It’s not often that a comet older than the Sun smashes through the solar system at more than 130,000 miles per hour, leaving behind hints of a birthplace billions of years and light-years away. And yet that’s what astronomers are seeing with 3I/ATLAS, the third interstellar object to be confirmed ever found, and now one of the most chemically anomalous.
On 6 August 2025, the James Webb Space Telescope (JWST) pointed its 6.5-meter infrared mirror at the comet, subsequently 3.32 astronomical units from the Sun. With its Near-Infrared Spectrograph (NIRSpec), Webb obtained spectral images from 0.6 to 5.3 micrometers, separating molecular emissions by subtracting the local continuum. The findings surprised the monitoring team: the mixing ratio of the carbon dioxide and water in the coma was measured at 8.0 ± 1.0, a value six standard deviations above the trend for Jupiter-family and long-period comets. In everyday terms, this indicates 3I/ATLAS is releasing CO₂ at the rate of 129 kilograms per second and water vapor at only 6.6 kilograms per second a mere 5 percent of the CO₂ emission.
Such a ratio is almost unprecedented. The only comparable case in the cometary record is C/2016 R2, but unlike that solar system comet, 3I/ATLAS shows no extended dust tail. Hubble images confirm the absence of a broad tail, suggesting the object releases few dust grains of ~0.5 micrometer size, the scale that scatters visible sunlight. Instead, the glow may come from sunlight reflecting off CO₂ ice fragments that sublimate into the surrounding gas cloud.
The chemical peculiarity has a number of different explanations. One is that the nucleus of the comet is intrinsically CO₂-rich, produced close to the carbon dioxide ice line in its parental protoplanetary disk the area where temperatures were cold enough for CO₂ to condense but too hot for water ice to prevail. Another is that its ices received higher doses of radiation than normal solar system comets, changing surface chemistry on a timescale of billions of years. A third explanation is structural: a crust of insulating material may be holding back heat penetration, restraining water sublimation while freeing CO₂ and CO more easily.
Trajectory analysis throws further mystery into the mix. 3I/ATLAS travels an exceptionally flat, linear course through the solar system, consistent with having formed in the Milky Way’s thick disk a population of old stars and planetary systems that existed before the Sun by some 3 billion years. Its record-breaking speed, highest ever for a solar system visitor, is the product of gravitational slingshots from innumerable stars and nebulae along the way during its interstellar journey. “No one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second,” UCLA’s David Jewitt, leader of Hubble’s observing team, said.
Measures have become more precise with repeated observations. Early earthbased measurements had indicated a nucleus of up to 11 kilometers in diameter; Hubble’s clearer images cut that in half, to a maximum of 5.6 kilometers with a minimum around 320 meters. SPHEREx infrared observations using a low albedo of 5 percent permit a much larger 46-kilometer diameter, although this drops to ~10 kilometers if the surface is highly reflective. Even at the lower end, 3I/ATLAS is probably the largest interstellar object ever seen and, by radiometric dating of its probable stellar origin, the oldest comet ever observed.
The JWST spectra also picked up water ice, carbon monoxide, carbonyl sulfide, and dust, but in ratios never seen before in any known solar system comet. The absorption feature of the water ice is strong, but the vapor phase is sparse, which supports the notion of a thermally insulated interior. Such fingerprints of composition are precious: they provide a direct taste of volatile chemistry from a planetary system that could have developed under very different circumstances from those around our sun.
For planet scientists, 3I/ATLAS is an uncommon standard. Comparing its volatile budget and outgassing pattern to those of comets that evolved in the Sun’s protoplanetary disk allows scientists to cross-check models of disk temperature gradients, ice-line migration, and long-term processing by radiation. The results also highlight the value of rapid-response observing campaigns: interstellar visitors are transients, and once 3I/ATLAS whizzes around the Sun in October, it will disappear into deep space with its secrets intact.
