‘Oumuamua was a comet after all, study suggests – 03/24/2023 – Science

Was it alien space junk? A wandering interstellar asteroid? Or a strange comet from another sun? Since 2017, when astronomers in Hawaii discovered an object hurtling through the solar system they’ve named Oumuamua (Hawaiian for “scout”), they’ve been debating what it could be.

Telescopes only saw a moving dot that was already heading back into the interstellar darkness. 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 about 800 meters to hundreds of kilometers in diameter — Halley’s Comet is about 11 kilometers wide.

Initially, Oumuamua was classified as an asteroid, as it did not exhibit any of the hiss and glows typical of comets—comets are basically dirty snowballs; when heated by sunlight, they give off jets of vapour, carbon dioxide and dust, which create glowing tails or “comas”. There was no evidence of gas or dust surrounding the object, and radio telescopes heard nothing when pointed at it.

But deeper analysis revealed that something was causing ‘Oumuamua to accelerate as it exited the solar system, leaving scientists with a delightful puzzle.

Now, two astronomers have discovered 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 gushing 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 at Berkeley, and Darryl Z. Seligman of Cornell University, in a paper published on Wednesday (22 ) in Nature. “This provides further support that ‘Oumuamua originated as a planetesimal relic extremely similar to comets in the solar system.”

In a statement released by the University of California, Berkeley, Seligman said: “What’s beautiful about Jenny’s idea is that it’s exactly what should happen with interstellar comets. We had all these silly ideas, like hydrogen icebergs and other crazy things. , and it’s just the most general explanation”.

In an email, Karen Meech, a comet expert at the Institute of 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 ultimate proof would be to detect hydrogen spectroscopically,” she added. “But it’s very plausible, and if another object like ‘Oumuamua is discovered, all these models and explanations will provide a lot of guidance for observations. I’m amazed at how much work went into explaining this single object—a lot of creative effort went into getting the best understanding.” possible.”

The controversy is not likely to go away any time soon. Avi Loeb, the Harvard astronomer who proposed that Oumuamua could have been a light sail or some other alien artifact, quickly took issue with the new paper.

“The authors of the new paper claim it was a water ice comet, although we did not see the comet’s tail,” Loeb said in an email. He added: “That’s like saying an elephant is a zebra without stripes.”

Bergner and 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,” Seligman said. “So the non-gravitational acceleration was really weird.”

Bergner, a specialist in the chemistry of ice in outer space, questioned whether molecular hydrogen gas, the lightest, most abundant and most volatile element in the universe, could be responsible for the comet’s propulsion. But where would the gas have come from?

He found that laboratory experiments done since the 1970s have shown that when ice is hit by high-energy particles, their molecules can break apart, leaving small bubbles of hydrogen gas trapped several meters deep in the ice.

“A comet traveling through the interstellar medium is basically being cooked by cosmic radiation, forming hydrogen as a result,” Bergner said in a statement released by the University of California at Berkeley.

She added in an email: “Water ice in its amorphous form has a porous structure, containing pockets where other volatile molecules can become trapped. As the ice is heated, it rearranges itself into a more stable, 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 full-sized comet, such an outgassing would have a negligible effect, Bergner said. “But because Oumuamua was so small, we think it actually produced enough force to power that acceleration.”

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

Indeed, in recent years, astronomers like Seligman and colleagues have detected half a dozen “dark” comets: small bodies that exhibit acceleration but have no observable comas or tails. The hydrogen jets probably aren’t the culprit in all cases, Bergner said, but “together they reveal that there is much to be learned about the nature of small bodies in the solar system.”

^{Translated by Luiz Roberto M. Gonçalves}