A strange, city-sized comet seems to have woken up early as it drifts toward the sun — even though it's still much farther than Saturn in space.
Using a giant Chilean desert telescope, the Atacama Large Millimeter/submillimeter Array, scientists got a detailed look at C/2014 UN271. This comet, commonly known as Bernardinelli–Bernstein, is one of the largest and most distant ever detected, and it’s doing something no one saw coming. Despite being extremely far from the sun's heat in the freezing outskirts of the solar system, this 500 trillion-ton behemoth is already breaking down and releasing gas.
Scientists aimed ALMA at the comet in March 2024 and saw carbon monoxide — a gas that usually doesn’t show up in observations that far away — gushing out of its surface in jets. The team also picked up signs of dust and heat coming from its icy, 85-mile-wide nucleus.
It’s the farthest comet ever caught doing this in millimeter-wavelength light, which the ground telescope uses to look at cold cosmic objects.
"We’re seeing explosive outgassing patterns that raise new questions about how this comet will evolve as it continues its journey toward the inner solar system," said NASA's Nathan Roth, lead author of the new research, in a statement.
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Comets, known for their millions-of-miles-long streaks, are among the oldest objects in the solar system. These boulders of ice, dust, and rock are leftover from the early days of planet formation, about 4.6 billion years ago.
Astronomers often think of them as time capsules of the ancient solar system, perhaps holding the secrets of how primitive Earth got water and other critical chemistry to eventually spawn life. So far there are over 4,000 known comets that scientists continue to track.
As comets swing closer and closer to the sun, their ice disintegrates, converting from a solid to a gas in one fell swoop, skipping over the liquid phase. Their tails are made of vaporizing ice, dust, and carbon dioxide debris.
But this comet — still 1.5 billion miles away — is already active and has been for at least the past year, according to the new study published in The Astrophysical Journal Letters. The findings show complex and evolving jets of carbon monoxide bursting from the comet’s nucleus.
"These measurements give us a look at how this enormous, icy world works," Roth said.
Bernardinelli–Bernstein has an estimated 3-million-year-long orbit, taking it about half a light-year away from the sun. It’s thought to come from the so-called Oort Cloud, a sphere of ancient, icy objects surrounding the solar system. NASA says this cloud remains a theory because the comets there have been too faint and distant to be directly observed.
Bernardinelli–Bernstein is now traveling inward, with its closest point to the sun expected in early 2031. The gas coming off the comet mostly seems to be carbon monoxide, which can evaporate at low temperatures. Scientists also looked for formaldehyde but didn’t find any.
What's perhaps more intriguing than the gas itself is how it's coming out. The telescope's view showed that the jets of gas were spurting from different locations on the comet that shifted around over time.
Shortly before the team's observations, the comet had an outburst — a short, bright flash of activity — then calmed down a few weeks later. That kind of behavior suggests the comet has a complex, active surface with lots going on just beneath it.
As the comet keeps heading toward the sun, traveling into warmer regions, more types of gases might begin to escape. Scientists think methane could start to emerge soon, followed by other chemicals like ethane and perhaps ammonia or acetylene closer to 2031.
The researchers say the comet may provide a rare "window" into the composition of small Kuiper Belt Objects, ice-rock bodies orbiting the sun beyond Neptune, thought to be relatively unchanged over billions of years.
"As UN271 continues to approach the Sun," the authors wrote, "additional volatiles should be expected to activate, revealing the primitive chemistry preserved within."
