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The interstellar asteroid 鈥極umuamua has been making headlines since it was first discovered in 2017. Most controversially, Avi Loeb of Harvard University has suggested that it might be an alien probe, a claim that he has on in the face of criticism by other astronomers.

Loeb says no other explanation fits the asteroid鈥檚 unusual shape and acceleration, but now Amaya Moro-Martin at the Space Telescope Science Institute in Baltimore has an alternative suggestion: 鈥極umuamua could be a kind of giant interstellar snowflake.

The problem is, we didn鈥檛 get a very good look at 鈥極umuamua when it flew past in October 2017. Astronomers only saw that it brightened and dimmed by a factor of 10 every 8 hours, suggesting an elongated object tumbling end-on-end.

That led to a general agreement that it was a cigar-shaped asteroid, roughly 400 metres long. Yet one puzzle remained: the asteroid wasn鈥檛 just passively passing through the solar system, but actively accelerating, meaning its trajectory can鈥檛 be explained by momentum and gravity alone.

Loeb suggested that 鈥極umuamua was being pushed along by pressure exerted by the sun鈥檚 radiation, but that only works if it is a large, flat sheet less than 1 millimetre thick 鈥� in other words, on object of artificial origin, perhaps a solar sail.

Fractal structure

Moro-Martin has a more natural solution. She suggests that 鈥極umuamua be made from an icy, porous material, giving the asteroid an unusually low density and a fractal structure.

Fractals are self-similar objects, meaning their structures repeat at different scales, and are often found in nature. Moro-Martin thinks 鈥極umuamua could have been formed by the collision of icy particles in the discs of gas and dust surrounding newly formed stars, building a fractal structure much like a snowflake.

On Earth, we have only seen porous fractals on micrometre scales. 鈥滻t鈥檚 not really clear whether you can maintain fractal growth at metre or larger sizes鈥�, says Richard Alexander of the University of Leicester, UK, but it is plausible, he adds.

Michele Bannister of Queen鈥檚 University Belfast, UK says Moro-Martin鈥檚 idea extrapolates from what we know about planets form, though exactly how small particles become larger, asteroid-sized objects is still something of a mystery, because it鈥檚 hard for astronomers to observe objects of that size. 鈥淎 lot of what we can study is larger than 鈥極umuamua,鈥� she says, which is partly why its detection was so exciting.

For Moro-Martin鈥檚 idea to work, the asteroid would need to have a density around 0.01 kg/m³ 鈥� less than a hundredth that of air. Loeb says that such a low-density material would break up if it was spinning every 8 hours, as suggested by the brightness data, so the idea doesn鈥檛 work. 鈥淭he object cannot be held together by gravity,鈥� he says.

But Moro-Martin says this is only the case if we assume that 鈥極umuamua has a uniformly reflective surface. Get rid of that assumption, and 鈥極umuamua doesn鈥檛 have to be a fast-spinning cigar, she says.

There鈥檚 still work to be done to prove that such a low-density object could survive the journey into our solar system, including whether it would get condensed by pressure or could withstand impact from dust and gas, Moro-Martin admits.

Unfortunately, we can never be sure about 鈥極umuamua鈥檚 true nature, because the asteroid is now too far away for astronomers to observe. 鈥淲e will have to await future encounters with other interstellar interlopers,鈥� says Moro-Martin.

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