n 2017, a mysterious comet dubbed ‘Oumuamua fired the imaginations of scientists and the public alike.
It was the first known visitor from outside our solar system—and it was decidedly odd:
it had no bright coma or dust tail as most comets do, it had a peculiar shape, and its small size more befitted an asteroid than a comet.
But the fact that it was accelerating away from the sun in a way that astronomers could not explain perplexed scientists, leading some to suggest that it was an alien spaceship.
Now, a new study from astrophysicists affiliated with the University of Chicago argues that the comet’s mysterious behavior can be explained by a simple physical mechanism likely common among many icy comets: outgassing of hydrogen as the comet warmed up in the sunlight.
“There have been a lot of exotic theories for this behavior, but with this model, we can explain it with generic physics and chemistry,” said the study’s lead author, Jennifer Bergner, then a postdoctoral researcher with the University of Chicago and now a UC Berkeley assistant professor of chemistry.
“As a scientist, you’re always looking for the simplest explanation, so it’s very satisfying to see all the pieces fall into place.”
A messenger from afar
On Oct. 19, 2017, on the island of Maui, astronomers noticed what they thought was either a comet or an asteroid.
Once they realized that its tilted orbit and high speed implied that it came from outside our solar system, they gave it the name 1I/’Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawai’ian for “a messenger from afar arriving first.”
It was the first interstellar object aside from dust grains ever seen in our solar system.
As more and more telescopes focused on ‘Oumuamua, the astronomers were able to chart its orbit and determine that it had already looped around the sun and was headed out of the solar system.
Because ‘Oumuamua’s brightness changed periodically and varied asymmetrically, it was assumed to be highly elongated and tumbling end over end. Astronomers also noticed a slight acceleration away from the sun, larger than seen for asteroids and more characteristic of comets.
All of this was highly strange to astronomers. Most comets are essentially dirty snowballs that periodically approach the sun from the outer reaches of our solar system. When warmed by sunlight, a comet ejects water and other molecules, producing a bright halo around it and often tails of gas and dust.
The ejected gases act like the thrusters on a spacecraft to give the comet a tiny kick that alters its trajectory slightly from the elliptical orbits typical of other objects, such as asteroids and planets.
But when ‘Oumuamua was discovered, it had no tail and was too small and too far from the sun to capture enough energy to eject much water, which led astronomers to speculate wildly about its composition and what was pushing it outward. Was it a hydrogen iceberg? A large, fluffy snowflake pushed by light pressure from the sun? A light sail or spaceship created by an alien civilization?
Bergner, however, studies the chemical reactions that occur on icy rocks in the cold vacuum of space. She thought there might be a simpler explanation, and decided to test it together with Darryl Seligman, an expert in interstellar objects, then a postdoc at UChicago and now a National Science Foundation postdoctoral fellow at Cornell University.
Perhaps, they wondered, its strange acceleration actually came from hydrogen.
“A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result,” Bergner explained. “Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?”
If so, perhaps the force produced by the hydrogen outgassing could explain ‘Oumuamua’s odd movement.
When she checked the literature, Bergner found that experimental research published in the 1970s, ‘80s and ‘90s demonstrated that when ice is hit by high-energy particles akin to cosmic rays, H2 is abundantly produced and trapped within the ice. In fact, cosmic rays can penetrate tens of meters into ice, converting a quarter or more of the water to hydrogen gas.
Normally, the scientists said, comets are so large that the hydrogen doesn’t really affect their movement. But not so for the tiny ‘Oumuamua.
“For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn’t necessarily expect that to be a detectable effect,” she said.
“But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration.”
“What’s beautiful about Jenny’s idea is that it’s exactly what should happen to interstellar comets,” Seligman said. “We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation.”
Because H2 should form in any ice-rich body exposed to energetic radiation, the researchers suspect that the same mechanism would be at work in sun-approaching comets from the Oort cloud at the outer reaches of the solar system, where comets are irradiated by cosmic rays, much like an interstellar comet would be.
Future observations of hydrogen outgassing from long-period comets could be used to test the scenario of H2 formation and entrapment.
‘Dark’ comets
Seligman said that their conclusion about the source of ‘Oumuamua’s acceleration should close the book on the comet.
Since 2017, he, Bergner and their colleagues have identified six other small comets with no observable halos, but with small non-gravitational accelerations, suggesting that such “dark” comets are common.
While H2 is not likely responsible for the accelerations of dark comets, Bergner noted, together with ‘Oumuamua they reveal that there is much to be learned about the nature of small bodies in the solar system.
One of these dark comets, 1998 KY26, is the next target for Japan’s Hayabusa2 mission, which recently collected samples from the asteroid Ryugu. The 1998 KY26 was thought to be an asteroid until it was identified as a dark comet in December.
Many more interstellar and dark comets should be discovered by the Rubin Observatory Legacy Survey of Space and Time (LSST), allowing astronomers to determine if hydrogen outgassing is common in comets.
Seligman has calculated that the survey, which will be conducted at the Vera C. Rubin Observatory in Chile and is set to become operational in early 2025, should detect between one and three interstellar comets like ‘Oumuamua every year, and likely many more that have a telltale coma, like Borisov.
Adapted from an article by Robert Sanders published by the University of California Berkeley.
