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One of The Largest Comets Ever Seen is Headed Our Way


A comet so massive that it was initially misidentified as a dwarf planet is on its way in from the outer Solar System.


There’s no need to be concerned; C/2014 UN271 (Bernardinelli-Bernstein), as the comet is known, will pass just outside Saturn’s orbit. However, its large size and close proximity will provide a once-in-a-lifetime opportunity to study a pristine object from the Oort Cloud and learn more about the formation of the Solar System.


“We have the privilege of having discovered perhaps the largest comet ever seen – or at least larger than any well-studied one – and caught it early enough for people to watch it evolve as it approaches and warms up,” co-discoverer and astronomer Gary Bernstein from the University of Pennsylvania said earlier this year.

“It has not visited the Solar System in more than 3 million years.”

The outer Solar System is, for the most part, a mysterious place. It’s very far away, very dark, and the objects in it are quite small, so seeing what’s out there beyond Neptune’s orbit is difficult.


We have a general idea of the architecture of that region of space, with the Kuiper Belt made up of small icy bodies and the Oort Cloud at much greater distances, but the specifics are more difficult to drill down into.


However, we’re learning more from an unexpected source: the Dark Energy Survey (DES), which took place between August 2013 and January 2019.


It pored over the southern sky in infrared and near-infrared for the course of several hundred nights, studying objects such as supernovae and galaxy clusters to try to calculate the acceleration of the Universe’s expansion, which is thought to be influenced by dark energy.


The depth, breadth, and precision of the survey proved to be excellent for identifying objects in the outer Solar System, beyond Neptune’s orbit at around 30 astronomical units from the Sun. Earlier this year, a group of astronomers announced the discovery of 461 previously unknown objects in the outer Solar System in DES data.


C/2014 UN271 was one of those objects discovered by Bernstein and fellow University of Pennsylvania astronomer Pedro Bernardinelli (Bernardinelli-Bernstein). They and their colleagues have now provided a more detailed description of the comet in a preprint paper accepted for publication in The Astrophysical Journal Letters.

This composite image from the Dark Energy Survey (DES) shows the comet Bernardinelli-Bernstein in October 2017, when it was 25 Astronomical Units (AUs) away. Source: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/P. Bernardinelli & G. Bernstein (UPenn)/DESI Legacy Imaging Surveys


“We conclude that C/2014 UN271 (Bernardinelli-Bernstein) is a ‘new’ comet in the sense that there is no evidence for previous approach closer than 18 au to the Sun since ejection into the Oort Cloud,” the researchers write.

“Indeed, this may be the most pristine comet ever observed, in that we have detected it before it comes within Uranus’s orbit, and it may never have done so on any previous orbit.”


C/2014 UN271 (Bernardinelli-Bernstein) began its inward journey at a distance of around 40,400 astronomical units from the Sun, according to the team’s analysis. That’s very much in Oort Cloud territory, a huge sphere of icy objects that extends anywhere from around 2,000 to as far as 100,000 astronomical units.


The comet was discovered at a distance of about 29 astronomical units from the Sun. It will make its closest approach to the Sun in 2031, at a distance of 10.97 astronomical units; Saturn’s orbit, for comparison, has an average distance of 9.5 astronomical units.


At a size of 155 kilometers (96 miles) in diameter, C/2014 UN271 (Bernardinelli-Bernstein) is an absolute chonker, but even so, will not be visible to the naked eye at that distance.


Scientists, however, will be taking every opportunity to study it using telescopes. They hope that learning more about its composition will be able to tell us more about the early Solar System, and its far reaches.


This is because icy rocks from the far reaches of our solar system are thought to have remained relatively unchanged since their formation around 4.5 billion years ago. As a result, the volatiles trapped in the comet’s ices should contain information about the chemistry of the outer Solar System during its formation.


Scientists have already detected signs of a coma, the cometary atmosphere that appears as a comet approaches the Sun. The rising temperature sublimates ices on the comet’s surfaces, resulting in the visible coma and, at closer ranges, cometary tails. The spectral analysis of these features will reveal a lot about what’s inside C/2014 UN271 (Bernardinelli-Bernstein).


Since we know very little about the Oort Cloud and the objects therein, C/2014 UN271 (Bernardinelli-Bernstein) represents a very rare window into this enigmatic region of our home in space.


The research has been accepted into The Astrophysical Journal Letters, and is available on arXiv.

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