A Cosmic Stranger of a Different Kind
Last July, astronomers employing the ATLAS sky survey in Chile reported a strange object entering our solar system. Identified as 3I/ATLAS (or C/2025 N1), it was flagged as interstellar — a traveler from beyond our solar system, only the third such visitor ever detected after ʻOumuamua and 2I/Borisov.
Ever since, 3I/ATLAS has been intensely probed by some of the world's most capable telescopes — including the James Webb Space Telescope (JWST), Hubble, and ESA's SPHEREx mission.
New data from JWST, along with corroborating datasets, indicate that 3I/ATLAS is much weirder — and more difficult — than initially anticipated.
Webb's Verdict: CO₂ Dominant, Water Nearly Absent
On August 6, 2025, JWST observed 3I/ATLAS using its Near-Infrared Spectrograph (NIRSpec) instrument.
The observations revealed a coma (the diffuse, gaseous “atmosphere” around a comet) dominated by carbon dioxide (CO₂), with negligible amounts of water vapor. In fact, the water outgassing (if any) appears to be an order of magnitude lower than earlier estimates.
The JWST analysis suggests strong CO₂ emission.
The water (H₂O) mass loss rate is bounded to an upper limit — much lower than previously ground-based estimates.
Other species found are carbon monoxide (CO), water ice, and carbonyl sulfide (OCS).
In short: This comet looks more like a massive carbon dioxide fog bank than a classic water-ice comet.
That's surprising, since most of the comets we see in our solar system depend very much on water ice sublimation (turning into gas) when they warm up. The predominance of CO₂ indicates that 3I/ATLAS developed under conditions quite different from most solar system comets.
What's "Worse" — Or More Worrisome — About These Findings?
"Worse" is not "about to destroy the Earth," but it does signal that our current models and assumptions regarding how comets (particularly interstellar comets) work are under attack. Here are a few unsettled consequences:
1. Origin Conditions Were Alien
The enormous CO₂ signature indicates that 3I/ATLAS can have formed in a realm with extremely atypical volatile chemistry — one that repressed water or enriched in CO₂. It can have originated from an area where carbon dioxide ice was much more abundant, or where UV/radiation processes significantly altered its surface chemistry.
That suggests that the star system it originated from had a considerably different chemical environment — perhaps very cold, lacking in water vapor, or exposed to high radiation. That spoils the story that exocomets tend to form like ours.
2. Model Predictions Flipped
Most comet models postulate that water is the main cause of outgassing and tail formation. But if CO₂ is predominant, then non-gravitational acceleration (small pushes from outgassing) acts differently. In fact, calculations of the motion of the comet suggest non-gravitational accelerations compatible with mass loss in the range of tens of kilograms per second, particularly of dust and gas emitted sunward.
Some authors suggest mass loss up to ~60 kg/s for 100-micron dust grains.
That requires re-calibrating how we understand orbital perturbations in interstellar objects.
3. The Nucleus Is Elusive
Even with the deep imaging, the core nucleus of 3I/ATLAS is still veiled and unresolved. The coma is seen as a point source with minimal extended structure, making it difficult to separate the solid body below.
Hubble estimates put an upper bound on nucleus size at around 3.5 miles (5.6 km), but it may be far smaller — potentially hundreds of meters in diameter.
Since CO₂ outgassing could erode or cover up the nucleus in an uneven manner, its true size and shape are all the more difficult to determine.
4. We Could Be Underestimating the Variety of Interstellar Objects
Prior to 3I/ATLAS, the handful of interstellar objects (ʻOumuamua, 2I/Borisov) were already confounding. Now, with this CO₂-rich example, it's evident that interstellar bodies could come in a great many chemical flavors than we had previously assumed. Many would not act like any comet or asteroid we are familiar with.
If so, our detection methods, classification systems, and theoretical frameworks will all require radical overhaul.
What It Doesn't Mean — And What We Need to Be Cautious About
Before ringing cosmic alarm bells, here are important caveats:
No current danger to Earth. 3I/ATLAS's orbit keeps its closest approach to Earth at around 1.8 AU (approximately 270 million km).
The comet's unusual behavior is not evidence of alien technology (much less science fiction-esque sensationalism). It's a naturally occurring object with extreme characteristics.
Several of the results are preliminary, and the entire data — including the JWST preprint — are in the process of being analyzed and peer-reviewed.
The absence of water from the coma does not mean there is no water below the surface. It may be covered by layers or thermally insulated.
Future JWST & Telescope Runs
As 3I/ATLAS moves closer to the Sun (perihelion on or about October 30, 2025, at ~1.4 AU)
, new observations will challenge whether more volatile species are showing up — or whether CO₂ is still the leader.
Trajectory Tracking & Non-Gravity Effects
Accurate astrometry (i.e., minute-by-minute tracking of its course) will assist in refining models for how gas jets and dust ejection are influencing its motion. Any deviations could indicate unknown processes at play.
Comparisons to SPHEREx & Hubble Data
Comparison of JWST's infrared spectra with observations of 3I/ATLAS by SPHEREx (August 7–15) will serve to validate or disambiguate volatile composition models.
Astronomers will attempt to trace back its interstellar path, and model its chemical signature, to speculate what sort of star system "kicked out" it. Some recent research suggests that it could have origins in the Milky Way's thick disk, around ~10 billion years old.
Search for More Interstellar Comets
If 3I/ATLAS is only one among a much more varied group of interstellar bodies, then next-generation sky surveys (e.g. the Vera Rubin Observatory) could find dozens more — each of them with their own revelations.
Final Thoughts: Strange, But Profound
The new JWST discoveries regarding 3I/ATLAS break the rulebook on what comets — and interstellar interlopers — can be. A composition dominated by CO₂, poor water sublimation, and an unresolved nucleus render this object more alien (in behavior, if not in origin) than many were anticipating.
Although it's not "worse" in terms of peril,
it is worse — or at least more perplexing — in the sense that it violates lots
of cozy assumptions among planetary scientists. It compels us to face the fact
that interstellar visitors might arrive in forms we hardly comprehend.
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