Scientists confirm that a comet observed in 2014 is the largest ever seen

It's so large it's nearly in the minor planet category.
Chris Young
An artist's impression of 2014 UN271NOIRLab/NSF/AURA/J. da Silva (Spaceengine)

Save this one for the general knowledge quiz. An international team of astronomers just confirmed that a comet first observed in 2014 is the largest comet ever observed by human telescopes, a press statement reveals.

The team, from the Paris Observatory and the Instituto de Astrofísica de Andalucía-CSIC, published a paper in Astronomy and Astrophysics Letters detailing how they confirmed the size and ranking of Comet 2014 UN271.

When it was first observed, researchers determined that Comet 2014 UN271, also known as Bernardinelli-Bernstein, was originated in the Oort Cloud. The comet was as far away as Neptune, however, meaning astronomers weren't able to determine its size. As the comet gradually drew closer over the following years, it became clear that it was unusually large. Researchers started to estimate 2014 UN271's size, suggesting it might be up to 230 miles (370 km) across.

How scientists measured the record-holding comet

The new study drew from data collected by the Atacama Large Millimeter Array to determine a more accurate reading of the comet's size. Using that data, the astronomers were able to study the wavelengths of light bouncing off of 2014 UN271 to take their measurements. This is an unusual method, as comets are typically measured by the amount of sky they cover compared to their distance from Earth.

Scientists confirm that a comet observed in 2014 is the largest ever seen
233 GHz total image of 2014 UN271, also known as Bernardinelli-Bernstein. Source: E. Lellouch et al

The researchers made their measurements by honing in on the wavelengths of the comet's microwave radiation that weren't emitted by the dust coming off the space rock. They observed that for the comet to be reflecting so much light, it would have to be approximately 85 miles (137 km) across. While that's not quite as large as that earlier 230-mile estimate, it is large enough to make it the biggest comet ever observed and measured by scientists. It also brings it close to the minor planet category. Prior to 2014 UN271, the record holder for the largest-ever comet observed was Hale-Bopp, at roughly 45 miles (74 km) across.

Interestingly, Comet 2014 UN271 won't keep its record size for long. The researchers, who pointed out that their measurement of the comet is the most distant ever performed of a comet's reflectivity (albedo), said it will continuously lose ice as it approaches the Sun, meaning it will be half its current size when it starts to travel away from our star once again.

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