Behold: the largest known comet in our solar system

New data shows that a comet discovered in 2014 is in the record books. This icy object, dubbed Bernardinelli-Bernstein, is the largest comet ever spotted.

Comets are pieces of rock and ice that orbit the sun. These “dirty snowballs” in space are often surrounded by clouds of gas and dust. These misty shrouds come from frozen chemicals that sizzle on comets as they pass near the sun. But when it comes to comparing comet sizes, astronomers focus on a comet’s icy core, or nucleus.

Telescope images now show that the heart of Bernardinelli-Bernstein is about 120 kilometers (75 miles) in diameter, says David Jewitt. It’s about twice as wide as Rhode Island. Jewitt is an astronomer at the University of California, Los Angeles. His team shared their news in the April 10 Astrophysical Journal Letters.

Jewitt and his colleagues measured the comet using new images from the Hubble Space Telescope. The researchers also looked at images taken at far infrared wavelengths. (Infrared waves are too long for the eye to see, but are visible through some telescopes.)

The new data revealed more than the size of the comet. They also suggest that the comet’s nucleus only reflects about 3% of the light that hits it. This makes the object “blacker than coal,” says Jewitt.

The new record holder is much larger than other well-known comets. Take Halley’s Comet, which passes Earth approximately every 75 years. This space snowball is just over 11 kilometers (7 miles) in diameter. But unlike Halley’s Comet, Bernardinelli-Bernstein will never be visible from Earth with the naked eye. It’s just too far. Right now, the object is about 3 billion kilometers (1.86 billion miles) from Earth. Its closest approach will be in 2031. At that time, the comet will still not approach the sun closer than 1.6 billion kilometers (1 billion miles). Saturn orbits around this distance.

Comet Bernardinelli-Bernstein takes about 3 million years to orbit the sun. And its orbit is very elliptical. This means that it has the shape of a very narrow oval. At its farthest point, the comet can reach about half a light-year from the sun. That’s about one-eighth the distance to the nearest star.

This comet is likely “just the tip of the iceberg” for finding huge comets, Jewitt says. And for every comet this size, he thinks there could be tens of thousands of undetected smaller ones circling the sun.

Arline J. Mercier