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Mysterious interstellar visitor ‘Oumuamua’ appears to be a comet after all

Oumuamua's weird cigar shape wasn't the only thing that made it stand out.

Oumuamua's weird cigar shape wasn't the only thing that made it stand out. Photo: ABC/ESA

Our first ever visitor from interstellar space left a trail of mystery in its wake when it zipped past late last year.

‘Oumuamua (oh-MOO-ah-MOO-ah) was first detected on October 19 using the Pan-STARRS telescope in Hawaii, as it streaked across our solar system.

Travelling at speeds of 26 kilometres-a-second on a journey away from the sun to never return, the fast-moving, cigar-shaped object clearly came from another solar system, according to astronomers.

At first it was thought ‘Oumuamua was a comet, a cosmic ball of ice and dust that develops a halo or coma when it gets close to the sun.

But follow-up observations of the mysterious object, which is about the length of two footy fields, found it didn’t develop a coma. So it was decided it was an asteroid.

“The consensus was that it was asteroidal because there was no gas and there was no dust,” said Karen Meech, an astronomer at the Institute for Astronomy in the University of Hawaii.

“However, the object looked reddish – in other words, it scattered the red end of the light spectrum better, rather than blue – and that’s what comets do.”

Now a study by Dr Meech and colleagues, published in the journal Nature, indicates ‘Oumuamua is indeed a comet – albeit a little bit different to those in our own solar system.

Model weighs up the evidence

The international team analysed data from ground and space telescopes such as the Very Large Telescope in Chile and the Hubble Space Telescope.

The data showed the object appeared to accelerate a little as it moved away from the sun, which suggested some force besides the sun’s gravity was pushing it away.

“The motion of the object is not behaving as though it is purely controlled by the sun’s gravity,” Dr Meech said.

'Oumuamua

‘Oumuamua was detected on October 19, 2017, when it was already on its way out of the solar system. Photo: ABC/Wikimedia Commons

Davide Farnocchia from NASA’s Jet Propulsion Laboratory said the team looked at what could cause this so-called non-gravitational acceleration.

One hypothesis is the effect of the sun heating up one side of an object as it rotates – which is known to give asteroids a bit of a push.

Other ideas included the impact of solar winds; the possibility the object fragmented; or it was magnetised.

“None of these hypotheses could even come close to explaining the observed deviation in the motion of ‘Oumuamua,” Dr Farnocchia said.

The only possibility, the team concluded, was that the object was “out-gassing” – a typical feature of comets.

'Oumuamua comet

Unlike comets from our solar system such as 67P, ‘Oumuamua had no gas or dust jets. Photo: ABC/ESA

“[Gases] often don’t leave the comet in a uniform manner, so you might get jets coming off the surface,” Dr Meech said.

But how can it be a comet if it doesn’t appear to have any gas or dust?

The researchers said the amount of gas needed to accelerate the object might be too small to be detected.

It might also have a different chemical composition to comets we see in our solar system, “which is not unreasonable if it formed somewhere else”, Dr Meech said.

Unlike most of the comets in our solar system, it might also be covered with larger grains of dust.

“There was a prediction … that anything that travels through interstellar space and passes through clouds of gas and dust, that would preferentially erode the small particles away,” Dr Meech said.

The debate is far from over

When the object was discovered last year, JJ Kavelaars was working with another team to observe its progress using the Gemini North telescope in Hawaii.

“The expectation is that such objects that might be found as interstellar would typically be comets,” Dr Kavelaars, an astronomer at the University of Victoria in Canada, said.

“This [study] is very strong evidence that this thing definitely had some gas coming off it and that would seem to indicate that it would be a comet.”

But Dr Kavelaars, who was not involved in the current study, said the debate about the object was far from over.

He said people would now be asking how such a weird-shaped icy object could form, and what could have stopped jets of gas escaping from its surface.

“This will really push people to think about what kind of surface you would need to provide the insulation from solar radiation,” Dr Kavelaars said.

“Does it form as it travels through interstellar space, perhaps through, say the cosmic ray environment of interstellar space chemically altering the surface of the object?”

The answers might come as more objects are found.

“We anticipate that over the next decade more of these things are going to be found because telescopes exist to find them,” he said.

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