“Alien Probe or Galactic Driftwood?” asked Scientific American in December 2017. ‘Oumuamua was the first object ever detected to have come from out of our Solar System, travelling far too quickly to succumb to the Sun’s gravitational pull. Astronomers had spotted our interstellar visitor less than two months earlier, already on its way out of the Solar System, and were scrambling to decipher its secrets before it sped out of sight. Was it a comet or an asteroid – or maybe an extraterrestrial spacecraft?


The Scientific American article was referring to Breakthrough Listen, a Search for Extraterrestrial Intelligence (SETI) project who had announced their plan to probe the mysterious object for radio signals which could hint at alien life. Disappointingly for alien hunters, they came up empty-handed.

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Now, a year and a half later, the final nail has been driven into the ‘alien hypothesis’ coffin with a paper titled ‘The natural history of ‘Oumuamua’. “For all of the available observations that were made, the best answer that we have is that it was a natural object,” says Dr Michele Bannister, one of the paper’s authors.

And yet, the idea that aliens could be traversing our Solar System was tenacious, mostly because ‘Oumuamua was completely unlike anything we had seen before. For one thing, its brightness varied by a factor of ten, like a slow-motion twinkle. This suggests it could be a cigar-shaped object, ten times longer than it is wide, reflecting more or less light as it tumbles end over end.

That’s the familiar image of ‘Oumuamua, but in fact, we don’t know for sure what it looks like. Even the most powerful of our telescopes couldn’t resolve its shape beyond a point of brightness. The same glimmer could easily be produced by a pancake shape. Either way, no comet or asteroid in the Solar System has such an extreme ratio of length to width.

Over the short time it was within sight of our telescopes, astronomers around the world grappled with ‘Oumuamua’s identity. It looked like an asteroid. It showed no signs of cometary ‘outgassing’, the visible tail of evaporating gas as the Sun melted its ice. But then, remarkably, it accelerated out of the Solar System, faster than could be accounted for by gravity. Various means of providing this acceleration were floated, including a collision with another object and a combination of magnetism and solar winds, but the most outlandish theory was, once again, that it was an alien spacecraft.

In a paper published in the Astrophysical Journal Letters, Dr Shmuel Bialy and Professor Abraham Loeb, at Harvard University, laid out their logic to suggest that ‘Oumuamua could have been a solar sail. Just as the wind filling a ship’s sail is made up of a huge number of air molecules, the tiny impact of each photon of starlight bouncing off a large, thin solar sail could theoretically add up to enough force to power a spacecraft.

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However, there is a key problem with this theory: the tumbling motion that revealed its extreme shape in the first place. To work effectively as a solar sail, a broad surface needs to be pointing at the Sun. “That hypothesis does not fit with the evidence,” says Bannister, though the reason for its unusual acceleration is still unclear.

So, if ‘Oumuamua is not an alien spacecraft, what is it?

When taking into account all the evidence, Bannister says that it becomes clear that it is a planetesimal: “This is a little building block of a planet that started its life around another star and has travelled to us,” she says.

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Even if it is an entirely natural phenomenon, the chance to study the first ever object from outside the Solar System is exciting in its own right. “In many ways, this was the gravitational waves moment for people who study how solar systems form and evolve,” says Bannister.

Even better, we could explore the Solar System’s next visitor in much more detail after the launch of the European Space Agency’s Comet Interceptor mission in 2028. Launched without a particular target in mind, Comet Interceptor will park itself far out from Earth, waiting to chase an interesting comet or interstellar object.

“The next time ‘Oumuamua comes, we might be able to have a spacecraft waiting to go visit it,” says Bannister. “I’m really, really excited by that. It’s going to be so much fun.”

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.