• Researchers have proposed an idea that could explain Oumuamua's acceleration
  • It's possibly from the outgassing of molecular hydrogen
  • "It's exactly what should happen to interstellar comets," study author Darryl Seligman says

Interstellar object Oumuamua has remained quite a mystery since it was discovered. A team of scientists has now found a rather simple explanation for its odd non-gravitational acceleration, showing it simply is a comet and not alien technology.

Oumuamua was the first interstellar object known to pay a visit to our solar system. Discovered in October 2017 using the University of Hawaii's Pan-STARRS1 telescope, it has been the object of interest for many scientists because of its rather unusual characteristics.

For one, it has a rather odd elongated shape, quite like a cigar, being 10 times as long as it is wide. It also has a reddish hue. It was initially identified as a comet, but was classified briefly as an asteroid because it had no tail or coma, noted NASA. It was later reclassified as a comet when measurements showed it was accelerating, thus behaving more like a comet.

With all these eccentricities, various theories about the object emerged. Some even said the interstellar visitor may actually be alien technology.

But a team of scientists has now shed light into why Oumuamua was accelerating like a comet even though it didn't produce the signature coma. It's possible that it was being propelled forward because it was simply passing gas, they said.

Usually, comets eject water and molecules when they are warmed by sunlight, thus producing their bright coma and tails, explained the University of California, Berkeley (UC Berkeley). This then helps propel the objects forward.

In the case of Oumuamua, it didn't have the coma or tail, so there was much speculation about its non-gravitational acceleration.

"So far, there has been no explanation that can reconcile these constraints," the study authors wrote.

But UC Berkeley's Jennifer Bergner, one of the authors of the study published Wednesday in the journal Nature, surmised that it had something to do with the hydrogen trapped in the object's body due to cosmic rays.

"A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result. Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?" Bergner said. "Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?"

Calculations by Bergner and colleague Darryl Seligman, of the National Science Foundation, showed the ice at the surface could "emit enough gas" to affect the orbit of small bodies like Oumuamua. It's possible that the release of entrapped molecular hydrogen in Oumuamua powered its acceleration without producing dust, the scientists suggested. This also helps explain why the interloper didn't have a coma.

Overall, the scientists suggest that Oumuamua started out as an icy planetesimal body that was irradiated during its interstellar journey, forming hydrogen. The gas was then released when it entered our solar system and warmed up, thus explaining its odd acceleration.

"The main takeaway is that 'Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing," Bergner said.

"What's beautiful about Jenny's idea is that it's exactly what should happen to interstellar comets," Seligman added. "We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it's just the most generic explanation."

The scientists' work sheds light on the mystery surrounding Oumuamua and other such dark comets — several of which have reportedly been discovered since 2017. Perhaps, it even closed the door on the idea that Oumuamua is a sign of alien technology.

But apart from understanding thes mysterious visitor better, such works may also help us understand the origins of our own solar system as interstellar comets can provide clues as to the conditions of the cosmos around the time when our solar system was formed.

The good news is that scientists may have many more chances to study them in the future. In a 2021 study, researchers suggested that interstellar visitors such as Oumuamua and Comet Borisov, which was discovered in 2019, may actually be more common than previously thought. We simply haven't had the technology to detect them.

ESO ‘Oumuamua
This artist rendering shows the highly unusual ‘Oumuamua interstellar asteroid. ESO