There’s a giant ball of ice barreling through the solar system right now, and it’s bigger than any we’ve seen before. It poses no threat to Earth, but this comet, called C/2014 UN271 (Bernardinelli-Bernstein), has enraptured astronomers ever since its discovery in 2021. The hulking object, sometimes jovially called a “megacomet,” is 100 times bigger than most comets we see in the solar system. And now we’re learning more about it than ever before as it zooms toward its closest approach to our sun in 2031.
In a study published in the Astrophysical Journal Letters on June 12, Nathan Roth of American University and his colleagues report the first conclusive detection of carbon monoxide on the megacomet. That’s a crucial finding because it might tell us more about the object’s origins, history and likely upcoming behavior as it dives deeper into the solar system. “We wanted to test what drives activity in this comet,” Roth says. “It’s so far from the sun and so cold that trying to explain what makes a comet ‘work’ at these distances is difficult.”
C/2014 UN271 was first imaged by chance in observations from 2014. Seven years later, when astronomers actually spotted it in their archives, the comet was at more than 20 times the Earth-sun distance, inside the orbit of Neptune. But they also found that it is on a path that will bring it nearly to Saturn’s orbit in 2031 before it heads out again. The comet’s orbit is huge, extending out to about 55,000 times the Earth-sun distance—87 percent of a light-year and well out into the Oort Cloud of icy objects that surrounds our sun.
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Following the comet’s discovery astronomers used various telescopes, including the James Webb Space Telescope and the Hubble Space Telescope, to scrutinize it from afar. The object was initially thought to be as big as 370 kilometers (230 miles) across. Revised observations showed it to be about 140 kilometers (87 miles) wide. But that’s still the biggest anyone has ever seen—most comets in the solar system are only one or two kilometers across. “It’s huge,” says Quanzhi Ye, an astronomer at the University of Maryland, who was not involved in Roth’s study. “It represents a part of the cometary spectrum that we don’t understand.”
Some of those observations revealed bursts of activity from the comet, which sprouted an enormous, enveloping “coma” of expelled gas that stretches some 250,000 kilometers (155,000 miles) across (more than half the distance from the Earth to the moon). To find out the cause of this activity, Roth and his team used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe the comet in radio waves for about eight hours in March 2024.
They found a clear trace of carbon monoxide spewing from the comet, suggesting that its sprawling coma is fueled, at least in part, by carbon monoxide ice sublimating—turning from solid to gas—as the comet approaches the sun. The carbon monoxide appears to be vented in jets from spots on the object’s surface, possibly the result of the overhead sun heating a localized region and causing the ice to sublimate.
“If you were standing on the comet, and the sun was right overhead, this is the area where the sun is heating the surface the most and the jet originates from,” Roth says. What’s not clear so far, however, is how fast the comet is spinning and whether the location of the jets is changing over time. “Are there different jets being activated at different times? We don’t know yet,” Roth says.
As C/2014 UN271 gets closer, other ices that are often found on comets, such as methane and hydrogen sulfide ice, might start to sublimate, too, and add their own contributions to the object’s activity. “As we continue to monitor it, we’ll be able to get a better idea of the chemical fingerprint that’s preserved inside the comet,” Roth says.
Rosita Kokotanekova, an astronomer at the Rozhen National Astronomical Observatory in Bulgaria, who was not part of Roth’s research team, says the detection of carbon monoxide is useful because it is “important to identify what prompts activity at these large distances.” Researchers have witnessed gas venting from other, much smaller comets at a similar distance, “which was very puzzling,” she adds. “People were trying to figure out what exactly is causing this activity [so far from the sun].”
C/2014 UN271’s size makes it an especially alluring target for study. The presence of carbon monoxide ice is doubly interesting: Analysis of available data about the comet revealed that it exhibited signs of activity when it was more than 25 times as far out as the Earth-sun distance. But according to theoretical models, its carbon monoxide ice should have been sublimated by the sun’s rays when the object was even farther out in the solar system. This discrepancy may mean the comet made a pass of the sun before, with sublimation first eating away at layers of ice on its surface and its current activity only being kickstarted at closer distances, when heat from sunlight reached ice deeper within the object.
Finding a behemoth like C/2014 UN271, Kokotanekova says, could hint at the existence of a whole class of gigantic progenitor comets. Such comets might have been the first large, icy objects to coalesce in the solar system, after which they may could have eventually broken apart to form smaller comets. “It’s possible that the small objects are mostly fragments, while the large ones, like UN271, have never collided with anything,” she says.
That might mean there are more primordial megacomets awaiting discovery. If so, the recently completed Vera C. Rubin Observatory in Chile, which will begin a 10-year panoramic survey of the heavens later this year, could find more of them. “It’s so sensitive that it will certainly pick up comets of this size, quite probably even further away from us,” Ye says.
Rubin’s wide eye on the sky should also give us more information on C/2014 UN271 itself, says Meg Schwamb, an astronomer at Queen’s University Belfast uninvolved with this latest finding. “Rubin’s going to watch it come in,” she says. That could help us get a better handle on its activity, in partnership with telescopes like ALMA. “You need both of those pieces of information—if it got brighter, and whether the amount of carbon monoxide changed—to tell you what’s going on,” Schwamb says.
For now Comet UN271 remains a fascinating target of study, a giant comet like no other that is giving us a unique window into the dark frontiers of the outer solar system. “This is just an incredibly exciting object,” Roth says. And, for astronomers eager to learn more about this and other mega comets, the best is yet to come.
