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Astronomers have confirmed that a 鈥渕ega comet鈥� flying towards the sun is the biggest comet from the outer solar system ever found.

In June 2021, astronomers announced the discovery of comet C/2014 UN271 (Bernardinelli-Bernstein) beyond the orbit of Uranus. Its brightness suggested it was a vast object originating from a cloud of icy bodies that surrounds our solar system, the Oort Cloud. It was estimated to be between 100 and 370 kilometres across, making it possibly the biggest comet ever seen.

at the Paris Observatory and his colleagues have now confirmed the object鈥檚 size. Measuring the heat radiating from the comet using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, they calculated that its solid nucleus inside the coma of dust and gas that surrounds it is about 137 kilometres across 鈥� more than half the length of Wales.

鈥淲e have confirmed the estimate,鈥� says Lellouch. 鈥淚t鈥檚 the biggest comet from the Oort Cloud ever found.鈥�

The comet reflects about 5 per cent of the light that hits its surface, making its reflectivity 鈥� or albedo 鈥� similar to other comets in the solar system and perhaps suggesting a 鈥渒ind of universal albedo鈥� among comets, says Lellouch. 鈥淭here is no trend that larger comets should be intrinsically brighter,鈥� he says.

The size of the comet, although lower than some upper estimates, still makes it twice the size of its closest known competitor, comet Hale-Bopp, discovered in 1995. There is another non-Oort Cloud comet that is technically larger 鈥� 95P/Chiron, which orbits between Saturn and Uranus and is thought to have a diameter of around 210 kilometres 鈥� but its status as a comet or minor planet is debated.

Nonetheless, comet Bernardinelli-Bernstein is a 鈥渞emarkable discovery鈥�, says Lellouch. It will make its closest approach to the sun in 2031 at 10 times the Earth-sun distance, and will be closely watched by telescopes 鈥� perhaps including the James Webb Space Telescope 鈥� before flying out into the solar system again on an orbital path that may last millions of years.

鈥淣ow that we know the size, we are going to be able to study how active this comet is,鈥� says Lellouch.

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