Our sun is a ship; our galaxy the sea. Moving in cosmic currents, our star completes a lap of the Milky Way every 230 million years or so, with its retinue of planets in tow. For the most part, this journey is solitary, save for the occasional close encounter with another star. But a few years ago, something remarkable seems to have occurred. While traversing this vast, magnificent ocean, our sun may have come across a cosmic iceberg, a sizable hunk of hydrogen ice adrift in space. As unlikely as this scenario might seem, given that it would involve a new type of astrophysical object that has never been seen before, the evidence is strangely compelling—and the implications are broad.
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 arXiv.org). They examined existing data on an object called ‘Oumuamua, which became the first interstellar object discovered in our solar system in October 2017. Since then there has been some debate over whether it was a comet or asteroid; no one is quite sure. Seligman and Laughlin, however, say the object was neither. “We’re proposing that ‘Oumuamua was composed of molecular hydrogen ice,” Seligman says. “Basically, it was a hydrogen iceberg.”
Astronomers first spotted ‘Oumuamua after it had already made its closest approach to our sun, when it was already on its way out of our solar system. That situation made observations somewhat difficult, but researchers were able to discern a few of the object’s features. 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 scenario, in itself, would have some pretty fascinating implications for where ‘Oumuamua came from. Hydrogen ice sublimates (turns from solid to gas) at an extremely low temperature of just –267 degrees Celsius—only slightly higher than the ambient temperature of space: –270 degrees Celsius. 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. Within such a core’s lightless, dense depths, conditions can be cold enough for hydrogen ice to form.
“If you want to get that amount of hydrogen ice, you want to start with a very, very cold environment,” says Shuo Kong of the University of Arizona, an expert in molecular clouds who provided feedback for Seligman’s and Laughlin’s research but was not directly involved in the study. “And the coldest environment that is not very far from us would be these starless cores inside molecular clouds. They have very low temperatures in their central regions. So they could be the promising place for the formation of ‘Oumuamua.”
If the idea is true, the object would offer an unprecedented glimpse into these cauldrons of stellar formation. “The reason why that star formation process is so inefficient in molecular clouds is not fully understood,” Laughlin says. “If these molecular hydrogen objects are being formed, what that is telling us is the temperature in some clouds has to get extremely low, and the densities have to get relatively high. It’s providing a very interesting calibration point as to what conditions are leading to the formation of stars and planets.”
Bizarre as it might seem, this theory appears to explain a lot of ‘Oumuamua’s oddities. 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.
‘Oumuamua’s unusual cigar shape, too, can be explained by the theory. It may actually have been three times larger and spherical in shape—and composed of 99 percent hydrogen ice—when it first formed, likely less than 100 million years ago. The ice would have been worn away as it approached our sun and was heated for the first time, eventually dwindling into its elongated shape in the same way that a bar of soap wears down into a thin sliver over time.
The fact that ‘Oumuamua was discovered so rapidly and easily—as part of a four-year survey—also posed a problem for theorists. If it was an interstellar comet or asteroid—like the undisputed interstellar comet Borisov found in 2019—that conclusion would suggest that such objects are up to 100 times more prevalent than had been thought. In contrast, the “molecular cloud” theory of ‘Oumuamua’s origins would suggest there might be billions upon billions of these objects in the galaxy, in accordance with its quick discovery. “Even though it’s only one object that we observed, the number density that is implied is too high,” says Amaya Moro-Martín of the Space Telescope Science Institute, who proposed a different theory for ‘Oumuamua’s origin last year. “This proposal might solve that problem.”
Testing the theory on ‘Oumuamua any further is now impossible: the object is long gone from our sights. But with a bit of luck, astronomers could sooner or later evaluate its predictions. If they spot a similar interstellar interloper entering our solar system, they could observe a telltale change in the object’s mass as its hydrogen ice sublimates away. Upcoming telescopes such as the Vera C. Rubin Observatory in Chile, set to begin a 10-year survey of the solar system in 2022, could look for more.
With proposals to visit some of these objects via missions such as Europe’s Comet Interceptor, along with continued remote observations, the scientific possibilities for new investigations of the theory are tantalizing. Floating on our cosmic sea, these hydrogen icebergs that formed inside failed stars may lie in wait for us, secrets and all. “And there’s so many of them that we can actually study them up close,” Seligman says.