A new paper says 'Oumuamua was a comet, not an alien spacecraft. Not everyone agrees

Nov 01, 2023

In 2017, a bizarre cigar-shaped object dubbed 'Oumuamua zipped through our solar system. It was the first time astronomers had seen an interstellar object pass through our solar system — meaning something that originated in another star system. The properties of 'Oumuamua, at least those which we could observe, were way out of the norm for what most asteroids and comets look like: it was unusually reflective; very oddly shaped (with one long axis and one thin one); and perhaps most bizarrely, it unnaturally accelerated as it left.

The researchers did calculations to see if 'Oumuamua was big enough to store enough entrapped hydrogen, and if that could account for its peculiar acceleration.

Naturally, the combination of multiple odd traits spurred a debate among astronomers over whether or not 'Oumuamua could have been artificially made — perhaps a light sail spacecraft built by an alien civilization — or the consequence of a natural phenomenon scientists didn't know existed.

But according to a new paper published in the journal Nature this week, some astronomers think the answer is less exciting than aliens: 'Oumuamua might have been a piece of debris from a water-rich comet, full of entrapped hydrogen. That hydrogen started to vent when it got near the sun, which explains the object's acceleration.

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"A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result. 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?" said co-author of the paper Jennifer Bergner, an astrochemist with the University of California, Berkeley, in a media statement. "Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?"

Bergner and Darryl Seligman, a postdoctoral researcher at Cornell University, were curious to run the numbers to figure out if perhaps 'Oumuamua was the leftover of a comet that was exposed to cosmic radiation, and which then released hydrogen trapped in its ice-water shield. To investigate this theory required a deeper understanding of what happens when radiation interacts with water and ice in space, taking into account the mass of an object like 'Oumuamua too.

The researchers did calculations to see if 'Oumuamua was big enough to store enough entrapped hydrogen and see if that could account for its peculiar acceleration. Indeed, the results supported their theory.

"What's beautiful about Jenny's idea is that it's exactly what should happen to interstellar comets," Seligman said in the media statement. "We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it's just the most generic explanation."

But the results of this study have yet to close this chapter of astronomy history. Instead, it has brewed up further debate surrounding the mystery of 'Oumuamua, with some questioning the paper's calculations.

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The theory that 'Oumuamua may be artificially constructed by an alien civilization was popularized by Harvard astronomer Avi Loeb, whose book "Extraterrestrial: The First Sign of Intelligent Life Beyond Earth" covered that theory in great detail. 'Oumuamua, which is Hawaiian for "an object from afar," was first observed by a postdoctoral researcher at the University of Hawaii who was sifting through the data stream from the Pan-STARRS astronomical survey of the sky. The researcher noticed the object was highly elongated, like a stick or a disk (the data allowed for either possibility), with a long axis 10 times longer than its short axis. Researchers suggested its shape would minimize abrasions from interstellar gas and dust, thus being an ideal shape for an interstellar spacecraft.

To date, two unexpected characteristics — its shape and how fast it accelerated after passing the Sun — have mystified astronomers. As 'Oumuamua whizzed by the Sun, it accelerated at a rapid speed, suggesting that it was propelled by sunlight as a solar sail spacecraft might have been — a type of spacecraft that would, indeed, be shaped like a disk. Meanwhile, there were no observed signs of cometary activity — such as a cometary tail, or gas emission absorption lines — which is why the possibility of it being a comet was initially ruled out by some.

In fact, a separate paper co-authored by Loeb speculated that the non-gravitational acceleration of 'Oumuamua was due to solar radiation pressure (as a solar sail would engender). In the same paper, Loeb theorized the acceleration could have been a result of cometary outgassing, but said it was unlikely because there was no evidence for a cometary tail around it.

Now, the researchers of the new Nature paper say otherwise.

"Here we report that the acceleration of 'Oumuamua is due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body," the researchers wrote in their paper in Nature. "In this model, 'Oumuamua began as an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellar journey, and experienced warming during its passage through the Solar System."

But what about the lack of evidence of a cometary tail?

Seligman said a coma, which is a cloud of gas surrounding the nucleus of a comet, consists of micron-sized dust particles that reflect sunlight and which travel with the gas.

"Typically what happens is you discover comets because they have a coma, and then you go and measure the gas via other means like getting spectroscopic measurements," he said. Seligman added that previous observations of comets show that it is possible to identify them based on outgassing, and not through a coma observation. "It's very plausible that there are lots of other objects in the solar system — and maybe in the interstellar comets — that don't cause a coma, but have accelerations that are due to outgassing and we just didn't see them before."

Seligman said one takeaway from their paper is that there could be more of these interstellar objects in space.

Loeb alleged that the Nature study "miscalculated the surface temperature of 'Oumuamua."

"It took something from interstellar space, the most intensely scrutinized small body over such a short timescale, to show us that there's this population of objects that are accelerating with no obvious dust coma," Seligman said. "And like there could be a lot more of them and they could teach us a lot about things like volatile delivery of planets and low levels of activity."

In an essay published on Meduim after the new Nature study adorned headlines, Loeb alleged that the Nature study "miscalculated the surface temperature of 'Oumuamua." Loeb said he and a colleague Thiem Hoang have submitted a new paper for publication to address the claim that the calculations are wrong.

"'Oumuamua did not exhibit any traces of carbon-based molecules or dust based on deep observations by the Spitzer Space Telescope," Loeb said. "It also did not show jitter from jets as a result of uneven sublimation of ice on its surface, nor a substantial evolution in its spin period, as often witnessed for evaporating comets."

In his essay, Loeb said suggesting that Oumuamua's acceleration can be explained by being a comet made out of water ice that was "dissociated into hydrogen by cosmic-rays in interstellar space" is incorrect because "their surface temperature calculation near the Sun ignored the crucial cooling effect of evaporating hydrogen."

"By adding the cooling from hydrogen evaporation, our new paper shows that the surface temperature of the iceberg is reduced by an order of magnitude," Loeb said.

In his essay, Loeb expressed disappointment that the astronomy community hasn't rallied more around the idea that 'Oumuamua could be of artificial origin.

"After I proposed the possibility that 'Oumuamua might be artificial in origin, there was a series of expert papers insisting that 'Oumuamua is a generic object of natural origin," Loeb said. "The experts disagreed with each other on what this generic object might be: a hydrogen iceberg, a nitrogen iceberg, a dust bunny, or a hydrogen-water iceberg in the paper that just appeared in Nature."

In a co-written response to Salon, Seligman and Bergner said they have "identified several reasons why the inclusion of effects such as H2 evaporative cooling and the heat released by the annealing of water ice are unlikely to alter our conclusions."

"While we are happy to enumerate these reasons, we believe that letting the independent peer review process evaluate the Hoang-Loeb paper is the best immediate course of action," Seligman and Bergner said.

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