The estimated diameter of the nucleus of comet C/2014 UN271 (Bernardinelli-Bernstein) is approximately 137 km (85 miles) across, making it the largest nucleus ever seen in a comet by astronomers. The previous record holder is comet C/2002 VQ94, with a nucleus estimated to be 100 km (60 miles) across.
C/2014 UN271 was first observed by the Dark Energy Survey (DES) at a heliocentric distance of 29.3 AU in 2014, with additional observations through 2018.
But it was only discovered in 2021 when a focused trans-Neptunian object search of the DES data was done.
“This is an amazing object, given how active it is when it’s still so far from the Sun,” said Dr. Man-To Hui, an astronomer at the Macau University of Science and Technology.
“We guessed the comet might be pretty big, but we needed the best data to confirm this.”
So, Dr. Hui and colleagues used the NASA/ESA Hubble Space Telescope to take five photos of the comet on January 8, 2022.
The challenge in measuring this comet was how to discriminate the solid nucleus from the huge dusty coma enveloping it.
The comet is currently too far away for its nucleus to be visually resolved by Hubble. Instead, the Hubble data show a bright spike of light at the nucleus’ location.
The astronomers made a computer model of the surrounding coma and adjusted it to fit the Hubble images. Then, the glow of the coma was subtracted to leave behind the starlike nucleus.
The team then compared the brightness of the nucleus to earlier radio observations from the Atacama Large Millimeter/submillimeter Array (ALMA).
The Hubble measurements are close to the earlier size estimates from ALMA, but convincingly suggest a darker nucleus surface than previously thought.
“It’s big and it’s blacker than coal,” said Professor David Jewitt, an astronomer at the University of California, Los Angeles.
C/2014 UN271 is coming from the hypothesized nesting ground of trillions of comets, called the Oort Cloud.
The comet has been falling toward the Sun for well over 1 million years.
“This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the Solar System,” Professor Jewitt said.
“We’ve always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is.”
The study was published in the Astrophysical Journal Letters.
Man-To Hui et al. 2022. Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein). ApJL 929, L12; doi: 10.3847/2041-8213/ac626a