Quirks & Quarks

Our solar system's first interstellar visitor has its own natural 'rockets'

Originally thought to be an asteroid, it turns out to be a comet and is outgassing vapour
An artist's illustration of the interstellar object 'Oumuamua, which appears to be outgassing material. Scientists now suspect it is a comet after all, and not an asteroid. (ESA/Hubble, NASA, ESO, M. Kornmesser)
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It's a comet, not an asteroid

Last October astronomers spotted the first object ever detected from outside of our solar system passing near to the  Earth. They named it 'Oumuamua, Hawaiian for "a messenger from afar arriving first."

When Quirks & Quarks spoke to Dr. Karen Meech, an astronomer at the University of Hawaii's Institute for Astronomy who led the observation efforts late last year, the general consensus was that it was an asteroid.  

But now, new position data collected from earlier this year as 'Oumuamua headed out of the solar system, suggests that it's not an asteroid, but a comet. Meech published the new findings this week.  

What's the giveaway?

"The surprising observation we discovered was that 'Oumuamua was accelerating," says Meech. "In other words, it wasn't just influenced by the gravity of the planets and the Sun, it was actually moving faster than it should have on its outbound journey."  

They explored the different scenarios that might have caused this unexpected motion, and came to the conclusion that it had to be outgassing. "There must be outgassing from this body that we didn't see but was strong enough to create this acceleration," explains Meech.  

Image of Comet Hyakutake discovered in 1996 when it passed close to Earth. A comet tail is formed when radiations from the sun melts the ice and pushes the released gas and dust particles away from the comet. (NASA/Bill Ingalls)

Outgassing is a unique feature of comets, which are often described as dirty snowballs. A comet is typically made of ice and dust. When it gets close to the sun, the ice gets heated up by the sunlight and turns into a gas that flows away from the comet and is visible as a tail.

The gas shooting off the surface of the comet also acts like a little rocket engine. And because outgassing is not uniform, it can give the comet a little push or a thrust, explains Meech.

How certain are the scientists?

Despite the lack of a gas and dust trail from 'Oumuamua — the reason why scientists initially thought it was asteroid — Meech is still confident that it is indeed a comet, and offers several explanations.

"We explored a handful of particular reasons and the only one that stood up to all of the tests was outgassing." says Meech.

This gas may not have been seen because it was simply too faint to be detected, and perhaps 'Oumuamua has lost much of its dust on its long trip through interstellar space. "It also gives us the exciting result that maybe its chemistry is quite different in a visitor from another solar system," she adds.