Last year, scientists announced the discovery of a colossal comet remaining within the orbit of Neptune. They calculated, based on its brightness, that its icy core would be between 99 and 200 kilometers long. If the estimates were accurate, it would be the largest comet ever discovered.
But scientists wanted to make sure the title was apt, so in January they pointed the Hubble Space Telescope at the comet and accurately measured its nucleus. As reported this week in The Astrophysical Journal Letters, the comet’s core can be up to 136 km in diameter, that is, more than twice the width of the US state of Rhode Island (northeast US). It also has a mass of 500 trillion tons, equivalent to about 2,800 Mount Everest.
“It’s a hundred times larger than the typical comets we’ve studied all these years,” said David Jewitt, an astronomer and planetary scientist at the University of California at Los Angeles and one of the study’s authors.
Despite its impressive dimensions, the comet — named C/2014 UN271 (Bernardinelli-Bernstein) after its two discoverers — will only be visible to the naked eye for a brief period. It is racing towards the Sun at 35,200 km/hour. But at its closest approach, in 2031, it will come within just 1.6 billion kilometers of the Sun – just behind Saturn’s orbit – where it will appear as a faint glow in the night sky before returning to the shadows.
With Hubble’s help, however, astronomers can see and study this effervescent extraterrestrial visitor in all its glory, almost as if they were flying alongside it — a spectral blue haze enveloping an apparently glowing white heart. “The image they have is beautiful,” said one of the comet’s discoverers, Pedro Bernardinelli, an astrophysicist at the University of Washington who was not involved in the study.
Despite its weight, measuring the size of this comet’s nucleus proved difficult. Though far from the Sun, just a trickle of sunlight suffices to vaporize the core’s volatile carbon monoxide ices, creating a blinding dusty atmosphere called a coma.
Hubble was unable to clearly see the comet’s nucleus through this haze. But by taking these high-resolution images of the comet with the space telescope, Jewitt and his colleagues were able to make a computer model of the coma, allowing them to digitally remove it from the images. With only the core remaining, it was easy to scale.
Their analysis also revealed that its icy core is blacker than coal. This could in part result from being “baked by cosmic rays,” Jewitt said. High-energy cosmic rays have been bombarding the core, breaking the chemical bonds on its surface. This allowed some of the lighter elements, such as hydrogen, to escape into space, leaving the dark-hued carbon behind — making the core look a bit like an overly toasted slice of bread.
This dark core suggests that the comet – despite its giant size – is not very different from the others. “Comets’ nuclei are almost always superdark,” said Teddy Kareta, a planetary scientist at Lowell Observatory in Flagstaff, Arizona, who was not involved in the study. He suggests comparing comets to roadside snow piles. “Even though it’s still mostly ice, just adding a little dirt and dirt can make a pile of snow look nasty and dark.”
More comet secrets will be revealed as it approaches Saturn’s orbit. But in 2031, when it starts back on its 3-million-year-old circuit of the Sun, astronomers won’t know much more about its provenance, which is likely from the Oort cloud — a hypothetical and currently unobservable bubble around the filled solar system. of primitive ice fragments of varying shapes and sizes.
The comet is a welcome preview of what lurks inside this bubble. But “finding this thing is a reminder of how little we know about the outer solar system,” Jewitt said. “There are a lot of objects out there that we haven’t seen and a lot of things that we can’t even imagine.”
He added: “Who knows what the hell is going on out there?”
Translated by Luiz Roberto M. Gonçalves
