A gigantic comet is actually the largest ever seen, new observations by the Hubble Space Telescope confirm.
Stretching about 80 miles (129 kilometers) across, the nucleus (or solid center) of the comet, known as C/2014 UN271 (Bernardinelli-Bernstein), is larger than the state of Rhode Island, according to a statement from NASA. And it's about 50 times larger than the average comet core.
"This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system," David Jewitt, co-author of a new study confirming the comet's size and a professor of planetary science and astronomy at the University of California, Los Angeles (UCLA), said in the NASA statement (opens in new tab). "We've always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is."
This comet is currently far from Earth, zooming along at about 22,000 mph (35,405 kph). Comet Bernardinelli-Bernstein has been falling toward the sun for over 1 million years. But don't worry; the closest it will get to us, according to NASA, is about 1 billion miles(1.6 billion km km), which it won't even reach until 2031.
Previously, the comet that held the title for "largest nucleus" was C/2002 VQ94, which was spotted in 2002 and estimated to be about 60 miles (96 km) across.
This new behemoth of a comet was first observed in 2010. A few years later, astronomers Pedro Bernardineli and Gary Bernstein found the object in archival data gathered by the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile. Since its original discovery, the object has been studied using a wide array of instruments including both ground-based telescopes and space-based telescopes like Hubble.
With the observations from Hubble, researchers were finally able to officially confirm the mammoth size of this "dirty snowball." (Comets are nicknamed "dirty snowballs" as they are made up of rock, ice and other materials and debris, though the objects can vary in composition.) At this point in Comet Bernardinelli-Bernstein's orbit, in which it is "only" about 2 billion miles (3.2 billion km) from the sun, the icy object is about minus 348 degrees Fahrenheit (minus 211 degrees Celsius).
While frigid, this temperature is warm enough to allow carbon monoxide to sublimate (a process during which solid material becomes gas) off the comet's rocky surface, creating a "coma," an envelope of dust and gas that surrounds a comet's solid center.
"This is an amazing object, given how active it is when it's still so far from the sun," study lead author Man-To Hui, a researcher at the Macau University of Science and Technology, said in the same NASA statement. "We guessed the comet might be pretty big, but we needed the best data to confirm this." So, his team used Hubble to take five photos of the comet on Jan. 8, 2022.
The main challenge the team had in confirming the nucleus' size was differentiating between the nucleus and the comet's coma.
Bernardinelli-Bernstein is too far away for Hubble to exactly define its nucleus, but the team detected a light signal with the telescope, showing the comet's location. They were then able to use what Hubble observations they did have and, by using a computer modeling technique to show where the object's coma would be, they were able to determine the size of its nucleus.
The team compared their data with earlier observations made by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and found that earlier size estimates made with ALMA aligned with the new Hubble findings. And ALMA's radio observations allowed them to hone in on the object's reflectivity, showing that the comet's surface is darker than they expected.
"It's big, and it's blacker than coal," Jewitt said.
Scientists think Comet Bernardinelli-Bernstein is traveling from the Oort cloud, the most distant region of our solar system where huge numbers of comets reside. It's thought that the comets that lie in this immense, diffuse cloud formed closer to the sun but were tossed much farther out by gravitational interactions with our solar system's newborn giant planets. And they tend to stay out there unless another gravitational push nudges them our way.
This comet, being so far from Earth and originating in the farthest-flung reaches of our solar system, is thought to travel on a 3-million-year-long elliptical orbit around the sun. Scientists think that it might travel about half a light-year away from the sun in the farthest parts of its orbit.
These findings were described in a study published today (April 12) in The Astrophysical Journal Letters.