Although comets are most recognizable for their streaming tails, which can stretch for millions of miles, the heart of a comet is its solid nucleus. This nucleus is made of ice and dust, which forms a dirty snowball.
While most of the known comet nuclei measure a few miles across, astronomers using the Hubble Space Telescope spotted Comet C/2014 UN271 with a nucleus that reaches 85 miles across. That’s more than twice the width of Rhode Island.
This nucleus is about 50 times larger than those of other comets, and it has an estimated mass of 500 trillion tons, which is 100,000 times greater than the mass of a typical comet.
The comet is moving at 22,000 miles per hour from the edge of our solar system and will make its closest approach to us in 2031. But it will never get closer than one billion miles away from the sun — just a little farther than the distance between Earth and Saturn.
“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,” said study coauthor David Jewitt, a professor of planetary science and astronomy at the University of California, Los Angeles, in a statement. “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.”
Comets are relics from the early days of the solar system, icy leftover pieces from when the planets were forming. The gravity of the largest planets kicked comets out to the Oort Cloud, and the cloud is now the home of distant comets on the edge of our solar system that extends out into deep space. Comets travel back toward the sun when their orbits experience the gravitational tug of passing stars.
In a few million years, Comet Bernardinelli-Bernstein’s orbit will return it to the Oort Cloud.
“This is an amazing object, given how active it is when it’s still so far from the Sun,” said lead author Man-To Hui, assistant professor at the Macau University of Science and Technology in Taipa, Macau, in a statement. “We guessed the comet might be pretty big, but we needed the best data to confirm this.”
The research team used Hubble data to distinguish the comet’s nucleus from the coma, or the dusty envelope that surrounds a comet as it gets closer to the sun.
The heat of the sun warms the comet as it approaches, causing parts of it to sublimate, or transition from a solid to a gas. This cloudy coma is why comets look fuzzy when we see them through telescopes.
The team’s analysis not only revealed the size of the nucleus, but also the fact that it’s darker than coal, Jewitt said.
The comet experiences a 3-million-year-long, oval-shaped orbit. It’s now less than two billion miles from our sun.
Astronomers hope that studying Comet Bernardinelli-Bernstein could reveal more about the Oort Cloud, first hypothesized by Dutch astronomer Jan Oort in 1950. The cloud remains a theory because it’s too distant to be observed, so the largest structure in our solar system is essentially invisible.
NASA’s Voyager spacecrafts won’t reach the inner Oort Cloud for another 300 years — and it could take them 30,000 years to pass through it. But each comet that approaches the sun reveals more details about their mysterious home.