New studies of comet C/2014 UN271 (Bernardinelli-Bernstein) confirm that it’s the largest known comet

Comet C/2014 UN271 (Bernardinelli-Bernstein)
An article published in “The Astrophysical Journal Letters” reports a study on comet C/2014 UN271 (Bernardinelli-Bernstein) conducted thanks to observations made with the Hubble Space Telescope. A team of researchers also used previous observations to estimate that this comet’s nucleus is about 119 kilometers in diameter. This result confirms that it’s the largest known comet. The studies will continue even if it will remain very far from Earth also because it almost certainly comes from the Oort cloud.

Discovered examining Dark Energy Survey images from 2014 by astronomers Pedro Bernardinelli and Gary Bernstein, comet C/2014 UN271 (Image: Science. NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA). Image processing: Alyssa Pagan (STScI)) showed unusual characteristics from the beginning. It showed a coma indicating cometary activity at a far greater distance from the Sun than Saturn’s, and observations made it possible to estimate that it was a truly gigantic comet. Its trajectory will not take it into the inner solar system, so even the most powerful telescopes will be able to offer limited images and that’s also why the estimates of its nucleus’s size have been complex.

C/2014 UN271’s activity adds difficulty to the measurements of its nucleus. At those distances, distinguishing the nucleus from the coma is really complicated, so astronomers try to use the most powerful and sensitive instruments available today. An article published in “Astronomy & Astrophysics” in February 2022 reported an estimate of its diameter at about 137 kilometers based on observations conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope. Now another team took that data and combined it with others obtained using the Hubble Space Telescope.

It took a sophisticated analysis based on a computer model of the comet with its various parts to try to distinguish the emissions coming from the coma from those coming from the nucleus. The result was an estimate of the nucleus diameter with a probability peak at around 119 kilometers with a margin of error of plus or minus 15 kilometers.

Knowing the physical characteristics of comet C/2014 UN271 would help to understand something about the Oort cloud, the solar system’s outermost area. Currently, it’s impossible to see it directly because it’s full of objects too small to be detected by even the most powerful telescopes. C/2014 UN271 is a gigantic comet but was only seen when it got very close to the solar system’s planets. Setting up a space mission to intercept C/2014 UN271 and study it like the Rosetta space probe did with comet 67P / Churyumov–Gerasimenko is very complex, so at the moment there are only hypotheses and no real projects.

Other comets from the Oort cloud that came closer to the Sun lost a lot of mass over time but C/2014 UN271 will maintain its considerable size because it will remain far from the Sun. Monitoring its activity will continue and offer new information very useful to various astronomical studies.

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