The first interstellar object discovered in the Solar System, 1I/Oumuamua, was discovered in 2017 by ESA/Hubble, NASA, ESO, and M. Kornmesser.
'Oumuamua,' a mysterious comet, captured the imagination of scientists and the general public in 2017, having no visible coma or dust tail, as most comets, and a peculiar shape — something like a cigar or a pancake — that resembled an asteroid.
Scientists were puzzled by the fact that it was moving away from the sun in a way that astronomers could not explain. It may be an alien spaceship.
Astrochemist at the University of California, Berkeley, and a Cornell University astronomer have all suggested that the comet's strange deviations from a hyperbolic route around the sun may be explained by a simple physical mechanism common among many icy comets: the outgassing of hydrogen as the comet warms up in the sunlight.
What made 'Oumuamua different from every other well-studied comet in our solar system? Its smallness allowed it to deflect gravitational force around the sun without being slightly altered by the tiny push that hydrogen gas pumped out of the ice.
Most comets are essentially dirty snowballs that appear regularly in our solar system's outer reaches. When heated by sunlight, a comet ejects water and other molecules, resulting in a bright halo or coma, as well as tails of gas and dust. The ejected gases act like thrusters on a spacecraft, altering the comet's trajectory somewhat from the elliptical orbits typical of other solar system objects, such as asteroids and
The interstellar comet 'Oumuamua' was depicted by an artist as it warmed up in its approach to the sun and outgassed hydrogen (white mist) – making it the first known object other than dust grains to visit our solar system from another star.
'Oumuamua' had no coma or tail and was too small and too far from the sun to absorb enough heat to eject much water, causing astronomers to ponder wildly about its composition and what was causing it to go outward, such as a hydrogen iceberg or an alien civilization? A spaceship under its own power?
Jennifer Bergner, a UC Berkeley assistant professor of chemistry who studies ice crystals in the cold vacuum of space, considered there might be a simpler explanation. Darryl Seligman, a National Science Foundation postdoctoral fellow at Cornell University, proposed the idea.
“A comet going through the interstellar medium is basically getting cooked by cosmic radiation, forming hydrogen as a result.” Bergner asked. “Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?”
According to her, high-energy particles like cosmic rays can penetrate tens of meters into ice, converting a quarter or more of the water to hydrogen gas.
"You wouldn't necessarily expect that to be a detectable effect for a comet several kilometers across," she said, adding that because 'Oumuamua was so small, we think it produced sufficient force to power this acceleration."
The comet, which was somewhat reddish, is thought to have been around 115 by 111 by 19 meters in length. The actual size was not straightforward because it was too small and distant for telescopes to determine. The size had to be derived from the comet's brightness and how the brightness changed as the comet tumbled.
Jenny's suggestion is beautiful because she knows exactly what should happen to interstellar comets, according to Seligman. "We had all kinds of stupid ideas, like hydrogen icebergs, and other crazy things, and it's just the most general explanation."
Both were postdoctoral fellows at the University of Chicago when they began working on the paper.
Comets are icy rocks left over from the 4.5 billion years ago solar system's development, which can reveal important information to astronomers about the conditions that existed before our solar system was formed. Interstellar comets may also reveal details about the conditions around other stars that are enclosed by planet-forming disks.
"Comets provide a snapshot of what the solar system looked like when it was in the evolutionary stage that protoplanetary disks are today," Bergner said. "Studying them is a way to look back at what our solar system was like in the early formation stage."
Jennifer Bergner is a Ph.D. student at Harvard University. In the background is a system for studying interstellar space's frigid temperatures. Credit: Luke Kelley
Many distant planet systems will be ejected because of gravitational interactions with other objects in the system, which astronomers know happened over the history of our solar system. Some of these rogue comets should occasionally enter our solar system, providing an opportunity to study about planet formation in other systems.
"I think that interstellar comets might reveal more about extrasolar planets than the extrasolar planets we're trying to determine today," Seligman said.
Astronomers have written many papers explaining what we can learn from failure to observe any interstellar comets in our solar system.
'Oumuamua' came along.
