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What sent an interstellar comet zooming by our sun 6 years ago

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In 2017, ‘Oumuamua became the first observed interstellar object to zip through our solar system, and its appearance sparked questions scientists are still trying to answer.

The object, similar in shape to a cigar or a pancake, was the size of an asteroid and moved in a somewhat cometlike way but lacked a cometary tail.

Unlike comets, which can be several kilometers wide, ‘Oumuamua is estimated to be 377 by 364 by 62 feet (115 by 111 by 19 meters) in size.

Perplexing movements led some to speculate that the mystery object could even be an extraterrestrial spacecraft.

Now, new research aims to close the book on the interstellar comet’s orbit with a simpler explanation. A study describing the findings was published Wednesday in the journal Nature.

An interstellar visitor

As planets form around stars, gravitational interactions tend to kick out some of the small “leftovers,” such as comets and asteroids.

“Comets preserve a snapshot of what the solar system looked like when it was in the stage of evolution that protoplanetary disks are now,” said study coauthor Jennifer Bergner, assistant professor of chemistry at the University of California, Berkeley, in a statement. “Studying them is a way to look back at what our solar system used to look like in the early formation stage.”

These interactions send the space rocks zipping out of their planetary systems and into interstellar space, where they can travel for millions of years.

Scientists think that’s how ‘Oumuamua ended up passing through our solar system six years ago. The space object was first observed by the University of Hawaii’s Pan-STARRS1 telescope. The object, which had looped around the sun and was leaving our solar system, was dubbed ‘Oumuamua, which means “a messenger from afar arriving first” in Hawaiian.

This artist's illustration depicts 'Oumuamua moving toward the outskirts of our solar system after being heated by the sun.

Astronomers used telescopes around the world to watch the interstellar visitor for four months before it became too faint to observe.

But ‘Oumuamua didn’t neatly fit the proposed scenarios. The object most resembled an asteroid, but space rocks like asteroids move due to gravity.

‘Oumuamua was accelerating as it moved, which would require more than just gravity. The object appeared to be pushed, like comets are when they near the sun and the evaporation of their gas and dust by the sun’s heat causes a propulsive effect. The force of the ejected material also slightly alters the trajectory of comets, distinguishing them from asteroids and planets as they orbit the sun.

However, ‘Oumuamua didn’t look like a comet, nor did it have a tail or an envelope of gas and dust, called a coma, that all comets have.

Unraveling a space mystery

‘Oumuamua’s appearance sparked debate among astronomers as soon as it appeared.

“It was an exciting time within astronomy when ‘Oumuamua was first discovered, and it just became more and more intriguing since the more we learned about it, the harder it became to explain its behavior,” Bergner said. “As an astrochemist, my own scientific interest in ‘Oumuamua developed as models started emerging to explain its acceleration, which implied pretty unusual chemical properties of the object.”

Bergner heard a talk by Darryl Seligman, National Science Foundation postdoctoral fellow at Cornell University in Ithaca, New York, about what kinds of molecules might provide the best acceleration. The two began to work together on ways to test acceleration theories for the interstellar comet.

Together, Seligman’s deep understanding of ‘Oumuamua and Bergner’s background in ice phase chemistry allowed them to come up with a new theory.

The duo envisioned ‘Oumuamua as a water-rich comet traveling through the interstellar medium, or the space between stars. Over time, the reddish object was bombarded by radiation, which formed hydrogen inside the comet.

Bergner found previous research from decades past that revealed ice could be converted into molecular hydrogen, suggesting that the snowball-like structure of a comet could trap the hydrogen in bubbles within the ice.

The heat of the sun would then force the bubbles to release the gas in a fan-shaped spray.

“When the body is warmed, the water ice restructures to a more stable and compact form, and the trapped gas can escape,” Bergner said. “This could explain ‘Oumuamua’s behavior but doesn’t need to invoke any exotic chemistry or physics.”

As hydrogen gas released from the interstellar comet, it accelerated the small object. The fact that there was no dusty coma around ‘Oumuamua can also be explained using the same scenario.

“Even if there was dust in the ice matrix, you’re not sublimating the ice, you’re just rearranging the ice and then letting (hydrogen) get released. So, the dust isn’t even going to come out,” 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.”

The search for interstellar objects

Observatories like the James Webb Space Telescope can help astronomers learn more about where comets formed in planetary systems beyond our own, as well as the composition of these exo-comets, Bergner said.

When the Vera C. Rubin Observatory in Chile begins operations in 2025, it will aim to detect a few interstellar comets like ‘Oumuamua each year, allowing astronomers to better understand the nature of comets from other planetary systems.

“The comets and asteroids in the solar system have arguably taught us more about planet formation than what we’ve learned from the actual planets in the solar system,” Seligman said. “I think that the interstellar comets could arguably tell us more about extrasolar planets than the extrasolar planets we are trying to get measurements of today.”

(Extrasolar planets exist outside of our solar system.)

The Rubin Observatory Legacy Survey of Space and Time, or LSST, program, will observe the skies over the Southern Hemisphere for 10 years.

“If we had the LSST online prior to `Oumuamua’s passage through the inner solar system, we would have discovered it much earlier in its trajectory,” Seligman said. “We could have had the opportunity to send a space mission to intercept and rendezvous with the object because it came so close to the Earth. Therefore, in the future when we discover more interstellar objects, we will be able to send a dedicated space mission to an interstellar object.”

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