The flyby of a mysterious celestial object through our solar system in 2017 puzzled astrophysicists, with one of them going so far as to suggest that it was an alien craft. However, a study published on Wednesday offers another explanation for this exotic visit, reports AFP.

With the help of a telescope in Hawaii, “Oumuamua” (which means “scout” in the local language) flew at such a high speed that it could only come from another star system, the first. The discovery caused a stir among the astronomical community, which has long been searching for comet-like objects entering our solar system from outer space.

But Oumuamua was unlike ordinary comets: it did not have the characteristic crest formed by their huge halo of gas and dust as they approach the Sun. It was more like a cigar, long and thin. The interstellar guest, whose diameter was about 100 meters, was also distinguished by its brightness and wide variations in brightness, giving the impression of a metallic object revolving around it.

But what is even more surprising is that after orbiting the Sun, Oumuamua accelerated and deviated from its planned trajectory, pushed by a mysterious force that threw it out of the Solar System. Scientists were stunned and rushed to study the incomplete and seemingly contradictory data collected over four months. A number of theories emerged. Some of them were “beyond imagination,” said Jennifer Bergner, an astrochemist at the University of California, Berkeley, who co-authored the study, published Wednesday in the journal Nature.

According to his explanation, regardless of the origin of Oumuamua, it is a water-rich comet-like object. During its journey through interstellar space, it was exposed to cosmic rays that “bombarded” the water, releasing the hydrogen that remained trapped in the object’s body. As the spacecraft approached the Sun, the heat in turn released the trapped hydrogen, acting as a “push” that sent it on an unexpected trajectory.

“Trapped hydrogen is just the most common explanation,” said Darryl Seligman of Cornell University and co-author of the study.

This work “probably offers the first simple and realistic explanation of the features of this object,” commented Marco Micheli, an astronomer at the European Space Agency (ESA).

But not everyone agrees with these conclusions, starting with Avi Loeb, the former head of the astronomy department at Harvard University. The prominent scientist argued that the most likely scenario is that Oumuamua is an extraterrestrial craft, a controversial thesis he supported in 2021 in a book titled The First Sign of Intelligent Extraterrestrial Life. Asked by AFP about the Nature study, the astrophysicist said saying a comet has no tail “is like saying an elephant is a zebra without stripes”. He points to another comet. 2I/Borisov, the second extrasolar visitor observed in 2019, had a long dust tail.

Jennifer Bergner’s response to Loeb’s objection is that if Oumuamua does not have a tail, it is probably much smaller than all comets observed so far, including 2I/Borisov.

The game may change soon. Observations with the Vera-C.-Rubin telescope in Chile, which will begin in 2025, should allow the discovery of many new comets both inside and outside our solar system. If the smaller ones show signs of releasing trapped hydrogen and don’t have a tail like Oumuamua, that would support the theory, Bergner added.

As for the possibility of the existence of intelligent extraterrestrial life, “it all depends on the level of evidence needed” to refer to it, the researcher comments. “We’ll never know for sure what Oumuamua is, we’ve lost our chance. But for now, I think we have a compelling non-alien explanation,” she concludes.