Might a comet from another star system force a rewrite of the rules of planetary formation? The interstellar comet 3I/ATLAS, only the third such visitor ever detected, is erupting with cryovolcanoes-ice volcanoes unlike any seen on most comets in our solar system. This process, along with a metal-rich composition, is offering an unprecedented glimpse into the chemistry and physics of ancient planetary systems.
First spotted in July 2025 by the ATLAS survey telescope in Chile, 3I/ATLAS has been tracked by ground-based and space observatories as it speeds through the solar system at roughly 137,000 miles per hour. Its trajectory is hyperbolic, confirming it is unbound to the Sun and originated far beyond our planetary neighborhood. Researchers led by Josep M. Trigo-Rodríguez used photometric data from the Joan Oró Telescope to document a sharp, sustained increase in brightness as the comet approached 2.5 astronomical units from the Sun. This surge was not a brief flare but a prolonged activation of volatile materials across the entire surface.
Unlike most comets of the solar system, 3I/ATLAS does not develop a dust mantle that would insulate subsurface ice; thus, its icy crust is exposed directly to solar heating. Without such a mantle, the widespread sublimation of water ice could occur and was able to trigger cryovolcanism. Internal pressure due to volatile gases and liquids in the case of cryovolcanic processes forces icy material through the surface, creating jets of vapor and dust. On 3I/ATLAS, the subsurface heating of water was likely to initiate chemical reactions with fine-grained metallic particles and release more energy, thus prolonging eruptions.
Comparisons of spectrophotometric measurements of reflected light from 3I/ATLAS and samples of carbonaceous chondrite meteorites from Antarctica showed a spectacular match. Carbonaceous chondrites, the team explained, are primitive, metal-rich meteorites that contain iron, nickel, and sulfides, which have been known to experience aqueous alteration. “The spectral similarities indicate that 3I/ATLAS may be a primitive carbonaceous object,” the team wrote, “likely enriched in native metal and undergoing significant aqueous alteration during its approach to the Sun, experiencing cryovolcanism as we could expect for a pristine Trans-Neptunian Object.” Such a composition would be unusual among comets, which are typically rather metal-poor.
The activity of 3I/ATLAS might depend precisely on the fact that it is so metal-rich. Solar heating would cause ice to melt into liquid water, and then the fluid can corrode metallic grains, driving energetic Fischer–Tropsch-type reactions. The product hydrocarbons and released gases, such as carbon dioxide, in addition to the carbon monoxide detected in the coma, help feed the cryovolcanic jets. Recent observations with the Very Large Telescope and James Webb Space Telescope have confirmed unusually high CO₂/water ratios-indeed among the highest ever recorded in a comet-consistent with unique chemical processing.
This is the chemical profile that may result from galactic cosmic ray processing during the comet’s multi‑billion‑year journey through interstellar space. Laboratory experiments show that cosmic ray irradiation is able to convert CO into CO₂ and synthesize organic-rich crusts, consistent with the red spectral slopes seen in 3I/ATLAS. Current outgassing likely samples only this processed outer layer at depths of about 15–20 meters, rather than pristine interior material.
Striking similarities to trans-Neptunian objects stand out. TNOs like Pluto host cryovolcanism in which internal heat melts subsurface ice, releasing vapor and dust. That an interstellar comet shares such a trait means similar processes have converged in icy bodies across planetary systems. “Being a comet formed in a remote planetary system, it is remarkable that the mixture of materials forming the surface of the body has resemblance with trans-Neptunian objects.”
noted Trigo-Rodríguez. These findings challenge conventional comet formation models, which assume low-metal, ice-rock compositions and activity driven solely by solar heating. Instead, 3I/ATLAS points to a more diverse set of formation environments where metal-rich, ice-laden bodies can evolve chemically over trillions of years before being ejected into interstellar space. Its survival through cosmic ray bombardment and mechanical stresses implies significant tensile strength, with size estimates ranging from 0.3 to 5.6 kilometers in diameter and a rotation period of about 16 hours.
With 3I/ATLAS continuing to make its way toward perihelion and its eventual exit from the solar system, astronomers are in a race to capture high-resolution spectra for an inventory of its volatile components, among them methanol, formaldehyde, methane, ethane, hydrogen cyanide, and ammonia. Each such detection will continue to refine insights into how such bodies form and evolve, and how they may deliver water and organics across the galaxy. Future missions will try sampling interstellar visitors directly, like ESA’s Comet Interceptor, thereby unlocking records of planetary formation from distant star systems.
