NASA has released the results of the James Webb Space Telescope's observations of 3I/ATLAS, and the data confirms the comet is like nothing we see in our own Solar System.
The interstellar visitor was observed by Webb towards the end of 2025, just after its close pass of our Sun.
What Webb found, scientists say, proves that comet 3I/ATLAS formed in a distant corner of the Galaxy that's markedly different from the conditions found within our own Solar System.
More on comet 3I/ATLAS
A cosmic relic from before the Solar System
Comet 3I/ATLAS is an interstellar comet, meaning it originated from beyond our Solar System. It's one of only three such interstellar bodies ever discovered.
The comet was first detected on 1 July 2025, and spent the latter half of 2025 passing through our Solar System and making its close approach to the Sun.
Observations have revealed that the interstellar comet could be as old as 10 billion years, making it the oldest comet ever found.
Our own Solar System is about 4.57 billion years old, meaning comet 3I/ATLAS may well be twice as old as the Sun.
It's now making its way out of our Solar System and, once it's lost from view, will be gone forever.
That's why scientists have been using humanity's most powerful telescopes to get a good look at 3I/ATLAS and learn as much about it as they can, while it's still in view.
Webb's December views of 3I/ATLAS
The James Webb Space Telescope observed comet 3I/ATLAS in August 2025, but then took another look in December 2025, after the comet had made its close approach to the Sun.
During the December 2025 observations, Webb collected its first ever mid-infrared chemical fingerprint of an interstellar object.
Webb observes the Universe in infrared, giving astronomers the chance to see bits of the cosmos that would normally be invisible to human eyes.
The observations were made using Webb’s MIRI (Mid-Infrared Instrument) on two different dates as the comet began to exit the Solar System following its close encounter with our Sun.
The first observation occurred 15–16 December 2025, when the comet was 329 million km (205 million miles) from the Sun.
Then on 27 December 2025, Webb observed 3I/ATLAS when it was about 379 million km (236 million miles) from the Sun.
Webb directly detected methane gas, making it the first ever direct detection of methane gas in an interstellar object.
The methane would have originally been held within comet 3I/ATLAS in ice form, out in the cold depths of interstellar space.
But as the comet approached the Sun, it heated up, causing the methane to sublimate into a gas.
Scientists say the methane's delayed appearance at comet 3I/ATLAS could be because it was buried so deep, and it took a while for heat from the Sun to penetrate the comet to that depth.
Unlike anything in our Solar System
The team behind this study say the results are further proof that comet 3I/ATLAS is unlike any comet that originated in our Solar System.
That can tell scientists a lot about what the conditions in which the interstellar comet formed were like.
They say the amount of methane relative to water found is surprisingly high.
Webb also found that comet 3I/ATLAS is unusually rich in carbon dioxide and releases much more carbon dioxide, relative to water, compared to typical Solar System comets.
The Webb telescope also detected a decline in gas production as comet 3I/ATLAS moved away from the Sun, water showing the most pronounced drop.
That's because as the comet received less heat from the Sun and began to cool down again, its surface got colder and less ice was vaporised out into space.
Read the full paper in the Astrophysical Journal Letters
