Oumuamua Was a Comet After All, a Study Suggests

Was it alien space junk? A rogue interstellar asteroid? Or a strange comet from another sun?

Since 2017, when astronomers in Hawaii discovered an object they called Oumuamua (Hawaiian for “scout”) traversing the solar system, they’ve been arguing over what it was.

The telescopes only saw a spinning dot that was already on its way back into the interstellar darkness. The astronomers deduced that it was reddish, cigar- or pancake-shaped, and perhaps a few hundred meters long. To date, all comets observed in our solar system have ranged from half a mile to hundreds of miles in diameter. (Halley’s Comet is about seven miles wide.)

Initially, Oumuamua was classified as an asteroid, since it did not exhibit the typical crackle and flash of comets. (Comets are basically dirty snowballs; when heated by sunlight, they emit jets of steam, carbon dioxide, and dust, creating glowing tails, or comas.) There was no evidence of gas or dust around the object, and the radio telescopes heard nothing when they aimed. in that

But further analysis revealed that something was causing Oumuamua to accelerate out of the solar system, leaving scientists with a delicious puzzle.

Now, two astronomers have found what they call “a surprisingly simple explanation” for Oumuamua’s behavior: The object was a comet, after all, powered by minuscule amounts of hydrogen gas spouting from an icy core.

“We show that this mechanism can explain many of the peculiar properties of Oumuamua without fine tuning,” Jennifer Bergner, an astrochemist at the University of California, Berkeley, and Darryl Z. Seligman of Cornell University, write in an article published Wednesday in Nature. “This provides further support that Oumuamua originated as a planetesimal relic very similar to comets in the solar system.”

In a statement issued by the University of California, Berkeley, Dr. Seligman said: “The beauty of Jenny’s idea is that it is exactly what should happen with interstellar comets. We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation.”

In an email, Karen Meech, a comet expert at the Institute for Astronomy at the University of Hawaii who has studied Oumuamua extensively, called the paper “a very interesting explanation.”

“I’m not willing to say that it ‘solves’ things: the irrefutable proof would be to have detected hydrogen spectroscopically,” he added. “But it is very plausible, and if another object that looks like Oumuamua is discovered, then all these models and explanations provide a lot of guidance for observations. I am in awe of how much work has gone into explaining this single object – a lot of creative effort has gone into getting the best possible understanding.”

The controversy is not likely to evaporate anytime soon. Avi Loeb, a Harvard astronomer who has proposed that Oumuamua could have been a light sail or some other alien artifact, was quick to disagree with the new paper.

“The authors of the new paper claim it was a water ice comet even though we didn’t see the comet’s tail,” Dr. Loeb said in an email. He added: “This is like saying an elephant is a zebra without stripes.”

Dr. Bergner and Dr. Seligman began collaborating on a solution to the Oumuamua mystery as postdoctoral fellows at the University of Chicago.

“We’ve never seen a comet in the solar system that didn’t have a dust coma,” said Dr. Seligman. “So the non-gravitational acceleration really was weird.”

Dr. Bergner, an expert in the chemistry of ice in outer space, wondered if gaseous molecular hydrogen, the lightest, most abundant and most volatile element in the universe, might be responsible for propelling the comet. But where would the gas have come from?

He found that laboratory experiments conducted since the 1970s showed that when ice is struck by high-energy particles, its molecules can break apart, leaving tiny bubbles of hydrogen gas trapped several meters deep in the ice.

“Basically, a comet traveling through the interstellar medium is cooked by cosmic radiation and forms hydrogen as a result,” Dr. Bergner said in a statement issued by the University of California, Berkeley.

She added in an email: “Water ice in its amorphous form has a spongy structure that contains pockets where other volatile molecules can get trapped. As the ice warms, it reorganizes into a more stable and compact structure.” This process, she said, “leads to the collapse of these pockets and the formation of channels within the ice, through which trapped gas can escape.”

For a normal-sized comet, this release of gas would have a negligible effect, Dr. Bergner said. “But because Oumuamua was so small, we think it actually produced enough force to drive this acceleration.”

And any dust in the ice would get trapped there, taking away much of the show from the comet’s tail.

In fact, in recent years, astronomers like Dr. Seligman and his colleagues have detected half a dozen “dark” comets: small bodies that exhibit acceleration but have no observable tails or comas. Hydrogen jets are probably not to blame in all cases, Dr. Bergner said, but “together they reveal that there is much to learn about the nature of small bodies in the solar system.”