The new record, reported on the preprint website arXiv and now accepted for publication in the journal Astronomy and Astrophysics Letters, moves comet Hale-Bopp from the top spot. Hale-Bopp was discovered in 1995 and became visible to the naked eye in 1996; it was about 46 miles (74 kilometers) in diameter. Bernardinelli-Bernstein, also known as comet 2014 UN271, has now been calculated to be about 85 miles (137 kilometers) in diameter.
Comet Bernardinelli-Bernstein is named after its discoverers, University of Pennsylvania cosmologist Gary Bernstein and University of Washington postdoctoral researcher Pedro Bernardinelli, who first spotted the comet in the Dark Energy Survey data set.
The images showing the comet are from 2014, which is why this year is included in the comet’s official scientific designation. Bernardinelli and Berstein noticed that the small dot was moving as they studied images from subsequent years.
At that time, the comet was too far away for researchers to have a good idea of its size, although they could tell it was probably quite large.
The comet originated in the Oort cloud, a cloud of chunks of ice and rock hovering at the edge of the solar system. Its orbit takes it to within a light year of the sun — and takes 5.5 million years to complete.
The comet is currently heading towards the interior of the solar system. It will come close to Earth in 2031, but not too close for comfort: the comet will stay just outside Saturn’s orbit, Live Science reported.
The new research was led by Emmanuel Lellouch, an astronomer at Paris Observatory, and used data from the Atacama Large Millimeter Array in South America, taken in August 2021 when the comet was 19.6 AU away. (An AU is the distance between Earth and the sun and translates to about 93 million miles, or 150 million kilometers).
The researchers studied microwave radiation from the comet’s mass. From these reflected light wavelengths, the team was able to deduce the size of the comet. This is the longest distance at which this type of measurement has been done before, the researchers wrote in their new paper.
It’s exciting to get a measurement while the comet is still so far away, the researchers added, because Bernardinelli-Bernstein will likely shrink considerably as it approaches Earth. As the comet gets closer to the sun, its tail of dust and gas expands and its main body melts and shrinks.
The comet will not be visible to the naked eye, as Hale-Bopp was at its closest approach, but scientists expect to learn a lot about the objects in the visitor’s Oort cloud.
Large telescopes like the Atacama Array will allow scientists to learn more about the comet’s chemical composition as it passes by, Lellouch and colleagues wrote. They should also soon learn more about the comet’s temperature, rotation and shape.