'Weird' interstellar asteroid yields no alien signals, resembles worlds beyond Neptune

It's like nothing we've ever seen: a bizarrely shaped rock ejected from another stellar system, just passing through ours. It's even had some scientists listening for alien signals. Now researchers reveal some more details about this interstellar visitor.

'It moves like a piece of driftwood on the tide,' astronomer says

'Oumuamua — the interstellar asteroid spotted passing through our solar system last year, and pictured in this artist's impression — was going too fast to be captured by the gravity of the planets. But an object with a slower velocity could conceivably be pulled into the orbit of a giant planet like Jupiter. (M. Kornmesser/ESO)

Shocked astronomers had never seen anything like it: a bizarrely shaped rock ejected from another stellar system, just passing through ours.

So strange was the discovery back in October, astronomers around the world turned their telescopes toward the cigar-shaped asteroid in the hope of uncovering its mysteries — including even listening for signs of alien life.

Breakthrough Listen, one arm of an international project dedicated to searching for signs of intelligent life in the universe, started eavesdropping earlier this month using a telescope in West Virginia. But the group says it has found no evidence of artificial signals coming from the rock known as 1I/2017 U1 'Oumuamua, which means "messenger from afar who arrived here first" in Hawaiian.

The organization says it continues to listen across other radio frequencies and analyze the collected data.

Other stars, other worlds

Michele Bannister of Queen's University Belfast says the chance to study something that came from another solar system can provide scientists with crucial information to test their theory of planetary evolution.

"We can do stuff here that we simply can never do in detail for other systems. And then a piece of one comes visiting!" said Bannister, the lead author of a new study about 'Oumuamua that was accepted for publication in the journal Earth and Planetary Astrophysics. 

"We can actually connect the theory to the reality."

'Oumuamua, which is about 400 metres long and 40 metres wide, raises many questions Bannister would love to answer. 

"I would like to know what its home star system looked like," she said. "I want another one. I want to know this object isn't unusual. Is it statistically a reasonable example of what we expect to see wandering the cosmos?"

The grainy blue speckled image shows Oumuamua as a tiny white speck in the centre.
This is 'Oumuamua, circled, as seen by the 4.2m William Herschel Telescope on La Palma, Spain. (A. Fitzsimmons / QUB / Isaac Newton Group, La Palma.)

She says with more and more telescopes dedicated to sky surveys, the chances of discovering more rocks drifting from other solar systems are increasing.

"That's something that I'm really looking forward to: the launch of the new field of interstellar asteroid research," she said. "['Oumuamua is] going to have friends."

'Weird' rock

Astronomers have discovered 'Oumuamua resembles worlds in our outer solar system, beyond the orbit of Neptune, which helps shed some light on planetary formation.

'Oumuamua, for example, has a significant carbon content. Astronomers believe that carbon-rich material, together with cosmic ray bombardment, should colour an object like an asteroid bright red.

But this didn't happen with 'Oumuamua.

"This object has been travelling between stars for millions, perhaps billions of years, so it would have had a lot of cosmic bombardment," Bannister said. "So [you'd expect] maybe it would be ultra-red. But it's not."

Instead, the colour is a more neutral red, similar to what you'd find on objects in the Kuiper Belt — the outer region of our solar system, home to icy worlds — such as Pluto and its largest moon Charon.

The moon Charon is seen here in a mosaic of photographs taken by the New Horizons spacecraft during its approach to the system from July 7-14, 2016. A study found that Pluto is constantly 'spray-painting' Charon's poles red due to its escaping atmosphere and a reaction with solar radiation. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute via AP)

"This object is very much similar to 15 per cent of the outer solar system," Bannister said. "It's a little weird, but it's familiar."

Bannister, who worked as a post-doctoral researcher at the NRC's Herzberg Centre in Victoria, said her study wouldn't have been possible without the work of a Canadian-led study called Colours of OSSOS, which has been mapping the objects in the outer reaches of the solar system. 

Another surprise

When 'Oumuamua was first discovered, astronomers believed it was likely a comet. That's because as stellar systems form, most of the objects thrown into space are comets, leftover bits made up of water and dust. Asteroids, on the other hand, are mostly rocky remnants and much less common. 

But as 'Oumuamua neared the sun, the familiar tail that comets produce didn't appear, an indication that it's mostly rock. 

As 'Oumuamua makes its way out of the solar system at roughly 38 kilometres a second, tumbling from end to end, researchers still aren't sure exactly where it came from.

"It moves like a piece of driftwood on the tide," Bannister said. "It's really celestial driftwood."


Nicole Mortillaro

Senior reporter, science

Based in Toronto, Nicole covers all things science for CBC News. As an amateur astronomer, Nicole can be found looking up at the night sky appreciating the marvels of our universe. She is the editor of the Journal of the Royal Astronomical Society of Canada and the author of several books. In 2021, she won the Kavli Science Journalism Award from the American Association for the Advancement of Science for a Quirks and Quarks audio special on the history and future of Black people in science. You can send her story ideas at