There’s a simpler answer to ‘Oumuamua’s weird orbit: Outgassing hydrogen

In late 2017, our Solar System received its very first known interstellar visitor: a bizarre cigar-shaped object hurtling past at 44 kilometers per second, dubbed 'Oumuamua (Hawaiian for "messenger from afar arriving first"). Was it a comet? An asteroid? A piece of alien technology? Scientists have been puzzling over the origin and unusual characteristics of 'Oumuamua ever since, most notably its strange orbit, and suggesting various models to account for them. But perhaps the answer is much simpler than previously thought. That's the conclusion of a new paper published in the journal Nature. The authors suggest that 'Oumuamua's odd behavior results from the outgassing of hydrogen as the icy body warmed in the vicinity of the Sun—a simple mechanism common among icy comets. As we reported previously, 'Oumuamua was first discovered by the University of Hawaii's Pan-STARRS1 telescope, part of NASA's Near-Earth Object Observations program to track asteroids and comets that come into Earth's vicinity. Other telescopes around the world soon kicked into action, measuring the object's various characteristics. Astronomers were startled to find that 'Oumuamua was accelerating away from our Sun much faster than could be explained by gravity alone—i.e., via a "rocket effect" that is common in comets, caused by sunlight vaporizing the ice such bodies are made of. While its odd orbit initially had it categorized as a comet, imaging didn't show any indication of gas and dust being released, as is typical when a comet approaches the Sun. Its elongated, cigar-like shape, combined with its relatively rapid rotation, led to an early suggestion that it could also be an asteroid. Several astronomers suggested that 'Oumuamua may be the fragment of another, larger parent body in another solar system—a long-period comet or debris disk, perhaps, or even a super-Earth planet—torn apart by tidal forces as it passed too close to its host star. Specifically, 'Oumuamua-like interstellar objects can be produced through extensive tidal fragmentation during close encounters of their parent bodies with their host stars, then ejected into interstellar space. Or perhaps it's a fragment of an exoplanet knocked off by an impact roughly a half-billion years ago, throwing it out of its parent system. Such a parent body would have had characteristics similar to Pluto, which is covered in nitrogen ice like Neptune's moon Triton. As for the strange acceleration and odd orbit, in October 2018, Harvard astronomer Avi Loeb and his then-post-doc, Shmuel Bialy, submitted a preprint (since published) to the Astrophysical Journal. As we reported then, much of their analysis discussed the possibility of solar radiation pressure, or the momentum transfer of photons striking an object—the driving idea behind "solar sails." Loeb and Bialy stirred controversy by suggesting that the object might be a very thin solar sail—specifically, "a fully operational probe sent intentionally to Earth vicinity by an alien civilization." Astronomers are pretty much in consensus that 'Oumuamua is a naturally occurring object. In 2021, scientists suggested that 'Oumuamua is composed of solid nitrogen ice, which can account for the object's strong push away from the Sun. They calculated the value for just how reflective the body would have to be to account for that strong push and how quickly various types of ice would sublimate. Solid nitrogen exactly matched their calculations. This also explains its unusual flattened shape. As the outer layers of nitrogen evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use. Enter Jennifer Bergner, an astrochemist at the University of California, Berkeley, who studies the chemical reactions on icy rocks in the cold vacuum of space. She approached her colleague Darryl Seligman—now a postdoc at Cornell University—with an even simpler alternative explanation. Seligman had previously suggested that 'Oumuamua might be a solid hydrogen iceberg capable of outgassing with sufficient force to affect its orbit. But such bodies have never been observed, and it wasn't clear how one might form or survive for 100 million years or so in interstellar space. "A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result," said Bergner. "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? Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?" The answer appears to be yes. Bergner dug up experimental research from the 1970s, 1980s, and 1990s describing how abundant amounts of molecular hydrogen are produced when ice is hit by high-energy particles like cosmic rays. That hydrogen could be trapped within pockets in the ice. When the ice warms, its structure rearranges, the pockets collapse, and channels form within the ice, allowing the trapped hydrogen gas to escape. Bergner and Seligman found there is enough ice under 'Oumuamua's surface to produce such an outgassing mechanism. One person who is not yet convinced: Avi Loeb. “The authors of the new paper claim that it was a water ice comet even though we did not see the cometary tail,” Loeb told The New York Times. “This is like saying an elephant is a zebra without stripes.” Bergner and Seligman say their model accounts for that. "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," said Bergner. "But because 'Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration." As a result, any dust in the ice would remain trapped there, so you wouldn't get that showy tail that is typical of comets. “I’m not willing to say it ‘solves’ things—the smoking gun there would be to have detected hydrogen spectroscopically,” Karen Meech of the University of Hawaii’, who is not affiliated with the study, told The New York Times. “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.” "If the authors’ model is correct, we should expect the effects of their mechanism to be observed in comets that are similar in size to ‘Oumuamua, but that originate in our own Solar System," Marco Micheli of the European Space Agency's Planetary Defense Office, wrote in an accompanying commentary. "We haven’t yet spotted such objects, but the hope is that future telescopes will find them, and that instruments such as the [Webb Telescope] will help us to investigate them in detail. Such discoveries would be welcome, given that ‘Oumuamua is no longer observable. And, now that we know what to look for, we are a step closer to the key observations that can conclusively prove whether we finally understand the nature of this fascinating object." DOI: Nature, 2023. 10.1038/s41586-022-05687-w  (About DOIs).