The mysterious interstellar object ‘Oumuamua that is shooting through our solar system is wrapped in a thick coating of carbon-rich gunk that built up on its cosmic travels, astronomers have found.

New observations of the cigar-shaped body found evidence for a deep surface layer that formed when organic ices – such as frozen carbon dioxide, methane and methanol – that make up the object were battered by the intense radiation that exists between the stars.



The 400-metre-long object is the first confirmed interstellar body to visit our solar system. It was spotted in October by researchers on the Pan-Starrs telescope in Hawaii after it barrelled past the sun on its way through. The name is taken from the Hawaiian word for “messenger” or “scout”.



The deep outer crust may have formed on the body over millions or even billions of years and gives ‘Oumuamua a dark red colour, according to researchers who investigated the object with the William Herschel Telescope on La Palma in the Canary Islands and the European Southern observatory in Chile’s Atacama desert.



“This is a slowly spinning skyscraper-shaped object with a greyish-red surface crust and potentially ice in its heart,” said Michele Bannister, a planetary astronomer who worked on the observations at Queen’s University in Belfast. When carbon-rich ice is baked in interstellar radiation, it creates a layer of “organic gunk,” she added.



The coating may explain why ‘Oumuamua did not billow clouds of gas and dust when it swept past the sun earlier this year, well inside the orbit of Mercury. Most comets shed vast plumes of gas when they are heated as they swing past the sun, including comet 67P which the European Space Agency’s Rosetta mission chaperoned around the sun in 2015.



“It may well be that this object is icy inside,” said Alan Fitzsimmons, an astronomer on the team at Queen’s University. “If the surface layer is a few tens of centimetres thick, then any ice underneath would not have been heated enough by the sun, because it takes time for the heat pulse to travel through.”

The astronomers began watching ‘Oumuamua days after it was first discovered when it was more than 40 million miles away and travelling at 130,000 miles per hour. The telescopes found no signs of rocky minerals or ice on the surface, but did find evidence for the carbon-rich crust.

“This surface layer is what happens if you take comet ices and comet dust grains and bake them with high energy particles for millions or even billions of years,” said Fitzsimmons.

The researchers went on to compare the measurements of ‘Oumuamua with those taken from asteroids and comets circling in our own solar system. They found that ‘Oumuamua resembles the so-called Trojan asteroids near Jupiter, but also objects in the Kuiper belt beyond Neptune, the most distant planet in the solar system. “Our observations are very consistent with this having started life as an icy body in its own system, similar to those found in the Kuiper belt or around Jupiter,” Fitzsimmons said. Details of the work are published in the journal Nature Astronomy.

Last week, astronomers on an alien-hunting project known as Breakthrough Listen used the huge Green Bank telescope in West Virginia to monitor ‘Oumuamua for radio signals in case it happened to be a passing spacecraft, and not an interstellar asteroid after all. To date, no signs of intelligence have been found.