Spotted by a telescope in Hawaii, "Oumuamua" - which means "scout" in Hawaiian - sped at such a high speed that it could only have come, and this was a first, from another star system.
The discovery has set the community of astronomers in turmoil, who have long been searching for comet-like objects penetrating our solar system from the vastness of space.
Except that Oumuamua did not look like the usual comets: it did not have the characteristic hair that forms their immense halo of gas and dust as it approaches the Sun.
The interstellar visitor, who was about 100 meters wide, also stood out for its brightness and a large variation in brightness, giving the impression of a metallic object turning on itself.
But stranger still, after circling the Sun, Oumuamua accelerated and deviated from the planned trajectory, propelled by a mysterious force that ejected it from the solar system.
The scientists were left stunned, left with four months of incomplete and seemingly contradictory data that they tried to understand. This gave rise to a whole series of theories.
Some "exceeded imagination," said Jennifer Bergner, an astrochemist at the University of California at Berkeley, co-author of the study published in Nature.
According to his explanation, whatever the origin of Oumuamua, it is a water-rich object, similar to a comet. During its journey through interstellar space, it was subjected to cosmic rays that bombarded the water, releasing hydrogen, which became trapped in the object's body.
When the bolide approached the Sun, the heat in turn released the trapped hydrogen, acting as a "thruster" that sent it on an unexpected trajectory.
The mystery Oumuamua © John SAEKI / AFP
"Trapped hydrogen is simply the most generic explanation," Darryl Seligman of Cornell University and co-author of the study said in a statement.
This work "undoubtedly provides the first explanation, simple and realistic, of the particularities of this object," commented Marco Micheli, astronomer at the European Space Agency (ESA).
But these conclusions do not make everyone agree. Starting with Avi Loeb, former head of the astronomy department at Harvard University: the eminent scientist had argued that the most credible scenario was that Oumuamua was an alien ship.
A controversial thesis, which he had defended in 2021 in a book entitled "The first sign of extraterrestrial intelligent life".
Asked by AFP about the Nature study, the astrophysicist says that saying that a comet has no tail "is like saying that an elephant is a zebra without stripes". He recalls that Comet 2I/Borisov, the second visitor from outside the solar system spotted in 2019, had a long hair of dust.
Jennifer Bergner's answer: If Oumuamua has no tail, it is probably because it is much smaller than all the comets observed so far - including 2I/Borisov.
That could soon change. Observations from the Vera-C. Rubin telescope in Chile, which will begin in 2025, are expected to detect many new comets, both inside and outside the solar system. If the smaller ones show signs of the release of trapped hydrogen, and have no tail, like Oumuamua, it would confirm the theory, the astrochemist added.
As for the possibility of extraterrestrial intelligent life, "it all depends on the level of proof required" to invoke it, comments the scientist.
"We will never know for sure what Oumuamua was – we lost our chance. But for now, I think we're providing a compelling non-alien explanation," she concludes.
© 2023 AFP