What would be the best way for an advanced alien civilization to make its presence known across the galaxy? It might be to make the planets in its star system dance in a mathematical sequence.
Many searches for intelligent life have been conducted to dates, such as looking for radio signals or even megastructures surrounding stars. Still, the results from these can be ambiguous and have potential natural explanations. That is why Sean Raymond at the University of Bordeaux, France, and his colleagues have proposed we could look for planets orbiting in specific patterns that could have only arisen artificially.
Their idea is that a sufficiently advanced civilization could alter the orbits of planets in its solar system to act as a beacon of its existence to an outside observer. Over thousands of years, the gravitational pull of asteroids could be used to slowly move planets into different positions, making them have complete orbits relative to each other in a particular set ratio or resonance. “In theory, it’s totally doable,” says Raymond.
Such resonances form naturally through gravitational interactions. For example, Jupiter’s moon Io completes four orbits of Jupiter for every two of Europa’s and one of Ganymede’s, a chain of 2:1 resonances. Patterns like these have been seen in other star systems, while those where the ratios differ between each body, rather than forming chains, have never been seen.
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Because of this, the team suggests aliens could arrange their planets in sequences that are unlikely to form naturally, such as a prime number sequence (1, 2, 3, 5, 7, and so on) or the Fibonacci sequence (1, 1, 2, 3, 5 and so on). Simulating these orbits, the researchers found they could last for 20 billion years before falling out of resonance – even surviving the death of their host star.
Jason Wright at Pennsylvania State University says the idea has similarities to monumental structures on Earth, such as the most giant pyramids of ancient Egypt, designed to endure beyond the lifetime of a civilization. “This beacon has the ability to greatly outlive its creators,” he says.
While only a thought experiment, for now, Raymond says it might be worth looking at exoplanetary systems to see if any show evidence of unnatural orbital architectures. “We can look through planet databases for resonant chains,” he says. For example, the TRAPPIST-1 system, which is about 39 light-years away, has an unusual resonance, and there are already plans to beam a message there from Earth.
Ultimately, even if we did find such a sequence, that alone wouldn’t be proof of intelligent life, and it is possible that any civilization behind it might be long gone. “But we could still find the relics of it,” says Raymond.
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