Scientists Have Seen the Largest Comet Ever: 15 Mount Everests

Scientists have confirmed the discovery of the largest “standard” comet ever spotted and further refined its dimensions at roughly 85 miles in diameter, which is about 15 times the altitude of Mount Everest and almost twice as big as the infamous comet Hale-Bopp, the previous record holder. 

Known as C/2014 UN271 (Bernardinelli-Bernstein), or BB for short, the icy behemoth is currently hurtling toward the Sun and will make its closest approach on January 21, 2031, offering an unprecedented view of an enormous comet from the far-flung reaches of our solar system.

In addition to being the largest known object of its kind, BB could be “one of the most ‘pristine’ comets ever observed,” according to a recent study posted on the preprint server arXiv that has been accepted for publication in the journal Astronomy and Astrophysics Letters. 

Led by Emmanuel Lellouch, an astronomer at the Observatoire de Paris in France, the new study follows up on the initial discovery of BB last year by its namesake astronomers, Pedro Bernardinelli and Gary Bernstein. Bernardinelli and Bernstein spotted the comet in archival images from the Dark Energy Survey that date back to 2014, which is why that year is part of the comet’s official scientific name.  

Lellouch and his colleagues said that BB is the largest “standard” comet yet found, but note that there is an irregular cometary body called Centaur 95P/Chiron that is bigger, at about 120 miles in diameter. However, Centaur 95P/Chiron is a weird object that is classified as both a comet and a minor body, so it does not fit into the typical mold of a comet.

Comet BB “is standard is the sense that its activity is continuous, with a coma that is present since 2017 and 2018 at 25 astronomical units and has steadily increased in brightness as the comet came closer to the Sun (currently at 20 astronomical units), which must be related to increased sublimation of volatile ices (NH3, CO2, CO...) as the temperature increased,” Lellouch explained in an email.

“Centaur 95P/Chiron also shows cometary activity, but in an irregular way, with large variations of the object magnitude, not correlated with the distance to the Sun,” he added, noting that comet BB is the largest comet from the Oort Cloud that surrounds the solar system, and the largest standard comet, but technically the second largest comet known to scientists, after Centaur 95P/Chiron.

Regardless, as the largest standard comet known to scientists, BB will be closely watched as it makes its approach toward the Sun in the coming decade. Monitoring its chemical composition as it approaches perihelion, which is the term for an object's closest pass with the Sun, “will be of high value,” the researchers added in the study.

“We hope to detect gasses from this comet (so far none have been detected) and measure their production rates as a function of distance from the Sun,” Lellouch said. “Because we now know the size, we will be able to investigate how active this comet is intrinsically,” he added, noting that this would allow for comparisons with comets such as Hale-Bopp

To get a better sense of the record-breaking hulk and characteristics of BB, Lellouch and his colleagues followed up on the initial sighting in August 2021 using the Atacama Large Millimeter Array (ALMA), which is the most sensitive telescope in the world at millimeter and submillimeter wavelengths. 

Not only did the researchers confirm and clarify the massive dimensions of BB, they were able to measure its albedo, or surface brightness, from a distance of 20 astronomical units, about twice as far as Saturn—the most distant measurement of a comet’s albedo on its inbound orbit ever performed. All of these observations will help scientists prepare for BB’s closest approach with the Sun in 2031, which will occur at about the distance of Saturn’s orbit, roughly a billion miles from Earth. 

While this is probably not close enough for naked-eye visibility from Earth, astronomers will no doubt point sophisticated observatories—including the recently launched James Webb Space Telescope (JWST)—at this huge object as it streaks across the sky, where it will potentially shed half of its size in the glare of the Sun. 

“Because the comet will remain distant (more than 11 astronomical units at perihelion), it will not be spectacular, ie. we expect that the production rates will remain modest, hence the largest telescopes will be needed: ALMA, JWST, the Very Large Telescope and ultimately the Extremely Large—which hopefully will be in service in time!”

Given that BB is estimated to complete its orbit once every three million years or so, this is a once-in-an-epoch chance to watch it up close. 

“Because the object is distant and therefore very slow on this orbit, the ‘close approach period’ will actually last quite some time,” Lellouch said. “We should have good quality observations for maybe ~10 years around perihelion (say, 2026-2036).”

Even so, save the perihelion date on your far-future calendar: January 21, 2031.

Update: This article has been updated to include comments from lead author Emmanuel Lellouch.