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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 one leading astronomer was convinced it was an alien vessel.
Key points:
- 'Oumuamua left scientists baffled when it was detected by an observatory in Hawaii in 2017
- A new study theorises that it started out as a water-rich, comet-like object
- However, some experts are "very suspicious" of such theories
Researchers have now come up with a simple and "compelling, non-alien explanation" for the interstellar interloper's bizarre behaviour, although not everyone is convinced.
The object christened 'Oumuamua — which means "scout" in Hawaiian — left scientists baffled when it was detected by an observatory in Hawaii six years ago.
Astronomers had long been searching for comet-like objects entering our Solar System from the vastness of interstellar space, but had never before observed one.
Unlike comets that travel in from the edges of the Solar System, 'Oumuamua lacked a tail and a fuzzy halo — known as a coma — which are formed by dust and gas warming in the Sun's heat.
It was also a peculiar, elongated shape, never before observed in comets or asteroids.
Its diameter was roughly 100 metres but, by some estimates, it was 10 times as long as it was wide, shaped either like a pancake or a cigar.
By the way light glinted off the object, it appeared to be tumbling end over end.
The strangest part was that once 'Oumuamua slingshotted around the Sun, it sped up and deviated from its expected trajectory, propelled by a mysterious force on its way out of the Solar System.
Scientists were left with four months' worth of seemingly contradictory data to try to make sense of, that led to a range of theories.
'Thruster boost'
Jennifer Bergner — who is an expert in astrochemistry at the University of California Berkeley and the co-author of a new study — said many of the theories "stretched the imagination".
She proposed that 'Oumuamua started out as a water-rich, comet-like object.
During its interstellar travels it was blasted by penetrating cosmic rays that converted some of its water into hydrogen gas that became trapped within the object's body.
When 'Oumuamua neared the Sun, the heat released the trapped hydrogen, acting as a "thruster boost" that propelled the object on its unexpected path, she said.
Cornell University's Darryl Seligman — who co-authored the study published in the journal Nature — backed up Dr Bergner's theory about entrapped hydrogen.
"We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it's just the most generic explanation," he said.
European Space Agency astronomer Marco Micheli — who was not involved in the research — said the study offered "the first simple and physically realistic explanation of the peculiarities of this object".
Experts 'suspicious' of theory
However, not everyone is convinced.
Avi Loeb — a lauded theoretical physicist who was the longest-serving chair of astronomy at Harvard University — maintains that the simplest explanation is that 'Oumuamua was alien technology.
Professor Loeb has rejected the new theory, and said claims of a comet without a tail "is like saying an elephant is a zebra without stripes".
He pointed to the large cometary tail seen on 2I/Borisov, the second known visitor from outside the Solar System, which was seen in 2019.
Cambridge University's Roman Rafikov said he had previously demonstrated that, if trapped gas were behind 'Oumuamua's acceleration, it would have "dramatically" changed the rate at which it was spinning.
Professor Rafikov said he was "very suspicious" of such theories, adding nonetheless that he preferred "an explanation that does not involve aliens or divine forces".
Dr Bergner suggested the reason 'Oumuamua did not have a tail or coma was that it was far smaller than any comet that had ever been observed.
She said that if small comets showed signs of releasing trapped hydrogen — and lacked tails and comas — that could help confirm her theory.
When it came to ideas involving extraterrestrial life, she said, it "depends what standard of proof you require to invoke aliens".
"We'll never know for sure what 'Oumuamua was — we lost our chance," she said.
"But, for now, I think here we have a compelling non-alien explanation."
AFP