Last year, scientists announced that they had discovered a massive comet lingering within the orbit of Neptune. They estimated its ice cores to be between 62 and 125 miles long, based on their brightness. If the estimates are accurate, this would be the largest comet ever discovered.
But scientists wanted to make sure the preference stuck, so in January they pointed the Hubble Space Telescope toward the comet and measured its nucleus accurately. As reported this week in The Astrophysical Journal Letters, a comet’s core can be up to 85 miles wide, making it more than twice the width of Rhode Island. It also has a mass of 500 trillion tons, which is approximately 2,800 Mount Everest.
“They’re 100 times larger than typical comets that we’ve been studying for all these years,” said David Jewett, an astronomer and planetary scientist at the University of California, Los Angeles, and author of the new study.
Despite its impressive dimensions, this comet – named C/2014 UN271 (Bernardinelli-Bernstein) after its discoverers – will only be visible to the naked eye for a brief period. It is moving toward the sun at 22,000 miles per hour. But at its closest approach, in 2031, it will get just a billion miles from the sun – just behind Saturn’s orbit – where it will appear as a faint glow in the night sky before bouncing back into the shadows.
However, with the help of Hubble, astronomers can see and study this exciting extraterrestrial visitor in all its glory, as if they were flying right next to it – a spectral mist of blue enveloping a seemingly bright white core. “The picture they have is beautiful,” said Comet co-author Pedro Bernardinelli, a University of Washington astrophysicist who was not involved in the study.
Despite its weight, measuring the size of this comet’s nucleus has been difficult. Although far from the sun, a little sunlight is enough to vaporize the volatile carbon monoxide ice in the core, creating a dark, opaque atmosphere known as a coma.
Hubble could not clearly see the comet’s nucleus through the haze. But by taking such high-resolution images of the comet with a space telescope, Dr. Jewett and his colleagues were able to create a computer model of the coma, allowing them to digitally remove it from the images. With only the core remaining, scaling it was a breeze.
Their analysis also revealed that its icy cores are blacker than coal. Dr Jewett said this may be due in part to her being “cooked by cosmic rays”. High-energy cosmic rays bombard 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 the dark-colored carbon – making the core somewhat like a highly-burning roasted slice.
This dark core indicates that this comet – no matter how super massive – is not much different from the others. “Comet cores are always pretty much dark,” said Teddy Caretta, a planetary scientist at the Lowell Observatory in Flagstaff, Arizona, who was not involved in the study. He suggests comparing comets to snowpacks on the side of the road. “Although it’s still mostly icy, just adding a little bit of dirt and grime can make a pile of snow look nasty and dark.”
More secrets of the comet will be revealed as it approaches Saturn’s orbit. But in 2031, when the returning leg of the Sun’s three-million-year circle begins, astronomers won’t know much about its source, likely in the Oort Cloud – a hypothetical and currently unobservable bubble around the Solar System filled with primitive shards of ice of various shapes and sizes.
C/2014 UN271 is a welcome preview of what’s hiding inside that bubble. “Finding this thing is a reminder of how little we know about the outer solar system,” said Dr. Jewett. “There is a huge amount of things we haven’t seen, and a huge number of things we haven’t even imagined.”
“Who knows what the hell is going on there,” he added.