Recent observations of comet C/2014 UN271 Bernardinelli-Bernstein, now approaching the inner Solar System, reveal a nucleus approximately 150 km wide—officially the largest comet core ever detected.
Also known as Comet Bernardinelli-Bernstein, C/2014 UN271 was first identified in Dark Energy Survey data from 2014 to 2018. Its remarkable brightness immediately signaled it belonged among the giants, with initial estimates placing its nucleus at least 100 km in diameter. These early figures underestimated its true scale, largely due to its extreme distance—over 29 astronomical units (AU) from the Sun, nearly matching Neptune's orbit.
Since then, the comet has drawn closer. On June 23, astronomers in New Zealand detected its coma—a glowing envelope of gas and dust produced by solar heat sublimating surface ices—when it was about 19 AU from the Sun.
Refined measurements, again based on luminosity, now confirm the nucleus measures about 150 km wide, surpassing all known comets. The previous record holder, Comet Sarabat from 1729, had a nucleus estimated at about 100 km. Most comets are far smaller, with nuclei typically just a few tens of kilometers across.
Detailed orbital analysis, submitted to Astrophysical Journal Letters, traces its path to a perihelion beyond 40,000 AU from the Sun. Its last aphelion was roughly 1.5 million years ago.
The comet's previous Solar System visit occurred about 3.5 million years ago, nearing within less than 18 AU of the Sun. This time, it will pass even closer at 10.9 AU in 2031, grazing Saturn's orbit. Experts eagerly await detailed observations of this Oort Cloud visitor.
