Our solar systems 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 cookie.
Arizona State University astronomers report the strange 45-metre object appears to be made of frozen nitrogen, just like the surface of Pluto and Neptunes largest moon Triton.
The studys 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 cosmic radiation and, more recently, the sun.
Its named Oumuamua, Hawaiian for scout, in honour of the observatory in Hawaii that discovered it in 2017.
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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 wasnt 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.
But what is Oumuamua?
It didnt fit into known categories it looked like an asteroid but sped along like a comet. Unlike a comet, though, it didnt have a visible tail. Speculation flipped back and forth between comet and asteroid and it was even suggested it could be an alien artefact. It was ruled out as an alien spaceship in 2019.
Interstellar interloper Oumuamua was first described as cigar-shaped.
Everybody is interested in aliens, and it was inevitable that this first object outside the solar system would make people think of aliens, Desch said. But its important in science not to jump to conclusions.
It was classified as a comet in 2018 but using its shininess, size and shape and that it was propelled by escaping substances that didnt 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 shrinks with use.
Their two papers were published on Wednesday (AEDT) by the American Geophysical Union and also presented at the Lunar and Planetary Sciences Conference, typically held in Houston but virtually this year.
Not all scientists buy the new explanation. Harvard Universitys Avi Loeb disputes the findings and 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 newly published book Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, addresses the subject.
Given that Oumuamua is unlike comets and asteroids and something not seen before we cannot assume business as usual as many scientists argue, Loeb said by email. If we contemplate something that we had not seen before, we must leave the artificial origin hypothesis on the table and collect more evidence on objects from the same class.
When Oumuamua was at its closest approach to Earth, it appeared to have a width six times larger than its thickness. Those are the rough proportions of one wafer of an Oreo cookie, Desch noted.