Was it alien space junk? A wandering interstellar asteroid? Or a weird comet from another sun?
Ever since 2017, when astronomers in Hawaii discovered an object they called Oumuamua (Hawaiian for “scout”) zipping through the solar system, they have been arguing about what it was.
Telescopes saw only a tumbling dot that was already on its way back out into the interstellar dark. Astronomers deduced that it was reddish, cigar- or pancake-shaped, and perhaps a few hundred meters long. To date, all the comets observed in our solar system have ranged from around a half-mile to hundreds of miles across. (Halley’s comet is about seven miles wide.)
Initially Oumuamua was pegged as an asteroid, as it exhibited none of the sizzle and flash typical of comets. (Comets are basically dirty snowballs; when warmed by sunlight, they emit jets of steam, carbon dioxide and dust, which create bright tails or comas.) There was no evidence of gas or dust around the object, and radio telescopes heard nothing when pointed at it.
But further analysis revealed that something was making Oumuamua speed up as it exited 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, propelled by minuscule amounts of hydrogen gas spurting from an icy core.
“We show that this mechanism can explain many of Oumuamua’s peculiar properties without fine-tuning,” write Jennifer Bergner, an astrochemist at the University of California, Berkeley, and Darryl Z. Seligman, of Cornell University, in a paper published on Wednesday in Nature. “This provides further support that Oumuamua originated as a planetesimal relic broadly similar to solar system comets.”
In a statement issued by the University of California, Berkeley, Dr. Seligman said: “What’s beautiful about Jenny’s idea is that it’s exactly what should happen to 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 University of Hawaii’s Institute for Astronomy who has studied Oumuamua extensively, called the paper “a very interesting explanation.”
“I’m not willing to say it ‘solves’ things — the smoking gun there would be to have detected hydrogen spectroscopically,” she added. “But it is very plausible, and if another object is discovered that looks like Oumuamua, then all these models and explanations provide a lot of guidance for the observations. I’m amazed at how much work has gone into explaining this one object — a lot of creative effort has gone into getting the best understanding possible.”
The controversy is not likely to evaporate anytime soon. Avi Loeb, an astronomer at Harvard who has proposed that Oumuamua could have been a light-sail or some other alien artifact, was quick to take issue with the new paper.
“The authors of the new paper claim that it was a water ice comet even though we did not see the cometary 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 had never seen a comet in the solar system that didn’t have a dust coma,” Dr. Seligman said. “So the nongravitational acceleration really was weird.”
Dr. Bergner, an expert on the chemistry of ice in outer space, wondered if molecular hydrogen gas, the lightest, most abundant and most volatile element in the universe, could be responsible for propelling the comet. But where would the gas have come from?
He found that lab experiments done as far back as 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.
“A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming 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 fluffy structure containing pockets where other volatile molecules can be trapped. As the ice is warmed, it rearranges to 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-size comet, this release of gas would have a negligible effect, Dr. Bergner said. “But because Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.”
And any dust in the ice would remain trapped there, taking much of the show out of the comet’s tail.
Indeed, in recent years astronomers like Dr. Seligman and his colleagues have spotted a half-dozen “dark” comets: small bodies that exhibit acceleration but have no observable comas or tails. Hydrogen jets are probably not the culprits in every case, Dr. Bergner said, but “together they reveal that there is much to be learned about the nature of small bodies in the solar system.”
