleagues suggest that a sufficiently powerful and motivated alien civilization could arrange entire Solar Systems of planets into resonant orbits with more complex ratios — adding up to a mathematical concept sprinkled throughout nature.
Besides doing it for the aesthetic, this ambitious planet-shuffling could help ET advertise their presence to distant searchers like us — no radio signals or laser beacons needed.
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