An approximate diameter of 137 kilometers makes it the largest Oort cloud object known so far.

The ALMA (Atacama Large Millimeter Array) telescope, located in Chile, has enabled a team of European astronomers to precisely establish the size of comet Bernardinelli-Bernstein as it approaches Earth.

This celestial object, also designated as C/2014 UN271, was discovered in 2014 by scientists Pedro Bernardinelli and Gary Bernstein when it was 29 astronomical units from the Sun, that is, 29 times the distance between Earth and our star. Since then it is considered that it comes from the layer of trans-Neptunian bodies, the outermost layer of the solar system, the so-called Oort cloud .

Its cometary nature was not confirmed until June 2021, when scientists also saw, due to its extraordinary brightness, that it was  a huge object, and then preliminary estimates established that it was between 100 and 370 kilometers wide.

The new measurement, made from a distance of 20 astronomical units, shows that previous estimates were correct, since it puts the comet’s diameter at about  137 kilometers , with a margin of error of 17 kilometers.

A Franco-Spanish team led by astronomer Emmanuel Lellouch, from the Paris Observatory, has drawn two key conclusions from this data. The first is that Comet Bernardinelli-Bernstein is the largest Oort cloud object ever found, nearly twice the size of Comet Hale-Bopp. The second is that it is confirmed that the Bernardinelli-Bernstein is also the second largest known comet in the solar system , behind the space body 95P/Chiron, an object of about 206 kilometers that was initially identified as a centaur, as the bodies are called. smaller ones that orbit the Sun, but which was later classified as a comet because it was observed to emit a cometary tail. 

The Bernardinelli-Bernstein is heading towards the center of the solar system, but the researchers estimate that it will never reach it and that its perihelion (the closest point to our star) will be 11 astronomical units and that it will reach it in 2031 . So, without crossing the orbit of Saturn, it will go around and return to the Oort cloud.

Lellouch’s team also calculated how much material is emitted from the comet and concluded that all of the measured flux can be attributed to the nucleus. Scientists predict that Bernardinelli-Bernstein will have shrunk to half its current size by the time it begins its journey back to the Oort cloud .