Far beyond the orbits of Neptune and Pluto, a dark and mysterious expanse of space tantalizes astronomers. Here, as many as trillions of comets are thought to swarm, hurled to their present locale by Jupiter or other planets billions of years ago. They form a giant sphere known as the Oort cloud that envelops the solar system and stretches out to perhaps a couple of light-years from the sun. No one really knows just how many comets exist in the Oort cloud or its true extent because so little illuminating sunlight reaches that remote region. But occasionally a passing star or galactic tides will stir these icy leftovers from the solar system’s dawn, causing comets to fall toward the distant sun and into the observability of our telescopes. These so-called long-period comets have an orbit of thousands or millions of years and are predominantly small, no more than a few kilometers across. Yet last week astronomers announced the discovery of one with truly behemoth proportions: a giant comet that may measure hundreds of kilometers from edge to edge. “It was pretty shocking,” says Pedro Bernardinelli of the University of Pennsylvania, one of the researchers who found the object. Now efforts to train more telescopes in the comet’s direction to unearth its secrets of the deep are well underway.
Initially dubbed 2014 UN271, the object has been officially named C/2014 UN271 (Bernardinelli-Bernstein) for its discoverers: Bernardinelli and his University of Pennsylvania colleague Gary Bernstein. It was first observed in 2014 by a project called the Dark Energy Survey (DES), but Bernardelli and Bernstein only found the comet recently, after it popped out of their analysis of the 80,000 or so images taken by DES over the past several years. The images from 2014 revealed it to be lurking at about 30 times the distance between Earth and the sun, or 30 astronomical units (AU). Now, seven years on, the object is at 20 AU and continuing to approach us. Its closest point to the sun will be 10.9 AU, which it will reach in January 2031. That is not too much farther out than the orbit of Saturn—close enough that some have even envisaged sending a spacecraft to the object on a fleeting visit. Current estimates suggest the comet takes three million years to orbit the sun, traveling out to a distance of nearly 0.9 light-year—well into the Oort cloud—before swooping in again.
Both the object’s size and its looming proximity have captivated astronomers. “It’s very exciting,” says David Jewitt of the University of California, Los Angeles. Despite receiving 400 times less sunlight than Earth’s surface at its current location, the comet is bright enough to be seen by telescopes, which hints that its size must be somewhere between 100 and 370 kilometers. The uncertainty arises because of the object’s unknown reflectivity and shape. But at either end of the scale, this estimate would still make it much bigger than any previously known comet. The next largest in terms of its nucleus—Hale-Bopp, which wowed stargazers in 1997—measured a relatively paltry 60 kilometers across. The Bernardinelli-Bernstein comet is “certainly the largest comet we’ve seen in the modern astronomical era,” says Alan Fitzsimmons of Queen’s University Belfast. “We’ve had tremendously bright comets over recorded history, but that was before the invention of the telescope [in the 17th century].”
Efforts to study the object since it was announced have been swift. Already a team of astronomers has been able to detect signs of activity, most likely melting ices forming an atmosphere, or “coma,” around its solid nucleus, confirming it to be a comet. “Its brightness has increased a lot, which means that it’s active,” says Rosita Kokotanekova of the European Southern Observatory, who led the observations using a network of telescopes in the Southern Hemisphere. Getting continued rapid observations will be crucial in learning more about the comet. “There might still be a possibility we can see a rotational signal from the nucleus,” Kokotanekova says. “When the activity gets stronger, it will be completely obscured.”
Observing that activity will be enlightening, too, “because we’ve never observed a comet being active so far out [from the sun],” Kokotanekova says. This will allow researchers to probe the regions of the solar system where cometary activity begins. From the object’s initial apparition in DES optics in 2014 to 2018, it did not appear to show activity, meaning it likely “switched on” at some point in the past three years, Fitzsimmons says. “It’s going to give us a really nice ability to study what happens in this transition region—from being a frozen ice ball out in the Oort cloud to a fully active comet in the solar system.”
At its current distance, temperatures are too low for water ice to melt, so the Bernardinelli-Bernstein comet—which may be on its first foray into the inner solar system—must have some other type of ice that is melting. “The best guess would be carbon monoxide, because we know that’s present in comets, and it’s also very volatile,” Jewitt says.
In part because astronomers still know so little about the object and have never seen anything quite like it before, its exact nature remains unknown. Is it really a large comet or something else entirely? “Some people are speculating it could be round, almost in hydrostatic equilibrium, which makes it go in the direction of dwarf planets,” Kokotanekova says. This seems unlikely, however, given that most models suggest an icy object must be in the vicinity of 800 kilometers across before its own gravity begins sculpting it into a spherical shape. To pin down the object’s true size, Jewitt says the Hubble Space Telescope is the only current facility with sufficient power to peer through the coma and resolve the size of the nucleus. But as of this writing, his formal request to study the comet using the prized orbital observatory has not been approved. Other telescopes are capable of probing different features, though, such as its composition. “It’s so different from everything else we’ve observed that it’s very likely we’ll discover unexpected things,” Kokotanekova says.
Being able to observe the object for such a long time as it reaches its closest point to the sun, with a decade of observations ahead, will be hugely rewarding. Astronomers will be able to watch as it evolves, perhaps changing in its activity levels or even breaking apart. “The fact we can follow this thing for the next 10 years means there’s a lot of opportunity to discover more detail,” says Colin Snodgrass of the University of Edinburgh. And for the time being, a lot of what we might observe remains tantalizingly unknown, says Michele Bannister of the University of Canterbury in New Zealand. “This is something that’s been in the deep freeze for eons—hundreds of thousands of years at the very shortest,” she says. “And now it’s being heated by the sun. What’s going to happen? How active is it going to be? We don’t know yet. That’s going to be really fun to find out.”
The comet is also a taste of what is to come in the near future of solar system astronomy. In October 2023 a new telescope in Chile called the Vera C. Rubin Observatory will begin a 10-year survey of the entire overhead sky called the Legacy Survey of Space and Time (LSST). Thanks in part to its eight-meter mirror, Rubin will be able to discover much fainter objects than any of its predecessors, including many more expected large comets like this. “Typical telescopes find objects out to 50 or 60 AU,” says LSST team member Mario Jurić of the University of Washington. “With LSST, we can easily go out to 150 AU. We’re going to see things like [the Bernardinelli-Bernstein comet] maybe on a monthly basis.”
For the time being, C/2014 UN271 (Bernardinelli-Bernstein) remains the largest comet ever seen approaching the inner solar system, offering a glimpse into the secrets of our sun’s outermost reaches. How it behaves as it approaches Saturn’s orbit will be thrilling to watch, and the name Bernardinelli-Bernstein likely will not be forgotten any time soon. “It will be studied for years and years,” Kokotanekova says. “It’s only going to become more interesting. We’ll get to know it very well.”