No aliens needed: 'Oumuamua's behaviour has a natural explanation, scientists say
Release of hydrogen gas led to the unexpected acceleration of the interstellar object
A new theory about why the interstellar object 'Oumuamua acted so strangely when it descended into our solar system 2017 suggests a simple and natural explanation for its odd behaviour.
Two scientists are now suggesting that hydrogen gas released by solar heat escaping from the object could explain an unexpected acceleration — which others have speculated might be the sign that it was an alien spaceship.
'Oumuamua was the first interstellar object ever observed in our solar system. It was also unlike anything astronomers had ever seen before.
As it passed the sun it began to travel faster, as if something was pushing it. This is actually normal behaviour for a comet, which is pushed by water ice evaporating from its surface. But unlike a comet, 'Oumuamua didn't have any noticeable tail — the signature of that kind of propulsion.
"We couldn't detect any of the small dust that comes out with the gas [in comets] and we couldn't detect any of the gas molecules that are normally associated with this kind of behaviour," said Jenny Bergner, an astrochemist from the University Of California, Berkeley.
Bergner became increasingly interested in getting to the bottom of this mystery after learning about the exotic ideas others had to explain why 'Oumuamua sped up faster than could be explained by gravity alone.
A previous study suggested that 'Oumuamua wasn't exposed to enough solar heat to evaporate water ice and trigger its acceleration. The study suggested it might have been made up, in part, of frozen hydrogen and it was releasing hydrogen gas as the sun warmed it up.
However that was an unsatisfying explanation for some scientists. Bergner said not only have we never observed anything like a hydrogen iceberg before, but theoretical models can't explain how such an object could be formed in the first place. That got her wondering about research that had been done in her field of astrochemistry.
"In my field, a lot of people are really interested in what happens when you irradiate ices with types of radiation, like what you would find in interstellar regions," she said.
Previous research had found that when cosmic rays from interstellar space bombard water ice, it can knock hydrogen off some water molecules and transforms the ice into a fluffy, porous structure where hydrogen can get trapped. Then as the ice is heated, the hydrogen escapes, providing thrust.
"We thought this could be a way to produce hydrogen, which is a really compelling candidate to explain the outgassing (...), but using a more normal, natural scenario to explain why it's there in the first place," Bergner said in an interview with Quirks & Quarks.
Bergner and co-author Darryl Seligman's research was published in the journal Nature.
Not everyone agrees
Avi Loeb, the Harvard University astronomer who's been arguing since 'Oumuamua first appeared that its bizarre behaviour could be best explained by invoking an artificial alien source, wrote that he doesn't buy Bergner and Sligman's new theory.
In a blog post on Medium, and in a yet-to-be-published research paper, he writes that their theory — and the hydrogen iceberg theory — both miss a key point.
"If hydrogen evaporates easily from water ice, then it would not survive the [interstellar] journey, just as in the case of a pure hydrogen iceberg. These issues are not addressed in the new paper," wrote Loeb.
In an interview with The Current's Matt Galloway back in January 2021, Leob said other scientists should open their minds to a potential artificial origin for 'Oumuamua.
"They just dismiss it because of prejudice, because it takes them out of their comfort zone," he said.
A testable theory but, as yet, no conclusive proof
Bergner admitted that we may never really know what 'Oumuamua was because it's now long gone. But she said we should be able to test the process she's proposing.
"We expect that any icy planetesimal things like comets in the outer solar system should also experience this conversion of water to molecular hydrogen," she said.
Seeing this phenomenon in one of these objects if it were to travel to the inner solar system "would be as close to conclusive proof as we can get," she said.
Written by Sonya Buyting.