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Cosmic interloper Oumuamua may not have been aliens, but a strange ice cube

The first interstellar visitor ever observed may have been the most pristine frozen object humans have seen.

This artist's impression shows Oumuamua, an interstellar asteroid.
ESO/M. Kornmesser

It's been over two and a half years since astronomers first spotted Oumuamua, a weird oblong object from beyond our solar system cruising through our cosmic neighborhood. Its shape and sudden acceleration away from our planet had some scientists suggesting it could be anything from a bizarre asteroid to an alien ship of some kind. 

Now, yet another explanation could account for all of Oumuamua's strangeness: astronomers from the University of Chicago and Yale believe it was the cleanest, most pristine ice cube ever seen.

"It's a frozen iceberg of molecular hydrogen," Darryl Seligman, an incoming University of Chicago postdoctoral fellow, explained in a release. "This explains every mysterious property about it. And if it's true, it's likely that the galaxy is full of similar objects."

Seligman authored a paper accepted for publication in a future issue of Astrophysical Journal Letters laying out the theory.   

One of the more baffling properties of Oumuamua was that it seemed to speed up like a comet, which is propelled by ice being burned up by the sun, but it had no tail made up of the resulting gases like a comet does.

But it could be that Oumuamua's tail was just invisible to the telescopes that observed it. Seligman and his colleagues found  molecular hydrogen ice is the only substance that can explain all the weirdness. It doesn't produce or reflect any light as it burns up, which would explain why Oumuamua had no observable tail. The scientists also calculated that it could have given the object the burst of speed seen as it flew back out to deep space.

Molecular hydrogen freezes solid just a bit above absolute zero and this only happens in odd places in the Milky Way, like a giant molecular cloud of freezing hydrogen and helium. 

"It would be the most pristine primordial matter in the galaxy," Seligman said. "It's like the galaxy made it, and FedExed it out straight to us." 

He adds that the interaction between energetic particles in space and the frozen hydrogen could also explain Oumuamua's unique shape. 

"Imagine what happens to a bar of soap," he said. "It starts out as a fairly regular rectangle, but as you use it up, it gets smaller and thinner over time." 

Seligman thinks it was particles from our own sun that probably had the most impact in reshaping Oumuamua. That means that if astronomers catch an incoming hydrogen iceberg in the future, it might be possible to witness it being whittled down and prove the theory. 

Unless, of course, the real explanation is that some distant alien civilization is using molecular hydrogen ice as rocket fuel.