The exact nature of the bizarre interstellar object ‘Oumuamua remains a mystery. There’s been no shortage of ideas, but now astronomers at Yale and the University of Chicago have proposed that it’s a brand new type of object: a hydrogen iceberg.
‘Oumuamua was discovered in 2017 after it whipped round the Sun, and it was quickly determined by its trajectory that it wasn’t from around here. After spending millions of years in intergalactic space, the object was just passing through our neighborhood.
But its origin isn’t the only odd thing about it. As opposed to the roughly round shape of most asteroids and comets, ‘Oumuamua is a cigar shape estimated to be about 900 ft (274 m) long, tumbling end over end through space. And weirder still, astronomers observed it accelerating on its way out of our solar system.
This all makes it unlike anything we’ve ever seen. So what is it? The most obvious answers are that it’s a comet or an asteroid, but they don’t explain all of these oddities. If it’s a comet, gas could be released from beneath its surface as the Sun warms it, causing it to accelerate. But then why is it so long?
It could be more asteroid-like, a fragment of a planet torn to shreds by a close encounter with a star. That would account for its stretched shape. But it still leaves unanswered questions. Astronomers even checked to make sure it wasn’t some kind of alien spacecraft broadcasting radio signals. It wasn’t.
But maybe it’s something completely new. According to researchers at Yale and the University of Chicago, it could be a hydrogen iceberg. This would account for all of ‘Oumuamua’s weird properties.
The idea goes that in the dense cores of molecular clouds in deep space, temperatures could get so extremely low that hydrogen freezes solid. These icebergs would then drift around for potentially millions of years, until they happened to pass close to a star like ours.
“As ‘Oumuamua passed close to the Sun and received its warmth, melting hydrogen would have rapidly boiled off the icy surface, providing the observed acceleration and also winnowing ‘Oumuamua down to its weird, elongated shape — much as a bar of soap becomes a thin sliver after many uses in the shower,” says Gregory Laughlin, co-author of the study.
While it’s too late to catch up to ‘Oumuamua – it’s already beyond the orbit of Saturn on its outward journey – future interstellar visitors could be investigated. A second one was discovered in 2019, suggesting they’re common, and an MIT team has proposed using spacecraft that can deploy at short notice to intercept such visitors.
The new study has been accepted for publication in The Astrophysical Journal Letters.
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