Maybe ‘Oumuamua was a hydrogen iceberg. Not a comet. Not an asteroid. Hydrogen.

Scientific American has an unexpected explanation for the mysterious interstellar object dubbed “‘Oumuamua” that drifted into our solar system in 2017. Maybe the monolithic object is something we’ve never seen before, a solid chunk of frozen hydrogen that otherwise would have helped form a new star:

The idea is the conclusion reached by Darryl Seligman of the University of Chicago and Gregory Laughlin of Yale University in a paper to be published in the Astrophysical Journal Letters (a preprint is available at

It measured about 400 meters long, was shaped like a cigar and was spinning rapidly at roughly one revolution every eight hours. Based on its extremely high-speed trajectory through our solar system, astronomers deduced that it was born elsewhere, because it was moving too fast to be bound to our sun. But somewhat surprisingly, ‘Oumuamua exhibited a slight but significant acceleration as it moved away—the exact opposite of what would be expected to happen to an outbound object fighting against the sun’s gravitational grip. “It was extremely weird,” Seligman says. “This was a force continuously pushing away from the sun with a magnitude of about one one-thousandth of the solar gravitational acceleration.”

Efforts to explain this anomalous acceleration suggested it may have been linked to vaporous jets of sunlight-warmed water ice blasting into space and pushing the object along. But that event alone could not have produced a force large enough to account for the observed acceleration, Laughlin and Seligman claim. “It would require more than 200 percent of the surface to be covered in water,” Seligman says. Seeking more plausible explanations, the researchers examined other types of ice that might have produced sufficiently potent jets to account for the acceleration. And the thing that worked best was hydrogen. “Because molecular hydrogen ice is held together so loosely, you only need 6 percent of the surface to be covered in [it],” Seligman says.

That fact suggests that a hydrogen-ice-rich ‘Oumuamua must have formed somewhere extremely cold. The best bet for such a chilly birthplace would appear to be within a giant molecular cloud—accumulations of dust and gas tens to hundreds of light-years wide where star formation takes place.

Over many millions of years, about 1 percent of the material in a typical giant molecular cloud will come together under the force of gravity to form stars. Before dissipating, each cloud can create thousands of stars—as well as myriads of protostellar cores—half-baked clumps of gas, roughly the size of our solar system, that never get compact enough to begin nuclear fusion and “switch on” as full-fledged stars.

Aside from the unusual acceleration, it would reveal why it entered our solar system at 26 kilometers per second—close to the speed at which the sun travels relative to the average velocity of other nearby stars. The object was not moving toward us. Rather we sailed toward it as it simply sat motionless, following its initial protostellar core’s failure to become a star.