Mysterious space object ‘Oumuamua not alien, scientists say

A ‘weird, crazy’ object the size of a football field baffled researchers as it flew through our solar system - but it turned out to be something extremely common.

an artist's impression of the first interstellar asteroid, called ‘Oumuamua. When the first object ever known to have visited our Solar System from outer space zoomed past in 2017, it was so strange that at least no leading astronomer was convinced the most logical explanation was that it was alien technologY.

Nearly six years ago, a space object roughly the size of a football field baffled scientists as it zipped through our solar system. The irregular shape and motion of the object, dubbed ‘Oumuamua, led to one of the most controversial astronomical mysteries, with theories that ranged from asteroid to alien probe.

But a study published Wednesday in the journal Nature offers another answer: ‘Oumuamua is a typical comet that expelled gas in an odd way as it traveled through our solar system.

“We’ve gone through every weird, crazy, possible theory -- ideas that stretch the imagination to match all of these observed things, and then we figure out that it is just the simplest thing you could possibly imagine, which is just a water-rich comet,” said Darryl Seligman, a postdoctoral researcher at Cornell University and co-author of the new study.

Astronomers first spotted ‘Oumuamua at an observatory in Hawaii in October 2017 as it careened through the planetary neighborhood. It passed by the sun at speeds of nearly 196,000 miles an hour (315,431.4 km/h). The body’s trajectory indicated that it came from across the stars, having likely traveled for many millions of years before arriving.

The mysterious object was observed moving past the sun at speeds around 196,000 miles/h.

Observatories on the ground and in space had just weeks to characterize the interstellar object before it made a hook turn past the sun and continued its high-speed journey back into the greater Milky Way, beyond the reach of our most powerful instruments.

In that short window of scrutiny, scientists could collect only a limited amount of information about the orbit, color, shape and brightness of the tiny ‘Oumuamua. Pronounced “oh moo-uh moo-uh,” the name means a messenger from afar arriving first in the Hawaiian language, according to the National Aeronautics and Space Administration.

But this interstellar messenger -- the first known of its kind -- defied explanation, according to Davide Farnocchia, a navigational engineer at the agency’s Jet Propulsion Laboratory in Pasadena, Calif., who wasn’t involved in the research. Observations showed ‘Oumuamua appeared to move like an icy comet, yet lacked the telltale tail and head of dust typical of such space objects. Its long, pancake-like profile, which appeared inert, was more reminiscent of an asteroid.

Among the largest of its mysteries, Dr. Farnocchia said, was that ‘Oumuamua was being propelled by forces beyond the gravity of the sun or planets. Nongravitational forces are observed in typical comets -- the closer a comet gets to the sun and heats up accordingly, the more of its ice transitions into gas -- creating its tail while changing its motion in space. This process is known as outgassing.

Evidence suggested ‘Oumuamua was also under the influence of nongravitational forces, yet lacked the tail and other characteristics typical of an outgassing comet. The contradictory nature of these observations prompted a gamut of ideas as to what the interstellar object could be. Some scientists, including Dr. Seligman, first suggested it was an iceberg made of pure hydrogen that had been outgassing over time. Arizona State University astrophysicist Steve Desch co-led research that posited ‘Oumuamua is part of a Pluto-like exoplanet from outside our solar system that sloughed off layers of nitrogen ice.

By contrast, Harvard University astrophysicist Avi Loeb has been an outspoken advocate of a nonnatural origin for ‘Oumuamua. He posited in multiple scientific papers and a book published in 2021 that its odd acceleration could be because it is a piece of alien technology.

But the new study used modeling to determine that ‘Oumuamua is in fact a comet that was restructured during its long journey through space. The object was born in another solar system as a normal comet made of ice, said study co-author Jennifer Bergner, an astrochemist at the University of California, Berkeley.

As the comet traveled through interstellar space toward our part of the Milky Way, it was hit with cosmic rays. The radiation from those rays separated the hydrogen from some of the comet’s ice-forming water molecules. That hydrogen was then trapped inside tiny pockets within the ice deep inside ‘Oumuamua, according to Dr. Bergner.

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Once ‘Oumuamua approached the sun, the heat from our star rearranged its icy structure -- leading to the collapse of those pockets and the formation of channels within the ice through which trapped gas escaped to the comet’s surface and accelerated it.

“It is sort of the cleanest explanation for a lot of the strange behavior,” she said.

Matthew Knight, an astronomer and physicist at the U.S. Naval Academy who wasn’t involved in the research, said this theory “feels closer to being possible, and more plausible,” than the nitrogen or hydrogen ice ideas.

Drs. Loeb and Desch, neither of whom were involved in the new study, aren’t convinced.

Dr. Desch said he didn’t find any issues in the authors’ calculations, but that he didn’t understand their motivation to look for another solution to the mystery: “This problem is as solved as it gets in astrophysics: ‘Oumuamua is a fragment of an exo-Pluto’s surface,” he added.

Dr. Loeb said he takes issue with the authors’ characterization of ‘Oumuamua as a typical comet. “It must be a very unusual comet if it is a comet at all,” he said, adding that an interstellar comet named Borisov, which was discovered in 2019, “has a beautiful huge, cometary tail.” The new theory can be tested once new, more powerful telescopes like the one at the Vera Rubin Observatory -- currently under construction in Chile -- come online, Dr. Knight said. Ultimately much of the debate around this object, he added, is being driven by the fact that most scientists aren’t happy with the claim that ‘Oumuamua is alien in nature.

“That is not something that most scientists want to go with,” he said. “So we are pushing to try and come up with ideas that work.” Dr. Loeb said the topic of extraterrestrial intelligence remains taboo among his astronomy colleagues. But he continues to explore the idea that ‘Oumuamua is, for example, a piece of alien technology that harnesses the pressure of sunlight to move. “Science should be guided by evidence, rather than prejudice,” he said.

Yet NASA’s Dr. Farnocchia said a natural origin for the object seems far more plausible.

“While there is nothing wrong with considering the possibility of ‘Oumuamua as alien technology, such an exotic hypothesis would need to be backed up by strong evidence that simply isn’t there,” he said.

WSJ