On July 1, 2025, the ATLAS survey in Chile discovered 3I/ATLAS, the third confirmed interstellar object to enter our solar system. NASA’s James Webb Space Telescope identified a carbon dioxide-rich coma and methane gas, while the SETI Institute ruled out artificial radio signals, confirming the ancient comet is a natural object.
The arrival of 3I/ATLAS wasn’t a slow drift. It entered the system on a strongly hyperbolic orbit with an eccentricity of 6.14—the highest of the three known interstellar visitors—and an excess speed of roughly 58 kilometres per second. This trajectory is the definitive signature of an object not bound by the Sun’s gravity.
While its closest approach to Earth on December 19, 2025, remained a safe 270 million kilometres away, the object provided a rare, high-resolution window into the chemistry of another star system. Unlike 1I/’Oumuamua, which left astronomers puzzled by its lack of a coma, 3I/ATLAS behaved unmistakably like a comet, streaming a halo of gas and dust that served as a chemical ledger for the James Webb Space Telescope (JWST).
The Methane Mystery and Carbon Dioxide Ratios
Data from NASA’s James Webb Space Telescope revealed a composition that defies the norms of our own solar system. Using the Mid-Infrared Instrument (MIRI), astronomers found a coma dominated by carbon dioxide, with roughly eight times as much CO2 as water. This ratio is among the highest ever measured in any comet.
The most surprising discovery, however, was the delayed appearance of methane. Methane is highly volatile, meaning it typically sublimates from ice into gas quickly. Yet, Webb only detected it during post-perihelion visits on December 15-16 and December 27, 2025.
According to the European Space Agency, this suggests the methane was buried beneath the comet’s surface, protected from evaporation until the heat from its close pass to the Sun penetrated the icy outer shell. The resulting methane-to-water ratio was found to be surprisingly high, with very few analogs existing within our own stellar neighborhood.
As 3I/ATLAS retreated into deep space, Webb observed a sharp decline in gas production. Water showed the most pronounced drop, which is expected behavior; as the object cools, less volatile ices like water “shut off” their gas production faster than more volatile species like methane or carbon dioxide.
An Object Older Than the Sun
The composition of 3I/ATLAS doesn’t just tell us where it is going, but when it began. Two independent lines of evidence suggest this object is an ancient relic, potentially predating the birth of our own star.
- Kinematic Age: Based on its speed and galactic motion, estimates place its age between 3 and 11 billion years.
- Isotopic Analysis: The comet’s low level of heavy carbon suggests it formed before moderate-mass stars had enriched the galaxy with that element. Furthermore, its water contains more deuterium than any comet previously studied.
When combined, these isotopic markers point toward a formation time of 10 to 12 billion years. Given that the Sun formed approximately 4.6 billion years ago, 3I/ATLAS is likely a wanderer from a much earlier epoch of galactic history.
The SETI Hunt and the Household Appliance Limit
Because interstellar objects are prime candidates for technosignatures—evidence of artificial technology—the SETI Institute deployed the Allen Telescope Array (ATA) to scan the object. The team monitored frequencies between 1 and 9 gigahertz for more than seven hours, searching for narrowband radio signals that do not occur naturally.
The search began with a massive dataset of nearly 74 million narrowband signals. After filtering for human interference and the object’s specific movement, the list dwindled to about 200 candidates. Ultimately, every single one of those was traced back to Earth-based technology or orbiting satellites.
“The results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today. That is why it is important to keep searching for technosignatures, even from objects we might not expect to have signals.”
The study established a concrete upper limit on any potential radio transmitter on or near the object. The ATA ruled out any signal stronger than about 10-110 watts—roughly the power consumption of a common household appliance—across the detected frequencies.
While the “alien spacecraft” hypothesis remains unsupported, researchers argue that these searches are vital for establishing a baseline of what “natural” looks like in the interstellar medium.
“Eventually, our own Voyager spacecraft will be extraterrestrial artifacts in other stellar systems. Given that, it is important that we understand the natural distribution of interstellar objects so that we will be able to identify any anomalies that could one day be signs of an artificial interstellar object.”
Dr.
Comparing the Interstellar Trio
3I/ATLAS is only the third confirmed visitor from another star system, and it stands in stark contrast to its predecessors, 1I/’Oumuamua (2017) and 2I/Borisov (2019). While all three are believed to be natural objects booted from their home systems via gravitational interactions, their characteristics vary wildly.
| Object | Discovery Year | Coma Present? | Key Characteristic |
|---|---|---|---|
| 1I/’Oumuamua | 2017 | No | Puzzling lack of gas/dust; unusual shape |
| 2I/Borisov | 2019 | Yes | First confirmed interstellar comet |
| 3I/ATLAS | 2025 | Yes | Ancient (10-12bn yrs); extreme CO2/Methane levels |
The higher eccentricity of 3I/ATLAS (6.14) compared to the previous two suggests a more violent or distant origin. By capturing the chemical fingerprint of an object that may have formed 12 billion years ago, astronomers are effectively sampling the chemistry of the early universe.
As 3I/ATLAS continues its exit from the solar system, it leaves behind a blueprint for future detections. The ability of the ATA to respond within a day of the comet’s announcement and the precision of Webb’s mid-infrared spectroscopy demonstrate that humanity now possesses the tools to analyze these rare visitors in real-time, rather than playing catch-up after they have already vanished into the void.
