An object from outside our Solar System, currently moving away from us, has been explained by new research as having a natural origin, despite its unusual features.
The anomalous acceleration of ‘Oumuamua has been explained by the release of molecular hydrogen gas, which is now confirmed by recent research.
Jennifer Bergner, an astrochemist from the University of California, Berkeley, and Darryl Seligman, an astrophysicist from Cornell University, have suggested that ‘Oumuamua was initially a planet seed that was ejected from its star and has been wandering the galaxy without being tethered to any star since then.
The researchers propose that the release of molecular hydrogen gas can account for many of ‘Oumuamua’s peculiar properties, and there is no need for any extraordinary claims about the nature of the object.
‘Oumuamua was first observed in October 2017, shortly after it passed closest to the Sun and returned back into space on its journey through the universe. It is still a matter of great interest to astronomers as it is unlike anything else we have seen in our Solar System.
One of the most notable features of ‘Oumuamua is its shape – it is a long, cigar-shaped object that is up to 400 meters in length, which is unlike any other asteroid or comet observed in our Solar System.
‘Oumuamua also rotates as it moves, similar to a spinning bottle on its side. Despite the fact that it appears to have no detectable ice or gas emissions like a typical comet, its trajectory cannot be explained by gravity alone as it would be for an asteroid.
Comets experience an additional acceleration due to outgassing as their ice sublimates, which astronomers have observed with ‘Oumuamua. This suggests that it shares characteristics of both comets and asteroids.
Over the years since its discovery, researchers have come to believe that ‘Oumuamua is most likely a fragment that broke off from a planetesimal, a small planet still in the process of forming, after a collision with another object.
It is not uncommon for planetary systems to experience collisions during their formation, as evidenced by the theory that our own Earth was formed after a planet-sized object collided with it, causing a chunk to break off and form the Moon. Similarly, in the case of ‘Oumuamua, a shard of a planetesimal was ejected from its system entirely.
In 2020, Darryl Seligman co-authored a paper suggesting that the acceleration of ‘Oumuamua could be explained by the sublimation of molecular hydrogen (H2).
Molecular hydrogen is difficult to detect in space because it does not emit or reflect light. If ‘Oumuamua was outgassing molecular hydrogen, it would not be visible in the same way that cometary activity tracers are usually detected.
However, it is unlikely that ‘Oumuamua is a molecular hydrogen iceberg, as proposed by researchers in 2020. Therefore, Jennifer Bergner and Darryl Seligman returned to modeling to investigate how the object could contain and sublimate molecular hydrogen.
Bergner and Seligman proposed that ‘Oumuamua’s acceleration was caused by the sublimation of molecular hydrogen gas.
Molecular hydrogen is not easily detected in space since it doesn’t emit or reflect light. The researchers explained that ‘Oumuamua might contain molecular hydrogen through modeling.
The researchers found that the explanation for ‘Oumuamua’s acceleration is plausible through the irradiation of a body rich in water ice. As ionizing radiation hits the object, radiolytic processes split water molecules to produce molecular hydrogen.
According to the researchers’ model, ‘Oumuamua was initially an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellar journey, and it warmed up during its passage through the Solar System.
There is experimental evidence that processing water ice can separate out molecular hydrogen consistently and efficiently. As the water is heated and annealed, the molecular hydrogen escapes.
The sublimation of water ice alone would only produce up to 50 percent of the observed acceleration, while molecular hydrogen explains it more completely.
‘Oumuamua is now too far away and moving too fast for scientists to study it any further beyond the observations they have already made.
The question of whether or not the team’s hypothesis about the molecular hydrogen is correct will remain unanswered.
The researchers note that their proposed explanation fits the available evidence, and they suggest that it can be tested by observing other objects that show non-gravitational acceleration without cometary activity.
They recommend that future detections of small bodies exhibiting similar behavior could provide important information about the origins of ‘Oumuamua, despite the fact that it has already left the Solar System.
The research has been published in Nature.