What James Webb Telescope Found About the Interstellar Comet 3I/ATLAS

Updated on: August 27, 2025 | By: Jameswebb Discovery Editorial Team

What Has the James Webb Space Telescope Discovered About the Interstellar Comet 3I/ATLAS? 

The James Webb Space Telescope, often called JWST or simply Webb, has given astronomers an extraordinary new look at one of the rarest visitors in our solar system — the interstellar comet known as 3I/ATLAS. This comet is only the third confirmed object to travel into our solar system from another star system, following the famous ʻOumuamua in 2017 and comet 2I/Borisov in 2019. Its full name is 3I/ATLAS because it is the third interstellar (I) object ever discovered, and it was first detected by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey in July 2025.

Unlike regular comets that orbit the Sun, 3I/ATLAS is traveling on a hyperbolic trajectory, meaning it is just passing through and will never return. This makes every observation of it extremely valuable. Scientists quickly turned the James Webb Space Telescope toward this rare object, and what Webb has found has surprised researchers all over the world.

The Discovery of Comet 3I/ATLAS

On July 1, 2025, astronomers using the ATLAS survey in Chile first detected the comet. Right away, it was clear that this was no ordinary comet. Its path showed it was not bound to the Sun, and its extreme speed confirmed that it came from beyond the solar system. Estimates suggest that 3I/ATLAS is moving at more than 130,000 miles per hour, making it one of the fastest objects ever observed in the solar system.

Astronomers believe the comet may have originated in the thick disk of the Milky Way Galaxy, which means it could be more than 7 billion years old. For comparison, our solar system is about 4.6 billion years old. This means 3I/ATLAS might carry material from an even earlier generation of stars.

Webb’s Infrared Observations of the Comet

The James Webb Space Telescope observed the comet on August 6, 2025, using its powerful Near-Infrared Spectrograph (NIRSpec). Webb’s infrared vision is especially useful for studying the gases and ices in comets, since many of these chemicals show up strongly in infrared light.

One of the biggest surprises was the measurement of the comet’s carbon dioxide (CO₂) to water (H₂O) ratio. Webb found that this ratio was about 8 to 1, meaning that for every molecule of water, there were eight molecules of carbon dioxide. This is one of the highest CO₂/H₂O ratios ever recorded in a comet.

Most comets in our solar system have much more water than carbon dioxide, so this discovery immediately told scientists that 3I/ATLAS formed in a very different environment. It might have been born in a region of its original star system that was extremely cold and rich in CO₂ ice. Another possibility is that its surface has developed an insulating crust that prevents water from escaping while letting carbon dioxide leak out.

Webb also detected other chemicals in the comet’s coma (the cloud of gas and dust that surrounds the nucleus). These include water vapor, carbon monoxide, carbonyl sulfide, and even traces of nickel. Such a wide variety of gases suggests that the comet is made from a very complex mixture of ices, perhaps very different from anything in our solar system.

How Big Is 3I/ATLAS?

Measuring the size of the comet’s solid nucleus is challenging because it is hidden inside the coma. Early estimates suggested that the nucleus could be as large as 11 kilometers (7 miles) across. Later data from the Hubble Space Telescope refined this to about 5.6 kilometers (3.5 miles).

However, some more speculative estimates suggest it could be much larger, possibly up to 46 kilometers (28 miles) wide. If true, this would make it the largest interstellar object ever seen. But most astronomers think the smaller size is more realistic.

Activity Patterns of the Comet

Comets normally develop bright tails of gas and dust as they approach the Sun. Surprisingly, 3I/ATLAS does not show a clear visible tail. Instead, it appears as a glowing fuzzy ball with a faint, nearly spherical coma.

Ground-based telescopes studied the comet in July 2025 and found that it rotates once every 16 hours. During this time, it releases small amounts of dust — only about 0.3 to 4 kilograms per second, which is far less than typical bright comets. This weak dust activity may explain why the comet does not have a dramatic tail.

Interestingly, even when the comet was still more than three times farther from the Sun than Earth is, astronomers detected water activity. The Neil Gehrels Swift Observatory found hydroxyl (OH) in its spectrum, which comes from the breakdown of water molecules. This confirmed that the comet was releasing water vapor, though much less than carbon dioxide.

What Makes 3I/ATLAS Different?

The key discovery about 3I/ATLAS is its unusual chemical composition. The dominance of carbon dioxide over water is almost unheard of. This tells us that comets in other star systems may form under very different conditions from those in our solar system.

Other unique features include:

• The lack of a visible tail despite active outgassing.
  
  

• The detection of unusual gases such as carbonyl sulfide and nickel, rarely seen in comets.
  
  

• Its extremely high speed and clear interstellar origin.
  
  

• Its possible great age, making it a time capsule from the early galaxy.
  
  

Together, these make 3I/ATLAS one of the most scientifically important comets ever studied.

Why the James Webb Space Telescope Is Crucial

Without Webb, we would never have been able to measure the chemical details of this interstellar comet. Its infrared instruments are powerful enough to detect faint signals of gases in a small, distant, and fast-moving target.

The discovery of a CO₂-dominated comet challenges our ideas about how planetary systems form. It may mean that in some parts of the galaxy, comets are very different from those we see around the Sun. This could also affect our understanding of how water and other molecules needed for life are spread between star systems.

Webb will likely continue to observe 3I/ATLAS later in 2025 as the comet travels deeper into the solar system. These follow-up studies may confirm whether the high carbon dioxide levels are due to a crust or to its original composition. Other telescopes, including Hubble, are also planning observations, giving scientists multiple perspectives on this extraordinary object.

Conclusion

The James Webb Space Telescope has revealed that 3I/ATLAS is unlike any comet in our solar system. With an unusually high carbon dioxide content, weak water activity, and a strange lack of a tail, it is rewriting what astronomers thought they knew about interstellar comets.

Every observation of 3I/ATLAS is precious, because it is passing through the solar system only once. In a few months or years, it will disappear back into interstellar space forever. But thanks to Webb, scientists now have a detailed record of its chemical fingerprints.

This discovery shows that the universe still holds countless surprises. Each interstellar visitor gives us a glimpse of worlds beyond our own, and Webb’s success with 3I/ATLAS proves just how powerful this telescope is for exploring the unknown.