Oumuamua: Alien spaceship or comet? Study sheds light on interstellar visitor’s icy origin

'Oumuamua: Experts discuss 'unusual' interstellar object

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Oumuamua came flying into the solar system from interstellar space in 2017 and was detected by the Pan-STARRS observatory in Hawaii. At the time, it was the first-known object from beyond our system’s borders to pay us a visit. The object was formally christened 1I/2017 U1, although it is better known by its Hawaiian name ‘Oumuamua, which means “scout” or “messenger”.

On the one hand, Oumuamua appeared to behave like an interstellar comet.

On the other hand, the object was too slow and its unusual shape further complicated matters.

Astronomers at first described Oumuamua as cigar-shaped, although subsequent studies support a flatter, more rounded look like a pancake.

Further adding fuel to the mystery, renowned astronomer Avi Loeb of Harvard University’s astronomy department publically speculated about the object’s supposed extraterrestrial origin.

After a study published last year refuted the claims, the astronomer said: “We should still keep an open mind about Oumuamua’s origin.”

New research has now been published by Arizona State University astrophysicists, which has also rejected the alien hypothesis.

Instead, Steven Desch and Alan Jackson of the School of Earth and Space Exploration have proposed Oumuamua is an icy chunk dislodged from a Pluto-like planet from another solar system.

Professor Desch said: “In many ways ‘Oumuamua resembled a comet, but it was peculiar enough in several ways that mystery surrounded its nature, and speculation ran rampant about what it was.”

The astrophysicists presented their findings in two papers published in AGU Journal of Geophysical Research: Planets.

The pair analysed the comet hypothesis by highlighting Oumuamua’s unusual characteristics.

Oumuamua is slower than expected of a comet, indicating it had not been tumbling through interstellar space for more than a billion years.

The researchers also failed to detect any gas signatures coming from the object, which usually comes in the form of a comet’s tail.

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Oumuamua was also being pushed away from the Sun like a rocket, as the star’s heat vaporised ice into gas, but the push was stronger than expected.

The scientists then hypothesised the object was made of ice and ran the numbers on different chemical compositions to see how quickly they would evaporate under the Sun’s glare.

Professor Desch said: “That was an exciting moment for us. We realized that a chunk of ice would be much more reflective than people were assuming, which meant it could be smaller.

“The same rocket effect would then give ‘Oumuamua a bigger push, bigger than comets usually experience.”

What the researchers found was nitrogen ice – a gas associated with frozen worlds like Pluto – best matched Oumuamua’s behaviour in space.

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Dr Jackson said: “We knew we had hit on the right idea when we completed the calculation for what albedo (how reflective the body is) would make the motion of ‘Oumuamua match the observations.

“That value came out as being the same as we observe on the surface of Pluto or Triton, bodies covered in nitrogen ice.”

According to the astrophysicists, Oumuamua was most likely knocked off from a planet’s surface about half-a-billion years ago.

An impact sent the chunk of nitrogen into interstellar space and towards our system.

Dr Jackson said: “As the outer layers of nitrogen ice evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use.”

So no, Oumuamua is most likely not an alien spaceship or probe sent out to gather intel on the human race.

The researchers acknowledged there is a lot of interest surrounding the prospect of life beyond Earth.

But Professor Desch noted scientists should not jump to conclusions in the face of lacking data.

He said: “It took two or three years to figure out a natural explanation – a chunk of nitrogen ice – that matches everything we know about ‘Oumuamua.

“That’s not that long in science, and far too soon to say we had exhausted all natural explanations.”

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