A pair of scientists may have figured out interstellar asteroid ‘Oumuamua’s secret — and it’s not an alien spaceship after all.
When the first known interstellar object passed through our Solar System in 2017, it got a lot of attention. Astronomers noticed that it was accelerating away from the Sun in a way that shouldn’t have been possible, leading to wild speculation from some big names who suggested it might even be part of an alien spacecraft.
In a recent paper in the journal Nature, University of California Berkeley geophysicist Jennifer Bergner and Cornell University astronomer Darryl Seligman suggest a simpler explanation: hydrogen gas escaping from beneath ‘Oumuamua’s surface.
An Interstellar Mystery
In 2017, astronomers realized that ‘Oumuamua was a visitor from outside our Solar System. And very soon after that, they realized that it seemed to move in a way physics said it shouldn’t.
As an object gets closer to the Sun, the pull of the Sun’s gravity gets stronger, which pulls the object inward even faster; if the object speeds up enough, it can overcome the pull of the Sun’s gravity and slingshot around the Sun in an elliptical orbit instead; if not, fiery doom awaits. That’s why most objects on elliptical orbits speed up as they approach the Sun and slow down as they move away.
And that’s what astronomers expected ‘Oumuamua to do — but in late June 2018, it abruptly sped up. Gravity couldn’t account for the sudden burst of acceleration.
Comets do that sort of thing pretty often, as the Sun’s heat vaporizes their upper layers of ice, causing jets of gas and dust to erupt from their surfaces. The result is like a rocket firing its thrusters; the comet speeds up, and it also rotates. But astronomers, peering into telescopes around the world and in orbit, couldn’t see any sign of gas erupting from ‘Oumuamua: it had no tail, and it had no coma (the hazy halo of gas and dust that surrounds the body of a comet).
Hypotheses abounded, each stranger than the next. Notably, Harvard University astronomer Avi Loeb suggested that ‘Oumuamua’s sudden acceleration might be thanks to the pressure of solar wind. And that’s where the aliens enter the picture. In a 2018 paper, Loeb and a colleague wrote, “One possibility is that '‘Oumuamua is a lightsail, floating in interstellar space as debris from advanced technological equipment.”
Other theories included the thought that it might be a solid chunk of frozen hydrogen; Seligman, a coauthor on the latest study, proposed that one in [year], but now says he was wrong.
“We had all these stupid ideas, like hydrogen icebergs and other crazy things,” says Seligman in a recent statement. But instead, it turns out, the answer is “just the most generic explanation,” as Seligman puts it.
Cosmic Rays and Weird Ice
Bergner unearthed several experiments dating back to the 1970s, which showed that if you zap a chunk of water ice with high-energy particles similar to cosmic rays, some of the water gets converted into molecular hydrogen (two hydrogen atoms, bonded by their shared custody of the same electron). Those hydrogen molecules end up trapped in the ice. Out in interstellar space, an object like ‘Oumuamua would be constantly bombarded by cosmic rays, so it makes sense that it would have a lot of hydrogen trapped in its upper layers of ice.
The Sun’s heat didn’t quite melt ‘Oumuamua’s icy outer layers — it never got hot enough to sublimate water or most organic molecules, and that’s part of why astronomers doubted that ‘Oumuamua had sped up thanks to erupting gas. But it did get warm enough for the ice to rearrange itself, shifting from amorphous ice (which seems to be more common in interstellar space) to the more familiar crystalline form. In the process, that shifting dislodged the bubbles of molecular hydrogen trapped in the ice, ejecting them from ‘Oumuamua with just enough force to act like very weak rocket thrusters, putting on a bit of extra speed and baffling astronomers.
Bergner and Seligman say it only worked because ‘Oumuamua is very small: about 115 meters long on its longest axis and just 19 meters across on its shortest. Most of the comets we know of in our Solar System are many times larger, ranging from half a mile to a few hundred miles wide, too massive to budge for such a faint exhalation of hydrogen.
And molecular hydrogen gas, especially in relatively small quantities, wouldn’t have shown up in the images and data astronomers collected.
“The main takeaway is that ‘Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing,” says Bergner in a recent statement.
What Is a Dark Comet?
Bergner and Seligman’s model would make ‘Oumuamua something called a “dark comet,” one without a visible tail or coma. They and their colleagues have found six other “dark comets” in the last few years. One key thing they’ve all got in common is a lack of dust.
Normal comets release huge clouds of dust along with their escaping gases; the coma is a mix of gas and dust, and typically comets have two tails – one of which is made entirely of dust. But astronomers saw no dust anywhere around ‘Oumuamua, and the same thing seems to be true of Bergner and Seligman’s other six “dark comets.”
That may be because all seven objects are so small. Smaller objects, with weaker gravity, may have a hard time holding onto enough dust to form a tail or a coma. And that may explain why most of the comets we know of are so much bigger than ‘Oumuamua: they’re not trailing dust behind them like Pigpen from the Peanuts comics, so they’re harder for astronomers to spot or to recognize as comets.
“The lack of known small Solar System comets could reflect an observational bias due to intrinsically lower dust production rates,” write Bergner and Seligman in their recent paper.
If that’s the case, it highlights how much we still need to learn about the smallest objects in our Solar System — and about the population of the Oort Cloud, which we’ve only observed a small fraction of. Some astronomers have even suggested that many of the objects in the Oort Cloud may actually be interstellar wanderers that our Sun’s gravity has temporarily captured.
What’s Next
‘Oumuamua is long gone, back to the cold darkness of interstellar space, but the questions (and sometimes heated debates) it left in its wake will be with us for the foreseeable future.
Loeb, for one, isn’t giving up on his lightsail hypothesis — whether it means ‘Oumuamua was alien debris or just a very shiny natural object getting shoved around by solar winds.
“Regarding my proposal of the acceleration being triggered by the reflection of sunlight: in September 2020 the same telescope that discovered ‘Oumuamua, Pan STARRS in Hawaii, observed another object exhibiting such a push without a cometary tail,” Loeb tells Inverse. “This object, named 2020 SO, was a rocket booster made of stainless steel, launched by NASA in 1966.”
The best way to test Bergner and Seligman’s idea is to study other small comets, which means keeping an eye out for the next interstellar visitor, but also looking for small comets from our own Solar System’s Oort Cloud.
Because the Oort Cloud lies outside the reach of the Sun’s solar wind, objects there are pummeled with cosmic rays just like objects drifting in interstellar space. If Bergner and Seligman are right, that means those comets’ upper layers should also contain pockets of molecular hydrogen. Larger comets won’t be able to tell us much about that; as they swoop closer to the Sun, they shed so much gas and dust that the hydrogen would be lost in the mix. They’re also too large for such a small amount of light gas to affect their motion.
Smaller comets, however — things around the size of ‘Oumuamua — could tell the tale. If astronomers spot other small objects accelerating strangely but without any sign of a tail, that will lend weight to Bergner and Seligman’s model.
“We haven’t spotted such objects, but the hope is that future telescopes will find them, and that instruments such as the James Webb Space Telescope will help us to investigate them in detail,” writes European Space Agency astronomer Marco Micheli, commenting on the recent paper. One of the future telescopes that might help spot dark comets is the Vera Rubin Observatory.
Meanwhile, the truth is out there.
