spaceSpace and Physics

Mysterious Makeup Of Interstellar ‘Oumuamua May Have Just Been Explained


Madison Dapcevich


Madison Dapcevich

Freelance Writer and Fact-Checker

Madison is a freelance science reporter and full-time fact-checker based in the wild Rocky Mountains of western Montana.

Freelance Writer and Fact-Checker

Artist's concept of interstellar object1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system. ESO/M. Kornmesser

The mysterious makeup of the ‘Oumuamua (oh-MOO-a-MOO-a) interstellar object may have finally been explained in a new astrophysical analysis.

‘Oumuamua – Hawaiian for “messenger from afar arriving first” – may just be a big, giant “frozen iceberg of molecular hydrogen,” which could explain the object’s bizarre characteristics and, if true, suggests that the galaxy may be “full of similar objects,” according to a study published in Astrophysical Journal Letters.


On October 19, 2017, telescopes picked up our Solar System’s first interstellar visitor characterized by its elongated shape, measuring 10 times as long as it was wide. The strange object moved like a comet but with no apparent evidence of the tell-tale tail, leading some skygazers to speculate that it was an alien spacecraft (an idea quickly dismissed by further study and confirmed by the Search for Extra-Terrestrial Intelligence Institute). Its trajectory indicated that it had come from interstellar space, but the interstellar comet accelerated in a way that couldn’t be explained by gravity with chaotic spinning suggesting a violent past, perhaps once as a piece of a yet-to-be-discovered planet or a larger comet or planetoid.

This very deep combined image shows the interstellar object ‘Oumuamua at the center of the image. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving comet. ESO/K. Meech et al.

Typically, comets with a surface made of ice that pass close to the Sun will start to vaporize. Most comets have tails formed by this escaping gas emitted from the object reflecting in the sunlight, yet no such tail was visible on ‘Oumuamua. Building on research published arguing that the tail simply wasn’t visible to telescopes on Earth, astrophysicists from the University of Chicago and Yale worked backward to determine what the unseen substance might have been.

"The only kind of ice that really explains the acceleration is molecular hydrogen," said study coauthor Darryl Seligman, an incoming UChicago postdoctoral fellow, in a statement.

Molecular hydrogen (H2) is formed when the temperature is slightly higher than absolute zero and does not reflect light or produce light as it burns, explaining why telescopes wouldn’t have been able to detect it. Sublimation, the transition of a solid to a gas state, of H2 could explain ‘Oumuamua’s never-before-seen trajectory if a “significant fraction” were to have “continuously sublimated.”

'Oumuamua's trajectory through the Solar System. NASA

There are few places in the galaxy where H2 could have been made, among them being a dense core of massive, freezing clouds of hydrogen and helium where stars are believed to be created. Scientists cannot see within the cores of these clouds that Seligman describes as the “most pristine primordial matter in the galaxy”.

"That we saw one at all implies that there's a ton of these things out there," Seligman said. "The galaxy must be filled with these dark hydrogen icebergs. That's incredibly cool."

Studying ‘Oumuamua is perhaps the closest astrophysicists have come to exploring such celestial environments.

“It’s like the galaxy made it, and FedExed it out straight to us,” adds Seligman.


As for ‘Oumuamua’s unheard of shape, Seligman suggests that the object has likely moved through space for millions of years until it came into our Solar System, getting “smacked like a bug on a windshield” that resulted in intense radiation from the Sun after having been whittled away by energetic particles found in space and sunlight.

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

And if there are more objects like ‘Oumuamua in existence, the study authors conclude that a "galactic sea of unbound planetesimal-sized objects has potential consequences for star and planet formation” that may soon be detectable by Earth-bound telescopes. 



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