Astronomers make incredible discovery in the largest Oort Cloud comet ever detected

Astronomers are reporting a detailed analysis of the largest Oort Cloud comet ever detected – “C 2014 UN271.” Carbon monoxide gas is venting from its surface while it is still more than a billion miles from the Sun.

Comet C 2014 UN271, also called “Bernardinelli Bernstein.” It is active at a distance where most comets should be quiet, or so it was believed.

Nathan X. Roth, a planetary scientist at NASA’s Goddard Space Flight Center, led the team behind this remarkable discovery.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful network of 66 radio antennas that observes invisible light from cold gas and dust, his group focused on the comet long before it neared the inner solar system.

They observed it from more than a billion miles away, in a region where sunlight is faint and temperatures are hundreds of degrees below zero.

Even in that extreme cold, they detected bursts of gas escaping from beneath its frozen surface, proof that the largest known comet is far from dormant.

Why C 2014 UN271 is special

This comet is about 85 miles wide, which makes it larger than many small cities. Earlier measurements showed a nucleus 137 ± 17 kilometers across and a dark, comet-like surface.

It comes from the Oort Cloud, a distant shell of icy bodies that surrounds the solar system. Objects from that region spend most of their time far from the Sun, so they tend to be chemically primitive.

According to a peer reviewed study, ALMA detected carbon monoxide jets at 16.6 astronomical units from the Sun, roughly 1.54 billion miles.

The gas shows up in the radio spectrum as a sharp signal at 230 gigahertz that is impossible to confuse with noise.

The data reveal multiple jets that changed between observations separated by days. That tells us the nucleus has active regions and perhaps rotation that brings vents into and out of sunlight.

Why distant comets wake up

At these distances, water ice stays frozen solid, locked beneath the comet’s surface like rock.

Activity instead is driven by more volatile ices such as carbon monoxide and carbon dioxide, which can vaporize under much weaker sunlight. 

These gases escape through cracks in the surface, creating faint jets that push dust outward and form a thin atmosphere called a coma around the nucleus.

Even a small amount of warming can trigger this process, showing that comets can come alive long before they enter the warmer regions near Jupiter or Mars.

Measuring C 2014 UN271

The team saw a compact coma, the hazy atmosphere around the nucleus, and carbon monoxide jets that extended well away from the central source.

They also set a strict upper limit on formaldehyde, a simple organic molecule made of carbon, hydrogen, and oxygen, which helps rule out chemistry in the coma as the main source of the gas and points to venting from the surface.

The radio signal also carries heat information, which helps estimate the dust near the nucleus. The thermal emission was consistent with a large, cold body plus a small amount of dust close to the nucleus.

“These measurements give us a look at how this enormous, icy world works. We are seeing explosive outgassing patterns that raise new questions about how this comet will evolve as it continues its journey toward the inner solar system,” added Roth. 

The quote sums up the scientific surprise. The comet is venting long before water ice can take over, revealing that frozen gases buried deep inside can become active even in the dim outer reaches of the solar system.

C 2014 UN271’s surprisingly active

These observations offer a clean look at the raw materials that built the planets. Molecules like carbon monoxide can carry clues about the early solar system that water rich comets closer to the Sun may have lost.

They also help explain how such a large body can stay active at long range. Knowing the trigger for distant activity sharpens models of comet evolution.

ALMA used spectroscopy, the measurement of light at very specific frequencies, to pick out carbon monoxide from the faint background. The accuracy is high enough to see tiny shifts in frequency that show how fast the gas is moving.

The telescope’s resolution also lets astronomers map where the jets point. That combination of speed and position paints a dynamic picture of the vents.

Where C 2014 UN271 is headed

This comet will make its closest pass to the Sun, called perihelion, on January 29, 2031, at a distance of about 10.9 astronomical units, a little beyond Saturn’s orbit. As it moves closer, sunlight will grow stronger, and more types of ice will begin to turn into gas.

Scientists expect to see new materials like methane and carbon dioxide start to escape, giving them a chance to watch how the comet changes step by step as it warms on its long journey inward.

Future observations will look for carbon dioxide and other volatiles as sunlight increases. If those species turn on, the balance of gases will shift and the coma will change shape.

The team’s methods make it possible to compare this object with others from the Oort Cloud, a vast spherical region of icy bodies that surrounds the solar system and marks its outermost boundary.

Careful, repeatable measurements will tell us whether this giant is typical or an outlier.

Much more to learn about comets

The confirmed size from earlier ALMA work lines up with the new thermal data. That consistency strengthens the case that we are seeing the largest active comet known from the distant reservoir beyond the planets.

The changing jets show that even very cold comets are not static. They respond to sunlight in complex ways, and that behavior is now measurable with precision.

A comet this big and this far out is not supposed to be so lively. Yet the radio data prove that physics does not wait for water to boil.

Careful observations, clear definitions, and strong evidence let scientists separate signals from hype. That is how a distant speck becomes a detailed story about chemistry and time.

The study is published in The Astrophysical Journal Letters.

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