Right now, a colossal ice ball is hurtling through our solar system, exceeding the size of any previously observed comets. Known as C/2014 UN271 (Bernardinelli-Bernstein), this comet, often affectionately referred to as a “megacomet,” doesn’t pose any danger to Earth. However, it has captivated the astronomical community since its discovery in 2021. As it makes its way towards a close encounter with the sun in 2031, this giant comet, which is 100 times larger than the typical comets observed in our solar system, continues to be a subject of intense study.

A recent publication in the Astrophysical Journal Letters on June 12 highlighted breakthrough findings by Nathan Roth from American University and his team. They have made the first definitive discovery of carbon monoxide on the megacomet. This discovery is pivotal as it provides insights into the comet’s origin, its historical trajectory, and its expected behavior as it plunges deeper into the solar system. “We aimed to understand what stimulates activity in such a distant, cold comet,” Roth explains. This is challenging given the comet’s significant distance from the sun.

C/2014 UN271 was initially captured in images back in 2014 and was only identified seven years later in archival data. At that time, the comet was over 20 times the distance between Earth and the sun, well within Neptune’s orbit. Its path is projected to bring it close to Saturn’s orbit by 2031 before it journeys back out. The orbit of the comet is immense, stretching to about 55,000 times the Earth-sun distance—nearly 87 percent of a light-year, reaching into the distant Oort Cloud that encircles our sun.

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Since its discovery, astronomers have employed various telescopes, including the James Webb Space Telescope and the Hubble Space Telescope, to study the megacomet from a distance. Initial estimates pegged its diameter at about 370 kilometers (230 miles). However, more recent measurements adjusted its width to approximately 140 kilometers (87 miles), still marking it as the largest comet ever observed, with most solar system comets measuring only a couple of kilometers in diameter. “This comet is immense,” notes Quanzhi Ye, an astronomer at the University of Maryland, who was not involved in Roth’s study. “It represents an aspect of the cometary spectrum that is not well understood.”

Observations have shown that the comet exhibits bursts of activity, releasing a vast coma of gas extending about 250,000 kilometers (155,000 miles) across—more than half the distance from Earth to the moon. To determine 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 roughly eight hours in March 2024.

Their observations confirmed that carbon monoxide was being emitted from the comet, suggesting that its extensive coma is fueled in part by the sublimation of carbon monoxide ice—from solid to gas—as the comet nears the sun. This carbon monoxide likely escapes through jets on the comet’s surface, possibly caused by the sun heating specific localized areas, leading to sublimation. “Imagine being on the comet, with the sun directly overhead; this is where the sun heats the surface the most, and where the jet starts,” Roth explains. However, the rate of the comet’s spin and whether the jet locations shift over time remain unknown. “Whether different jets activate at different times is still a question,” he adds.

As C/2014 UN271 approaches the sun, other common cometary ices like methane and hydrogen sulfide might also begin to sublimate, contributing further to the comet’s activity. “With continued monitoring, we hope to better understand the chemical makeup preserved within the comet,” Roth states.

Rosita Kokotanekova, from the Rozhen National Astronomical Observatory in Bulgaria and not part of Roth’s team, believes the detection of carbon monoxide is significant as it helps clarify what triggers activity in such distant comets. “It’s intriguing to see gas venting from other smaller comets at comparable distances, which has been baffling,” she notes.

The sheer size of C/2014 UN271 makes it a particularly enticing subject for further study. The presence of carbon monoxide ice is intriguing as analysis indicates that the comet showed signs of activity beyond 25 times the Earth-sun distance, yet theoretical models suggest this ice should have sublimated even farther out in the solar system. This discrepancy suggests that the comet may have previously passed close to the sun, with its current activity sparked by sunlight reaching deeper layers of ice.

Kokotanekova suggests that discovering such a massive comet could indicate the presence of a whole class of large progenitor comets, which might have been the initial large icy bodies to form in the solar system, later breaking apart to create smaller comets. “It’s conceivable that the smaller objects are mostly fragments, while the larger ones, like UN271, have remained intact,” she remarks.

The recent completion of the Vera C. Rubin Observatory in Chile, which is set to start a 10-year comprehensive survey of the skies later this year, might uncover more such primordial megacomets. “Given its sensitivity, it will likely detect comets of this magnitude, possibly even more distant ones,” Ye notes.

Rubin’s comprehensive view could also shed more light on C/2014 UN271, according to Meg Schwamb, an astronomer at Queen’s University Belfast not involved with this discovery. “Rubin will track its approach,” she says. This, along with data from ALMA, will provide a clearer picture of its activity. “We need both data points—if it brightened, and if the carbon monoxide levels changed—to understand what’s happening,” Schwamb explains.

For now, Comet UN271 remains a compelling focus of study, a unique giant among comets that offers a rare glimpse into the mysterious reaches of the outer solar system. “This is an incredibly exciting object,” Roth concludes. And for astronomers keen on learning more about this and potentially other mega comets, the journey of discovery is just beginning.