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Out There
Oumuamua: Neither Comet nor Asteroid, but a Cosmic Iceberg
A new study suggests the interloper may have arisen in an interstellar cloud, where stars are sometimes born.
It has been two and a half years since astronomers in Hawaii discovered a strange, cigar-shaped object speeding through the solar system on a trajectory from far away and toward even farther away. Today Oumuamua, the Hawaiian term for “scout,” as the object was named, is now long gone, somewhere between the orbits of Saturn and Neptune and on its way to the Great Out There, but astronomers are still wondering and debating what it was.
The cosmic interloper was first thought to be an interstellar asteroid, a chunk of rock shed by another star system. Then astronomers decided it must be a comet, likewise flung loose from some faraway star and planetary system. Briefly they speculated that it could be an alien artifact, a derelict probe like the giant spaceship in Arthur C. Clarke’s novel “Rendezvous With Rama,” or a fragment from a planetesimal that was ripped apart by a gravitational interaction or collision.
Now a pair of Yale astronomers have suggested that Oumuamua was neither an asteroid nor a comet. Rather, it was a cosmic iceberg: a chunk of frozen hydrogen.
Moreover, it was a primordial leftover, originating not from another planetary or star system at all but from a place and time where stars and planets didn’t exist yet: the deep, dark core of an interstellar cloud, one of the galumphing assemblages of gas and dust that shadow the starry lanes of the Milky Way, and where stars are sometimes born.
That might not sound as exciting as an alien spaceship, but if the assessment is accurate, it would provide astronomers with direct insight into stellar nurseries, a part of the universe that human technology cannot access.
These clouds, composed mostly of molecular hydrogen left over from the Big Bang, can contain the mass of tens of thousands of suns and span hundreds of light-years.
At their center, where no sun yet shines, protected from radiation, the temperature can plunge to a few degrees above absolute zero, cold enough for hydrogen itself, the lightest, most volatile and most common element in the universe, to freeze. In turn, these frozen particles, stick to small grains of interstellar dust, growing in a few thousand years into an ice cube 1,000 feet wide.
“Oumuamua is now long gone and is not observable in any way,” Darryl Seligman, postdoctoral researcher at the University of Chicago, said in an email. But, he added, if he and his colleagues are right, more cosmic icebergs will surely come to be detected, and inspected, by facilities like the new Vera Rubin Observatory in Chile.
The European Space Agency is mulling a project called the Comet Interceptor, a spacecraft that would be parked in space near Earth and could chase comets and other alien wanderers as they come through the solar system.
“If the hydrogen hypothesis is correct, we should be able to prove our theory by detecting future objects,” Dr. Seligman said.
Also, he added, it would mean “Oumuamua bears no genetic relation to Borisov,” referring to 2I/Borisov, an interstellar comet that visited our solar system late last year.
Dr. Seligman, then a graduate student at Yale and now a postdoctoral fellow at the University of Chicago, and his adviser, Greg Laughlin, came to this conclusion after studiously recreating Oumuamua’s trajectory and analyzing all the forces and influences on it during its journey. They published their results in the Astrophysical Journal Letters.
Karen Meech of the Institute for Astronomy at the University of Hawaii, whose team discovered Oumuamua, described Dr. Seligman as “a very creative and highly competent scientist” with whom she would happily collaborate, although she said she had not yet spent enough time with his paper to venture an opinion on his hypothesis about Oumuamua.
“There are lots of creative ideas out there about this object,” she said. She mentioned a suggestion by Sean Raymond, an astrophysicist at the University of Bordeaux, that Oumuamua was a fragment of alien planetesimal that had been sheared off by a close encounter in its home system.
Originally Oumuamua showed none of the sizzle and flash typical of comets — dirty snowballs that blow off gas and dust into tails and glowing comas when sunlight warms them. Astronomers concluded that it was inert, like an asteroid. Yet its brightness varied wildly, suggesting that it was irregularly shaped and tumbling.
Astronomers eventually concluded that the object was about 1,000 feet long and cigar-shaped, evoking the image of a rocket cylinder. But further analysis revealed that it was speeding up as it exited the solar system, evidence that gas boiling off the surface of the icy body was giving it a boost. That meant it was a comet after all.
But when Dr. Seligman and Dr. Laughlin tried to simulate the comet’s behavior based on calculations of how much energy it would have received from the sun, they found that water ice, the main constituent of ordinary comets, did not provide enough oomph to explain the comet’s acceleration.
“Frozen hydrogen does, however, offer a compelling mechanism for acceleration,” Dr. Seligman said. Being more volatile, it could easily supply the necessary energy boost.
It wouldn’t take much, he noted. The subliming hydrogen would add about 200 feet per second to the velocity of a cosmic ice ball that was already speeding along at 40 miles per second.
“This speed increment would be a big deal on the autobahn, but it’s a rounding error on the cosmic racetrack,” Dr. Seligman said.
Dr. Laughlin added in an email, “As Oumuamua passed close to the Sun and received its warmth, melting hydrogen would have rapidly boiled off the icy surface, providing the observed acceleration and also winnowing Oumuamua down to its weird, elongated shape, much as a bar of soap becomes a thin sliver after many uses in the shower.”
But that version of events would require rewriting Oumuamua’s origin story yet again. Forget about alien planet systems or even aliens. Solid hydrogen can exist only at temperatures below 6 degrees Kelvin, or 6 degrees Celsius above absolute zero. To find temperatures cold enough to freeze hydrogen out of the interstellar gas, you would have to go inside the coldest, densest lumps, called prestellar cores, in big, dark molecular clouds.
“In fact, you need to do it in a starless core, because if you formed a star, then the radiation would destroy all of the icebergs,” Dr. Seligman said.
Interstellar molecular clouds are interesting places. Radio astronomers from the University of Arizona recently detected the presence of many of the organic molecules thought necessary for the evolution of life, such as methanol and acetaldehyde, in the dark, icy cores of these clouds.
Some of these clouds eventually collapse when they are thrown off balance by the shock waves of supernova explosions or collisions with other clouds. The result can be a profusion of starbirth, as captured in the famous Hubble Space Telescope image known as the Pillars of Creation.
Luckily for those who would like to study a sample of these primordial prebirths and probe their chemical secrets, such clouds do not always collapse and bear stars. They can disperse, leaving behind a cosmic iceberg to float among the stars.
In this case, Dr. Seligman calculated, the object that would become Oumuamua wandered for less than 100 million years — not long, on the galactic scale — before encountering us.
If this version of events is accurate, telescopes like the Vera Rubin Observatory, which is designed to scan the entire sky every three days, are likely to find more wandering hydrogen icebergs entering the solar system, Dr. Seligman said. Astronomers will be able to watch them light up and evolve as the erosive power of sunlight gets to work.
For those of us stuck probably forever near our home star, you have to appreciate a universe that delivers takeout.
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