NASA

If you’ve been walking around thinking that the mysterious, interstellar space rock that astronomers discovered last year is an asteroid, I have some news for you: It’s probably a comet.

There is, of course, the chance that you haven’t been thinking about this space rock, or know the difference between a comet or an asteroid, or why any of it matters. So let’s return to October 2017, when an astronomer named Rob Weryk was looking at the observations recorded by a telescope perched on top of a volcano in Hawaii.

Weryk’s job, as a postdoctoral researcher at the University of Hawaii Institute for Astronomy, is to sort through the telescope’s nightly observations and identify objects passing near Earth. During one of his data sweeps, Weryk noticed an unusual object. Unlike everything else in our solar system, it didn’t seem to be orbiting the sun. It was zooming across the night sky, carving out its own path through our little home in the universe.

Weryk and his colleagues sounded the alarm, and soon astronomers around the world were tracking the object with powerful telescopes. A week later, they made the announcement: They had seen, for the first time, an object from another solar system. It had traveled the cosmos for perhaps hundreds of millions of years, entered our own solar system, whizzed past our sun, and now was on its way out, continuing its epic journey to who knows where. They named it ‘Oumuamua, a term of Hawaiian origin meaning “a messenger from afar arriving first.”

As ‘Oumuamua barreled away, scientists pored over their data. The object was unlike any asteroids and comets they knew. It was rusty red, shaped like a cigar, and tumbling uncontrollably. The asteroids and comets in our solar system formed when the system was young, alongside the planets. Asteroids are mostly rocky because they formed closer to the sun, where the heat melted away any ice. Comets are mostly made of ice because they formed farther out, safe from the glare.

Astronomers had predicted an interstellar object would someday stumble into our view, but they had expected it to be a comet. Since our comets reside toward the edge of the solar system, significant disturbances—like the violent movements of young planets swirling into shape out of gas and dust—could send them flying out into the universe. Here at the edge, it’s much easier to escape the gravity of the sun. Perhaps ‘Oumuamua was a relic of a young solar system jostling into arrangements. Perhaps some comets that once resided in our young solar system are now hurtling through others.  

But ‘Oumuamua appeared to be rocky, and it lacked signs of a distinctly cometary phenomenon. When comets get close to a star, the heat boils some of their ice. The reaction causes dust to fly off the comet and trail behind it, producing a glowing, white tail called a coma. ‘Oumuamua didn’t have a coma. So astronomers classified the mysterious space rock as an asteroid.

But then they started to see something strange in their data, particularly from the Hubble Space Telescope, which continued to track ‘Oumuamua after it faded from view from even the most powerful ground-based telescopes. As seen in the GIF at the top of this story, the sun’s power gravity bent ‘Oumuamua’s trajectory, acting like a slingshot. But “the path was not behaving as it would if it were just merely controlled by the sun’s gravity,” says Karen Meech, an astronomer at the University of Hawaii Institute for Astronomy, and one of the discoverers of ‘Oumuamua. Something else was giving ‘Oumuamua an extra push.

Astronomers used computer modeling to explore possible explanations for the mysterious acceleration, including potential effects of solar radiation, drag-like forces, and interaction with solar wind, the charged particles emitted by the sun. None of them fit. The best explanation, they found, was a process called outgassing. Outgassing occurs when an icy object gets too close to the warmth of a star and starts releasing gases that propel it forward, scattering dust off its surface as it goes.

This phenomenon occurs in comets. So if ‘Oumuamua got pushed around like this, then it had enough ice to get the interaction started. Which would make the mysterious space rock a comet, and not an asteroid.

The results, led by Marco Micheli, an astronomer with the European Space Agency’s Near-Earth Object Coordination Center, were published Wednesday in Nature.

The problem is, astronomers never saw a coma, the tail of dust, trailing behind ‘Oumuamua. “That has been something that has bothered us all along,” Meech says.

There might be an explanation, the astronomers say. ‘Oumuamua is much smaller than comets in our solar system, so it may not require a significant ejection of gas to alter its trajectory. The less gas a comet emits, the fewer dust particles get disturbed. And perhaps, Meech says, during the long journey between stars, interstellar radiation eroded the surface of ‘Oumuamua, reducing the amount of dust coating the object.

‘Oumuamua may also be coated in large dust particles—about 100 times larger than particles found on comets in our solar system. Larger particles are more difficult to detect in optical wavelengths, which Hubble is designed to see.

Coma or no coma, the results are convincing for comet researchers like Jessica Agarwal, a scientist at the Max Planck Institute for Solar System Research. The study authors “have very thoroughly investigated a range of conceivable processes that could explain non-gravitational acceleration of [‘Oumuamua],” Agarwal said. “The conclusion that outgassing is causing the non-gravitational acceleration seems solid to me, and in that sense I find the description as a miniature comet appropriate.”

The telescope survey that found ‘Oumuamua has been running for only a few years, and astronomers hope it can spot more distant visitors soon. Some estimate that there is at least one interstellar object between the Earth and the sun at any given time. If they’re anything like ‘Oumuamua, it may not be easy to detect them, says Andy Rivkin, a planetary astronomer at Johns Hopkins Applied Physics Laboratory who studies asteroids. “If it’s without a coma or tail, they’d be much harder to find than if it did have a coma or tail.”

Eight months after their historic discovery, astronomers are now done observing ‘Oumuamau in real time. At the time of this writing, the comet is about 595 million miles from Earth. The sunlight the space rock can reflect is about 5 billion times fainter than the naked eye can see, and about 13 times fainter than the Hubble Space Telescope, Meech said.

But the story of the mysterious space rock is far from over. Each new piece of information about ‘Oumuamua and its properties is another clue about its origins. “We’d still like to trace back to its home solar system,” Meech said. Astronomers are currently using observational data and computer simulations to run the clock backward and trace ‘Oumuamua’s path. The discovery of outgassing, however, throws a big wrench into those plans. Comets only release gases when they approach stars and get warmed up. The amount of gas varies, which means it’s difficult to predict the resulting acceleration.

In the meantime, Weryk says he is more vigilant about his daily searches of telescope data in Hawaii. “Every time we get a [near-Earth object] candidate now—that probably ends up being a normal asteroid—I actually think, oh, is it possible this could be like ‘Oumuamua?” he says.

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