Comet Hale-Bopp as observed from Earth in 1997
© Philipp Salzgeber
Comet Hale-Bopp as observed from Earth in 1997. The newly detected object could be even larger than Hale-Bopp, but it likely won’t be visible to the unaided eye.
An object of unusual size will make its closest approach to the Sun in 2031, during which time it will swing past the orbit of Saturn and possibly turn into an exceptionally large comet. Astronomers are already looking forward to the potential "fireworks" display.

The mystery object is called 2014 UN271, and it's currently 22 AU from the Sun (in which 1 AU is the average distance of Earth to the Sun), which means it has already passed the orbit of Neptune. The International Astronomical Union Minor Planet Center announced the detection on June 19. This object will travel another 11.1 AU before reaching its perihelion point (closest approach to the Sun) in 2031, after which 2014 UN271 will begin its long journey back to the Oort cloud.

Algorithms spotted the object in data collected by the Dark Energy Survey (DES) from 2014 to 2018, as Samantha Lawler, an astronomer at the University of Regina in Saskatchewan, Canada, explained in an email. Nearly 40 distinct observations of 2014 UN271 were made during this timespan, allowing astronomers to estimate its size, speed, and trajectory.

The object was initially spotted at a distance of 29 AU from the Sun, and it has traveled 7 AU during the past seven years. It follows an exceptionally elongated orbit, but the exact orbital period remains uncertain, with estimates ranging from 400,000 to 1 million years. Speaking to me over the phone, Meg Schwamb, a lecturer at Queen's University in Belfast and an expert on Kuiper Belt objects, said the estimated orbital periods are within the expected range for comets.

"There's a lot of uncertainty in how far away this object will get from the Sun at its most distant, but the close-approach distance is quite well measured," said Lawler. "This is because we're seeing it a lot closer to its closest approach than at its most distant point, and it's very weakly bound to the Sun because of its huge orbit, so small changes in close approach result in huge changes in the most distant point in the orbit."

Measuring somewhere between 62 and 230 miles (100-370 kilometers) in diameter, 2014 UN271 is one of the largest known objects to come in from the outer solar system. As citizen scientist Sam Deen noted in the Minor Planet Mailing List forum, it's "almost undoubtedly the largest Oort Cloud object ever discovered." Schwamb toned down the size estimate, saying 2014 UN271 is probably between 60 and 93 miles (100-150 km) in length, not even close to a dwarf planet in terms of size — nor is it likely to be spherical, she noted. Astronomers calculated its size by measuring the object's reflectivity; as it gets nearer, they'll be able to refine the size estimate even further.

That said, should the object develop a coma and tail during its perihelion in 2031, it will rank as among the largest comets ever recorded. Other examples of big comets include Comet Sarabat (observed in 1729), with an estimated nucleus around 62 miles (100 km) in diameter, and Comet Hale-Bopp, which had a nucleus from 25 to 50 miles (40 to 80 km). The newly detected object could very well be a record breaker, but we'll have to wait for more precise size estimates to be sure.

That 2014 UN271 will turn into a comet in 10 years is no guarantee, however. Comets are defined as celestial bodies that, as they approach the Sun, develop a coma, the result of sublimating ice (when solids turn directly into gas), and a tail.

"We expect that most bodies in the outer solar system have a large fraction of ices, so they should start sublimating if they get close to the Sun," Lawler explained. "But whether this particular object will have a coma as it gets to its closest approach will depend on how many times it has passed through the inner solar system and its exact composition — it may have less ice than we expect."

Interestingly, this object, should it turn into a comet, could be a pristine comet, meaning a comet that's experiencing its first flare-up during a close approach to the Sun. Observations of pristine comets are quite rare.

As Lawler pointed out, the object could be heated to far higher temperatures than anything it has experienced in the past million years or so. This could cause outgassing, or thermal stresses that will cause it to shatter and break up into tiny pieces. Or, it could do "absolutely nothing and remain a sturdy, unchanged ball of ice and rock," she said.

Astronomers also don't know if 10.9 AU — the closest this object will get to the Sun during its current orbit — is close enough to kindle cometary activity.

"Some cometary activity has been observed at these distances, but most comets need to get closer to the Sun before they grow a coma and a tail," said Lawler. "So, we really don't know. The cool part is, we're going to get to find out over the next decade!"

Should 2014 UN271 flare up into a comet, it likely won't be visible to the unaided eye or binoculars. But at an estimated magnitude between 16 and 17, it will most certainly be visible to telescopes.

Schwamb is looking forward to some potential "fireworks," including the possibility of the object breaking up, but we won't know until it happens. "Comets are like cats," she said. "You never know what they're going to do." Schwamb is particularly excited by 2014 UN271 because we could be witnessing the birth of a long-period comet. Lawler is curious to know its shape (she said it might be weird, like Arrokoth's pancake-showman shape) and whether it has rings like Chariklo, the first asteroid known to possess a ring system.

"It's cool that we're finding it now," said Schwamb. "With the Vera Rubin Observatory coming online shortly, we'll be able to create a movie of how this object will evolve over the next 10 years — we're going to keep our eyes on this object."

Lawler agreed that the Vera Rubin Observatory, set to begin operations in the next year or two, will be "perfect" for monitoring the object as it nears, but she expressed some concerns about our ongoing ability to make these sorts of astronomical observations.

"Depressingly, we know now that the Vera Rubin Telescope is going to be severely hampered by megaconstellations of satellites like Starlink, so we might not get as much information on this new object as we hoped, and we might miss discovering many more of them due to light pollution by satellites," Lawler explained. "Sorry to end on a depressing note, but I've been thinking a lot about how megaconstellations are going to really [hamper] the field of astronomy — discoveries like this are an excellent example of what we'll be missing if megaconstellation corporations get their way without any regulation on light pollution."

Indeed, astronomers have already expressed their concerns about the Starlink megaconstellation, currently being built by SpaceX, and its potential to impede work at the Vera Rubin telescope. Sad to think our view of the night sky is degrading as our most powerful ground-based telescopes finally come online.