Aliens could say hello by arranging planets in prime number pattern

A sufficiently advanced alien civilisation would be able to arrange the orbits of the planets in its star system in a pattern that could never form naturally, signaling its existence to others

By Jonathan O’Callaghan

Odd planetary motion could be worth a closer look

Shutterstock/Vadim Sadovski

What would be the best way for an advanced alien civilisation to make its presence known across the galaxy? It might be to make the planets in its star system dance in mathematical sequence.

Many searches for intelligent life have been conducted to date, such as looking for radio signals or even megastructures surrounding stars, but 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 civilisation 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 differs between each body, rather than forming chains, have never been seen.

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 largest pyramids of ancient Egypt, designed to endure beyond the lifetime of a civilisation. “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 having a look 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 civilisation behind it might be long gone. “But we could still find the relics of it,” says Raymond.

Reference: arxiv.org/abs/2204.14259

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