The Interstellar Comet That Showed Us Its Billion-Year-Old Interior: Then Started to Empty
Three interstellar objects have now visited our solar system. The third one just showed us something the first two never could: what an ancient, alien cometary interior looks like as it empties itself in real time.
3I/ATLAS, a 2.6-kilometer-wide comet that entered our solar system in mid-2025 and swung past the Sun in October, has been shedding its outer layers since perihelion. In the weeks after its closest solar approach, the James Webb Space Telescope caught it releasing methane at a rate that fell by nearly half in just twelve days — from 4.2 × 10²⁶ molecules per second in mid-December to 2.3 × 10²⁶ by month's end. The cause, according to a paper published this month in The Astrophysical Journal Letters: the comet had cracked open, exposing ice that had been sealed away from cosmic radiation for up to eleven billion years.
"This constitutes the first reported direct detection of CH₄ on any interstellar object," the authors write — meaning no astronomer had ever confirmed methane on a comet that came from another star. The detection comes from JWST's Mid-Infrared Instrument, which captured the comet's spectral fingerprint at wavelengths where methane emits strongly against the cold background of space.
The finding is more than a chemical catalog entry. It is the third data point in a project that is quietly reshaping planetary science: the empirical study of material from other stellar systems. 'Oumuamua, detected in 2017, was already leaving when astronomers spotted it, and its composition remained ambiguous — it behaved like both an asteroid and a comet, leading to speculation it might be something stranger. 2I/Borisov, detected in 2019, showed cometary activity but was faint and small. 3I/ATLAS was larger, brighter, and arrived with a window of observation that let astronomers watch its chemistry evolve. That sequence — ambiguous, then faint, then legible — is what turns a series of curiosities into a comparative dataset.
Matthew Belyakov, a graduate student at Caltech and lead author of the paper, put it plainly: "It's a very interesting object. It has been traveling through the galaxy for at least a billion years," as Universe Today reported. JWST is scheduled to observe it one more time this spring, though by then it will be out past Jupiter and the signal will be fainter.
The mechanism behind the methane surge is straightforward. For billions of years, 3I/ATLAS drifted through interstellar space with its surface battered by cosmic rays — high-energy particles that break apart volatile compounds on exposed ice. The outermost layers of the comet were depleted of their most easily vaporized ices, including methane. When 3I/ATLAS swung past the Sun, solar heating cracked or shed that ancient surface crust, exposing the interior. What JWST detected in December was not the comet's treated exterior but its raw, unprocessed interior venting into space.
The methane production did not hold steady. Between the first JWST observation on December 15 and the second on December 27, the comet's methane output dropped roughly 45 percent. The reservoir was depleting visibly. The H₂O production fell even more steeply — roughly 70 percent over the same interval — because water ice requires more heat to sublimate than methane, and the comet was already moving away from the Sun. The result is a composition snapshot at two moments: when the interior was freshly exposed and when it was already partially exhausted.
Those two ratios, known formally as the CH₄:H₂O and CO₂:H₂O mixing ratios, are what make 3I/ATLAS chemically interesting relative to what astronomers have catalogued closer to home. Typical solar system comets show CH₄:H₂O values between 0.1 and 10 percent. 3I/ATLAS measured 11 percent in the first epoch and 21.6 percent in the second — the increase reflecting water's faster decline as the comet cooled. The CO₂:H₂O ratio, at 2.3 to 5.2 depending on the epoch, is also enriched compared to most comets observed at similar distances from the Sun, though not uniquely so; the hypervolatile-rich comet C/2016 R2 shows comparable ratios.
What these numbers mean for models of how planetary systems form is where the comparative planetology angle becomes concrete. Comets are leftover material from the formation of a planetary system — the raw ingredients that never became planets. Their volatile chemistry encodes the conditions in the disk of gas and dust that surrounded the infant star. If volatile ratios in other stellar systems consistently differ from our own, that constrains where and how those systems formed. If they are similar, it suggests either common formation conditions or a mechanism that preserves a particular ice composition regardless of birth environment.
The detection of methane does not settle that question. The authors note that the pre-perihelion JWST spectrum, taken in August 2025 when the comet was still approaching the Sun, showed methane production an order of magnitude lower than the post-perihelion trend line would predict. They suggest the August measurement may have underestimated methane abundance due to possible underlying absorption from solid hydrocarbons — a caveat that means the true bulk methane content of 3I/ATLAS remains somewhat uncertain. And the CO₂ enrichment, while striking, has a parallel in at least one solar system comet, which tempers any claim of truly anomalous chemistry. As Astrobiology.com noted, the detection is the first of its kind but the interpretation is still being refined.
What is clearer is the observational methodology. JWST demonstrated it can do in months what ground-based telescopes spent years attempting with 'Oumuamua: characterize the volatile inventory of an incoming object from another stellar system at wavelengths inaccessible from the ground. The mid-infrared range is where many volatiles fluoresce most strongly, and the combination of JWST's mirror and MIRI's sensitivity is giving astronomers their first clean read on what these objects are made of.
Whether that capability leads to a dedicated ISO mission — the kind of flyby probe that planetary scientists have proposed under names like Comet Interceptor — is now a more concrete question than it was before. The scientific case for a mission that could meet an incoming interstellar object has always been strong in theory; 3I/ATLAS is the evidence that the objects are coming, and that they are worth the trip.
The next one may already be en route. Astronomers estimate that interstellar objects pass through the solar system more frequently than was assumed before 2017, but only the ones that happen to pass close enough to the Sun to activate — and that happen to be pointed at Earth when our telescopes scan the sky — get detected. 3I/ATLAS is the third confirmed. The fourth, whenever it arrives, will add another data point to a dataset that is no longer a list of one-offs. It is becoming a sample.