In October 2017, astronomers discovered ‘Oumuamuaan interstellar object that passed through our Solar System and is now on its way back through the cosmos.
Oumuamua is an unusual and fascinating object that has baffled scientists since its discovery. However, the new research suggests that Oumuamua’s strange acceleration is due to the release of molecular hydrogen gas, providing an entirely natural explanation despite its oddities.
A team of scientists, led by astrochemist Jennifer Bergner of the University of California, Berkeley, and astrophysicist Darryl Seligman of Cornell University, has determined that the acceleration of ‘umuamua can be attributed entirely to the release of molecular hydrogen gas. According to the researchers, this is an elegant solution that “can explain many of the peculiar properties of ‘Oumuamua without fine tuning» or resort to extraordinary statements about the nature of the object.
Astronomers discovered Oumuamua a few years ago, and since then, it has received a great deal of attention. On the one hand, its shape is unusual: is long and thin like a cigar, with a length of up to 400 meters. No other comet or asteroid in the Solar System has this shape. Also, Oumuamua seems to spin while moving, like a bottle lying down, which is also a rare phenomenon. Finally, Oumuamua’s trajectory cannot be explained by gravity aloneas in the case of an asteroid, but appears to have an additional source of acceleration, which is consistent with what astronomers observe in comets.
Scientists have also determined that Oumuamua likely originated from a baby planet (a planetesimal) still in the process of forming, which collided with another object and broke apart. Such collisions are common in forming planetary systems, and our own Earth is thought to have been struck by a planet-sized object, triggering the formation of the Moon. In the case of Oumuamua, a fragment of the planet was ejected from the system.
In 2020, Seligman proposed that Oumuamua’s acceleration was due to the sublimation of molecular hydrogen. Detecting molecular hydrogen in space is very difficult, as it neither emits nor reflects light, which means that if Oumuamua were emitting molecular hydrogen, we wouldn’t be able to detect it the way we normally detect cometary activity.
However, the researchers found that the irradiation of a body rich in water ice may explain the presence of molecular hydrogen in ‘Oumuamua. As ionizing radiation strikes the object, radiolytic processes split the water molecules apart to produce molecular hydrogen. According to the researchers, ‘Oumuamua started as an icy planetesimal that it was irradiated at low temperatures by cosmic rays during its interstellar journey and experienced heating during its passage through the Solar System.
A body of experimental evidence has already shown that processing water ice can efficiently separate molecular hydrogen. Most of the molecular hydrogen will remain trapped in the water matrix until it is heated to a range of temperatures; as the water heats and anneals, molecular hydrogen escapes.
The researchers note that sublimation of water ice itself would only produce up to 50 percent of the observed acceleration, but molecular hydrogen easily explains it.
Although ‘Oumuamua is now quite far away and traveling at considerable speed, the team’s research provides a possible natural explanation for the object’s acceleration. The team hopes that future detections of small bodies with non-gravitational acceleration and tenuous coma will provide a better understanding of the nature of ‘Oumuamua, even after it has already left our Solar System.
Image: (ESA/Hubble, NASA, ESO, M. Kornmesser)
