New data show that a comet discovered in 2014 is one for the record books. This frigid object, dubbed Bernardinelli-Bernstein, is the largest comet ever spotted.
Comets are chunks of rock and ice that orbit the sun. Such “dirty snowballs” in space are often surrounded by clouds of gas and dust. Those hazy shrouds arise from frozen chemicals sizzling off 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) across, says David Jewitt. That’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 sized up the comet using new images from the Hubble Space Telescope. The researchers also looked at pictures taken at far-infrared wavelengths. (Infrared waves are too long for the eye to see but are visible to some telescopes.)
The new data revealed more than just the comet’s size. They also suggest that the comet’s nucleus reflects only about 3 percent of the light that strikes it. That makes the object “blacker than coal,” Jewitt says.
The new record-breaker is way bigger than other well-known comets. Take Halley’s comet, which whizzes by Earth every 75 years or so. That space snowball is little more than 11 kilometers (7 miles) across. But unlike Halley’s comet, Bernardinelli-Bernstein will never be visible from Earth to the unaided eye. It’s just too far away. Right now, the object is about 3 billion kilometers (1.86 billion miles) from Earth. Its closest approach will be in 2031. At that point, the comet will still come no closer to the sun than 1.6 billion kilometers (1 billion miles). Saturn orbits at about that distance.
Comet Bernardinelli-Bernstein takes about 3 million years to circle the sun. And its orbit is highly elliptical. That means it’s shaped like a very narrow oval. At its farthest point, the comet may reach about half a light-year from the sun. That’s about one-eighth of the distance to the next nearest star.
This comet is likely “just the tip of the iceberg” for discovering huge comets, Jewitt says. And for every comet this size, he thinks there could be tens of thousands of smaller undetected ones circling the sun.