Interstellar visitor is a comet, not an asteroid, astronomers suggest

After some back and forth in the astronomical community, a new study suggests that the first object found to be from outside our solar system is a comet, not an asteroid.

Scientists find the clue: Its trajectory is changing ever so slightly

This artist’s impression shows the first interstellar object discovered in the solar system, 'Oumuamua as it vents gas and particles into space. (ESA/Hubble, NASA, ESO, M. Kornmesser)

It came as a surprise and left astronomers scratching their heads. The strange-looking rock that entered our solar system wasn't from the neighbourhood.

First, scientists thought it was an interstellar comet, then an asteroid. Now a new study published in the journal Nature suggests astronomers had it right the first time: 'Oumuamua is a comet.

'Oumuamua — a strange, elongated object — was discovered in 2017, by Canadian Robert Weryk.

Its name is a Hawaiian word for a scout or messenger from the distant past.

Weryk was sifting through telescope data at the University of Hawaii Institute for Astronomy and came across something on a trajectory that didn't quite match what was typical of asteroids or comets that originated from our solar system.

Followup observations confirmed that's because it hadn't. 

Unravelling the puzzle

Astronomers were able to trace its trajectory to somewhere toward the bright star Vega in the constellation Lyra (interestingly, Vega was the star where an alien signal was detected in the Carl Sagan book and film Contact), though the stars have moved since 'Oumuamua started its journey billions of years ago.

We were bound for some surprises— Robert Weryk, University of Hawaii

A comet, often referred to as a "dirty snowball," is made up of dust and ice. Solar heating causes comets to vent gas and particles into space as they near the sun, creating the tail we associate with them. This process is referred to by astronomers as outgassing.

Asteroids — large, rocky bodies, with little water — don't have these tails.

'Oumuamua didn't seem to have a tail, so it was believed to be an asteroid.

But there was something that didn't quite fit.

"Most theoretical models proposed, even before the discovery of 'Oumuamua, suggested that the objects ejected by planetary systems during their formation would mostly be cometary," said Marco Micheli, lead author of the paper and an astronomer at National Institute for Astrophysics Astronomical Observatory of Rome.

So the team of researchers studied it using ground-based and space-based observations, and found it was accelerating ever so slightly. If it was just orbiting as a result of the gravitational influence of the sun, it would orbit along a particular path. However, the researchers found that the trajectory was altered — a result, they concluded of outgassing. 

This animated viideo illustrates 'Oumuamua's orbit and path exiting our solar system.

As of June 1, 'Oumuamua was moving at 114,000 km/h at distance — though, inclined from the ecliptic where the planet orbits the sun — between Jupiter and Saturn.

"Our explanation of it as cometary outgassing shows that 'Oumuamua is actually even more similar to our own solar system's objects than we thought at first," Micheli said.

No tail

The reason we couldn't see 'Oumuamua's tail, as we do normally with comets, is that its particles and gases — water, oxygen and carbon dioxide — are difficult to see from the ground. Though the grains are believed to be 100 times larger, there are fewer of them.

"'Oumuamua passed so close to the sun, that if it was like a typical comet from our solar system, we should have seen the activity immediately. And that's what we didn't see," discoverer Weryk said.

"It's the first object we've seen from outside the solar system, so we were bound for some surprises," Weryk said.

Weryk is continuing to look for these interstellar wanderers. "I'm really hopeful to find another one, and then we can compare that to 'Oumuamua to see how it might differ or be the same," he said.

"And that will really help us understand where these objects come from."


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