Cambridge, Massachusetts — A new analysis published in Journal of Geophysical Research: Planets has cast significant doubt on what many researchers considered the leading explanation for 'Oumuamua's anomalous acceleration: the hypothesis that the interstellar object is composed largely of frozen hydrogen.
'Oumuamua, discovered in October 2017 as it passed through Earth's solar system, was the first confirmed interstellar visitor—an object born around another star, ejected by gravitational encounters, and now wandering the galaxy. Its trajectory through our solar neighborhood revealed something puzzling: the object accelerated slightly as it receded from the Sun, in a manner inconsistent with gravity alone.
The hydrogen iceberg hypothesis, proposed by researchers seeking to explain the acceleration via natural processes, suggested that 'Oumuamua contains a subliming layer of frozen hydrogen. As solar radiation warmed the object, the hydrogen would vaporize and escape, creating thrust—much like a comet sheds gas as it approaches the Sun. This mechanism could produce the observed acceleration without invoking exotic physics or technological sources.
"The hydrogen iceberg idea was elegant," said Thiem Houk, lead researcher on the new analysis. "It's a known physical process—sublimation of volatiles. It doesn't require us to abandon conventional physics. But when we ran the numbers carefully, they don't add up."
The Problem with Hydrogen
The issue centers on hydrogen's thermodynamic properties. Hydrogen ice is extraordinarily volatile—it sublimates at temperatures far lower than the ices of water, methane, or ammonia that populate solar system comets. Even in the cold of interstellar space, hydrogen ice gradually converts to vapor over long timescales.
'Oumuamua has been traveling through space for millions of years before reaching our solar system. If it contained a substantial hydrogen layer, outgassing would have slowly eroded that layer long before the object ever encountered the Sun. By the time 'Oumuamua reached our neighborhood, any hydrogen envelope would likely have been completely lost.
"The diffusion of hydrogen through 'Oumuamua's bulk and subsequent escape into space is not a slow process," Houk explained. "We calculated that a hydrogen-rich layer would sublimate over timescales of thousands to tens of thousands of years—cosmic time is long, but 'Oumuamua has been traveling for millions of years. Any hydrogen should have been gone."
The team also examined whether hydrogen could have been replenished somehow—if, for instance, 'Oumuamua had formed in an unusually hydrogen-rich environment and had been heavily shielded by a protective layer. But such scenarios required increasingly baroque assumptions with little observational basis.
"Every assumption we made in favor of the hydrogen hypothesis required additional assumptions to explain away contradictions," noted co-author Lisa Kaltenegger, director of the Carl Sagan Institute. "At that point, you're not explaining the data with hydrogen—you're just adding epicycles."
What Remains Unexplained
The acceleration is real. 'Oumuamua's trajectory shows acceleration inconsistent with solar gravity and the modest pressure of solar radiation—the "Yarkovsky effect," which pushes small objects very slightly off their purely gravitational paths. The observed acceleration is too large to be explained by the Yarkovsky effect alone.
Several competing hypotheses remain on the table:
The first invokes outgassing of more conventional volatile ices—water ice or methane ice—which could produce thrust via sublimation. But 'Oumuamua's observed brightness and lack of detectable spectral features have made it difficult to confirm the presence of such ices. The object appears remarkably dark and featureless in visible light.
A second hypothesis proposes an unusual shape—perhaps a highly elongated cigar or flat pancake—which could interact with solar wind in ways that produce the observed acceleration. But the object's rotation appears relatively stable, which creates geometric constraints on possible shapes.
A third hypothesis, more controversial, suggests an artificial origin: that 'Oumuamua might be a solar sail or spacecraft of some kind, accelerated by directed propulsion. This hypothesis has drawn criticism for invoking extraterrestrial technology before exhausting natural explanations. But it remains formally consistent with the observations and is being studied in peer-reviewed literature alongside more conventional hypotheses.
"The honest answer is we don't know," Kaltenegger said. "We have observational data that doesn't quite fit existing models, and we have multiple explanations that are all problematic in their own ways. That's not mysterious—that's science in progress. We look for ways to resolve the ambiguity."
Future Observations
Future observations with more advanced telescopes might clarify 'Oumuamua's composition, surface properties, and rotation rate—all of which could constrain the possible explanations. The James Webb Space Telescope has limited ability to observe 'Oumuamua, as the object is now quite distant and dim. But next-generation ground-based telescopes coming online in the next few years might offer improved measurements.
The rulingout of the hydrogen iceberg hypothesis narrows the field of possibilities, even as it leaves the core question unresolved.
"Eliminating a leading hypothesis is progress," Houk noted. "We're moving closer to the answer, even if we're not there yet. And sometimes the most interesting discoveries come from resolving mysteries that don't fit our expectations."