Last year, scientists announced that they had discovered a colossal comet that hovered just inside Neptune’s orbit. They estimate the icy core to be between 62 and 125 miles long, based on its brightness. If the estimates were correct, this would be the largest comet ever discovered.
But scientists wanted to make sure the superlative stuck, so in January they pointed the Hubble Space Telescope at the comet and measured its nucleus with precision. As reported this week in The Astrophysical Journal Letters, the comet’s nucleus could be up to 85 miles in diameter, making it more than twice as wide as the state of Rhode Island. It also has a mass of 500 trillion tons, which is equivalent to about 2,800 Mount Everests.
“It’s 100 times bigger than the typical comets we’ve been studying all these years,” said David Jewitt, an astronomer and planetary scientist at the University of California, Los Angeles, and an author of the new study.
Despite its impressive size, this comet – named C/2014 UN271 (Bernardinelli-Bernstein) after its two discoverers – will only be visible to the naked eye for a short time. It hurtles toward the sun at 22,000 miles per hour. But at its closest approach, in 2031, it will come just within a billion miles of the sun — just behind Saturn’s orbit — where it will appear as a faint glow in the night sky before booming back into the shadows.
With Hubble’s help, however, astronomers can see and study this vibrant alien visitor in all its glory, almost as if flying right next to it — a spectral blue haze enveloping a seemingly bright, white heart. “The image they have is beautiful,” said comet co-discoverer Pedro Bernardinelli, an astrophysicist at the University of Washington who was not involved in the study.
Despite its weight, it proved difficult to measure the size of this comet’s nucleus. Though far from the sun, just a ray of sunlight is enough to vaporize the core’s volatile carbon monoxide ice, creating an obscuring dusty atmosphere known as a coma.
Hubble couldn’t see the comet nucleus clearly through that haze. But by taking such high-resolution images of the comet with the space telescope, Dr. Jewitt and his colleagues created a computer model of the coma, which allowed them to digitally remove it from the images. With only the core left, it was a breeze.
Their analysis also revealed that the icy core is blacker than coal. This may be due in part to being “cooked by cosmic rays,” said Dr. jewitt. High energy cosmic rays have bombarded the nucleus, breaking the chemical bonds on its surface. This allowed some of the lighter elements, such as hydrogen, to escape into space, leaving behind dark-tinted carbon — making the core a bit like a heavily burned slice of toast.
This dark core suggests that this comet – despite its super size – isn’t too different from others. “Comet nuclei are almost always super dark,” said Teddy Kareta, a planetary scientist at the Lowell Observatory in Flagstaff, Arizona, who was not involved in the study. He proposes to compare comets to piles of roadside snow. “While it’s still mostly ice, adding a little bit of dirt and grime can make a pile of snow look filthy and dark.”
More of the comet’s secrets will be revealed as it approaches Saturn’s orbit. But in 2031, when it begins the return of its three-million-year orbit from the sun, astronomers won’t know much more about its origin, likely in the Oort Cloud – a hypothetical and currently undetectable bubble around the solar system filled with primitive icy shards of various shapes and sizes.
C/2014 UN271 is a welcome sneak preview of what’s inside that bubble. But “finding this thing reminds us how little we know about the outer solar system,” said Dr. jewitt. “There’s a huge amount of objects that we haven’t seen, and a huge number of things that we haven’t even imagined.”
He added: “Who knows what the hell is going on there.”
