It isn’t a bird, and it isn’t a plane. But quite what the cigar-shaped interstellar object ‘Oumuamua is has remained something of a mystery. Now research suggests it is a comet in disguise.

Named after the Hawaiian for messenger or scout – a nod to the fact it was initially spotted by researchers working at the Pan-Starrs telescope in Hawaii in October – ‘Oumuamua was quickly revealed to be a visitor from another solar system, making it the first known interstellar object to swing through our neighbourhood.



Scientists soon scotched ideas that it could be an alien spacecraft: despite having a shape akin to Battlestar Galactica, ‘Oumuamua showed no signs of radio signals and was found to be tumbling through space. It was subsequently discovered to be an object about 10 times as long as it is wide, and dark red in colour – likely down to the impact of cosmic radiation on a surface of frozen, carbon-based substances.



Interstellar visitor ’Oumuamua probably came from a two-star system Read more

But it remained unclear whether the object was an asteroid, a comet or something else. Although initially thought to be a comet, when ‘Oumuamua hurtled past the sun there was no sign of the gas and dust typically released as such icy bodies heat up.

“In the optical images we obtained with various telescopes, it did not show the coma and tail typically associated [with] comets,” said Marco Micheli, first author of the new research from the ESA European Space Agency’s SSA-NEO Coordination Centre in Italy.

That said, some have suggested that the object’s coating of “organic gunk” might have prevented heat from the sun reaching any ice lying beneath. With scientists puzzled, ‘Oumuamua was classed as an asteroid, and then when its origins were discovered it was relabelled as the first in the new class of “interstellar objects”.

Now scientists say that as well as this designation, ‘Oumuamua might be a comet after all.

Writing in the journal Nature, an international team of researchers reveal how they came to the conclusion by studying the path of ‘Oumuamua through to early January. When the team analysed the observations, they found the path could not be explained solely by the influence of the Sun, moon, planets and other large objects in the solar system. Instead they discovered another form of acceleration, not linked to gravity, also appears to be at play.



While the team offer a number of possibilities, including that this acceleration is down to pressure from solar radiation, they say the most plausible explanation is “outgassing”: in other words, ‘Oumuamua is partly propelled by the loss of water vapour and other gases, but at levels that would not be detected by instruments.

“We analysed other possible explanations in detail, and each of them turned out not to work, either because the effect was too weak, or because its behaviour did not match the data,” said Micheli.

The team say ‘Oumuamua is likely also losing dust, but suggest a number of reasons why that hasn’t been spotted, including that the grains might be too small to be detected, or that there is not much dust present relative to the amount of ice.



However, mysteries remain.

“People are still working to trace [’Oumuamua’s] motion back to its original

planetary system,” said Micheli. “But the existence of these non-gravitational forces acting on it would definitely make the extrapolation harder.”

But our interstellar visitor is only making a flying visit: it is already further away from the Sun than Jupiter is, and is heading out of the solar system. “It will reach the distance of Saturn in early 2019,” added Micheli.