On October 19, 2017, astronomers using the Pan-STARRS1 telescope at the University of Hawaii in Manoa discovered what they thought was a comet or an asteroid. They named it 1I/'Oumuamua (oh MOO-uh MOO-uh) which is Hawai'ian for "a messenger from afar arriving first." In 2019, a second, 2I/Borisov, was discovered, although it appeared and behaved more like a
The astronomers were able to observe 'Oumuamua''s orbit and see that it had already circled the sun and was heading out of the solar system as more telescopes focused on it.
The brightness of 'Oumuamua' was assumed to be long-lived and tumbling end to end, similar to that seen for asteroids and more typical of comets. When comets approach the sun, water and gases ejected from the surface form a glowing, gaseous coma and release dust in the process. Other substances, such as entrapped organic compounds and carbon monoxide, may also be released.
However, astronomers could not detect any coma, outgassed molecules, or dust around 'Oumuamua.' Furthermore, calculations showed that solar energy hitting the comet would be inadequate to sublimate water or organic compounds from its surface to give it the observed non-gravitational thrust. Only hypervolatile gases, such as H2, N2, or CO, might provide enough acceleration to match observations.
"We had never seen a comet in the solar system that had a dust coma." Seligman said the non-gravitational acceleration was "really strange."
This prompted much discussion about what kinds of volatile substances might be present in the comet that might be responsible for the strange acceleration. Seligman himself argued that if the comet was composed of solid hydrogen — a hydrogen iceberg — it would outgas enough hydrogen in the heat of the sun to explain the strange acceleration. Under the appropriate circumstances, a comet composed of solid nitrogen or solid carbon monoxide would also outgas with enough force to affect the comet's orbit.
astronomers must be patient in imagining what conditions might result in the formation of new hydrogen or nitrogen bodies, which have never been observed before. And how might a solid H2 body survive for maybe 100 million years in interstellar space?
Bergner considered that evaporating hydrogen trapped in ice might be sufficient to accelerate 'Oumuamua.' She is an experimentalist and a theoretician who studies the interactions between very cold ice and the kinds of energetic particles and radiation found in the interstellar medium.
Bergner and Seligman concluded that high-energy electrons, protons, and heavier atoms could convert water ice into molecular hydrogen, and that a comet's fluffy, snowball structure could trap hydrogen inside the ice. The gas could be released either by a collimated beam or fan-shaped spray, depending on how small a comet like ‘Oumuamua orbites.'
'Oumuamua' is consistent with being a standard interstellar comet that just underwent significant processing, according to Bergner. "The models we used are consistent with what we see in the solar system from comets and asteroids, so, you could essentially start with something that looks like a comet and have this scenario work."
The absence of a dust coma is explained by the notion.
"Even if there was dust in the ice matrix, you're not sublimating the ice," Seligman said.
Seligman said that their investigation into the origin of 'Oumuamua's acceleration should close the book on the comet. Since 2017, Bergner and his colleagues have identified six other small comets with no observable coma, but with small non-gravitational accelerations, suggesting that such "dark" comets are common. Together with 'Oumuamua, they reveal that there is much to learn about small bodies in the solar system.
1998 KY26, a dark comet, is the next target for Japan's Hayabusa2 probe, which recently collected samples from the asteroid Ryugu. The 1998 KY26 was initially thought to be an asteroid until it was discovered in December.
"Jenny is absolutely correct about the entrapped hydrogen," he said. "Between discovering other dark comets in the solar system and Jenny's fantastic suggestion, I think it's got to be correct. Water is the most abundant component of comets in the solar system and likely in extrasolar systems, as well."
Researchers speculate that the same mechanism might be at work in sun-approaching comets from the Oort cloud near the outer reaches of the solar system, where comets are irradiated by cosmic rays, much like an interstellar comet would be. Future studies of hydrogen outgassing from long-period comets might be used to test the hypothesis of H2 formation and entrapment.
The Rubin Observatory Legacy Survey of Space and Time (LSST) should uncover many more interstellar and dark comets, including one and three each year, according to Seligman.
Jennifer B. Bergner and Darryl Z. Seligman's Reference: "Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice" on 22 March 2023, Nature. DOI: 10.1038/s41586-022-05687-w
Bergner was aided by a NASA Hubble Fellowship grant. Seligman was aided by the National Science Foundation (AST-17152) and NASA (80NSSC19K0444, NNX17AL71A).