An interstellar object that is currently on its long journey back out of our Solar System has a completely natural explanation, in spite of its odd quirks.
The peculiar acceleration of 'Oumuamua, new research confirms, can be fully attributed to the release of molecular hydrogen gas.
This, according to astrochemist Jennifer Bergner of the University of California, Berkeley and astrophysicist Darryl Seligman of Cornell University, is further evidence that the cigar-shaped chunk of rock started off as a planet seed before being booted off to wander the galaxy untethered to a star.
It's an elegant solution, one that, the researchers write, "can explain many of 'Oumuamua's peculiar properties without fine-tuning" – or resorting to extraordinary claims about the object's nature.
'Oumuamua first appeared on our horizons in October 2017, a mere month after it made its closest pass to the Sun, looping around and speeding back out of the Solar System on its continuing journey through the cosmos.
We hadn't then – and we haven't since – seen anything like it in our home system, and it's still deeply interesting to astronomers.
First is its shape. 'Oumuamua is long and skinny like a cigar, measuring up to 400 meters (0.25 miles) in length. No other comet or asteroid in the Solar System has this shape.
'Oumuamua sort of spins as it goes, too, like a bottle on its side. And, although the object seems to contain no ice and emits no gasses that we can detect, as a comet would, its trajectory cannot be explained by gravity alone, as an asteroid's trajectory would.
Cometary outgassing as their ice sublimates gives comets an additional source of acceleration, consistent with what astronomers observed with 'Oumuamua. This suggests it is somehow similar to both a comet and an asteroid.
In the years since its visit, scientists have determined that 'Oumuamua is likely a shard broken off from a planetesimal, a baby planet still in the process of forming, that collided with another object.
Such collisions are not unheard of in a forming planetary system; our own Earth is believed to have been smashed into by another planet-sized object, breaking off a chunk that formed the Moon. In 'Oumuamua's case, the shard of planetesimal was ejected from its system entirely.
In 2020, Seligman co-authored a paper proposing that 'Oumuamua's acceleration could be attributed to the sublimation of molecular hydrogen (H2).
Molecular hydrogen is very difficult to detect in space, since it neither emits nor reflects light; if 'Oumuamua was outgassing molecular hydrogen, we wouldn't be able to see it the way we usually see tracers of cometary activity.
On the other hand, it's been suggested that it's not likely that 'Oumuamua is a molecular hydrogen iceberg, as the researchers proposed in 2020, so Bergner and Seligman went back to modeling to determine how the object might contain (and sublimate) molecular hydrogen.
They found that the explanation is plausible through the the irradiation of a body rich in water ice.
As ionizing radiation hits the object, radiolytic processes would split the water molecules to produce molecular hydrogen.
"In this model," they write in their paper, "'Oumuamua began as an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellar journey, and experienced warming during its passage through the Solar System."
An existing body of experimental evidence shows that the processing of water (H2O) ice can consistently and efficiently separate out H2. Most of the H2 will remain trapped in the water matrix until heated to a range of temperatures; as the water is heated and annealed, the molecular hydrogen escapes.
The sublimation of water ice itself, the researchers note, would only produce up to 50 percent of the observed acceleration. Molecular hydrogen, however, explains it quite neatly.
'Oumuamua is now quite a distance away, and traveling fast; there's no real possibility of taking a closer look at it now than the observations we've already had.
So whether the team is right about the molecular hydrogen is going to remain an open question.
However, it does, they say, tick all the boxes, and they can test it by looking at other objects – small outer Solar System bodies and other interstellar objects discovered in the future – that show non-gravitational acceleration without detectable tracers of cometary activity.
"Future detections of small bodies with non-gravitational acceleration and faint coma could provide insights into the origins of 'Oumuamua," Bergner and Seligman write, "even though it has long departed the Solar System."
The research has been published in Nature.
Update 29 March 2023: After this article was published, astrophysicist Avi Loeb from Harvard University – who has previously suggested 'Oumuamua could be an object of extra-terrestrial origin – contacted ScienceAlert, claiming the new study by Bergner and Seligman contained miscalculations regarding the surface temperature of 'Oumuamua.
Loeb's rebuttal – detailed in a new preprint paper co-authored by astrophysicist Thiem Hoang from the Korea Astronomy and Space Science Institute – suggests the miscalculations would affect the thermal speed of outgassing hydrogen and render their model untenable.
In response to this argument, Bergner told ScienceAlert: "We have identified several reasons why the inclusion of effects such as H2 evaporative cooling and the heat released by the annealing of water ice are unlikely to alter our conclusions. While we are happy to enumerate these reasons, we believe that letting the independent peer review process evaluate the Hoang-Loeb paper is the best immediate course of action."