Our solar system's first known interstellar visitor is neither a comet nor asteroid as first suspected and looks nothing like a cigar. A new study says the mystery object is likely a remnant of a Pluto-like world and shaped like a biscuit.
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Arizona State University astronomers reported this week that the strange 45-metre object that appears to be made of frozen nitrogen, just like the surface of Pluto and Neptune's largest moon Triton.
Study authors Alan Jackson and Steven Desch think an impact knocked a chunk off an icy nitrogen-covered planet 500 million years ago and sent the piece tumbling out of its own star system, towards ours. The reddish remnant is believed to be a sliver of its original self, its outer layers evaporated by the sun.
It's named Oumuamua, Hawaiian for scout, in honour of the observatory in Hawaii that discovered it in 2017.
Visible only as a pinpoint of light millions of kilometres away at its closest approach, it was determined to have originated beyond our solar system because its speed and path suggested it was not orbiting the sun or anything else.
The only other object confirmed to have strayed from another star system into our own is the comet 21/Borisov, discovered in 2019.
Oumuamua did not fit into known categories - it looked like an asteroid but sped along like a comet. Unlike a comet, though, it did not have a visible tail. It was even suggested it could be an alien artefact.
Using its shininess, size and shape - and that it was propelled by escaping substances that did not produce a visible tail - Jackson and Desch devised computer models that helped them determine Oumuamua was most likely a chunk of nitrogen ice being gradually eroded, the way a bar of soap thins with use.
Their two papers were published Tuesday by the American Geophysical Union and also presented at the Lunar and Planetary Sciences Conference, being held online this year.
Not all scientists buy the new explanation. Harvard University's Avi Loeb stands by his premise that the object appears to be more artificial than natural - in other words, something from an alien civilisation, perhaps a light sail. His new book Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, addresses the subject.
At its closest approach to earth, Oumuamua appeared to have a width six times larger than its thickness - the rough proportions of an Oreo biscuit wafer.
It's now long gone, beyond the orbit of Uranus, more than 3.2 billion kilometres away. It's far too small to be seen, so astronomers will need to rely on the original observations and continue to refine their analyses.
By the time the object starts leaving our solar system about 2040, the width-to-thickness ratio will have dropped to 10-to-1.
"So maybe Oumuamua was consistent with a cookie when we saw it but will soon be literally as flat as a pancake," Desch said in an email.
Australian Associated Press
