Astronomers have captured the most detailed spectroscopic data yet of interstellar comet 3I/ATLAS, a visitor from another star system currently traversing our Solar System. Observations from the Very Large Telescope (VLT) in Chile revealed the presence of atomic nickel and cyanogen gas in the comet’s coma—the hazy cloud of gas and dust surrounding its nucleus—making it only the third confirmed interstellar object observed to date.
A Rare Glimpse into Another Star System
Discovered in July 2025 by the ATLAS survey telescope, 3I/ATLAS originated from the direction of the Sagittarius constellation. Its arrival offers a unique opportunity to study the chemical composition of material formed around another star, essentially providing a pristine sample of building blocks from a distant protoplanetary disk. These disks are the swirling clouds of gas and dust where planets form.
The comet was observed as it approached the Sun at a distance of roughly 4.51 astronomical units (AU). High-resolution spectroscopy using the VLT’s X-Shooter and UVES instruments detected the emissions of nickel and cyanogen, while notably lacking the presence of iron. This suggests that nickel is being released from dust grains in the coma through a process influenced by solar radiation, which is unusual compared to typical cometary behavior.
Why This Matters: Tracing Origins Beyond Our Sun
Interstellar objects like 3I/ATLAS are valuable because they haven’t undergone the repeated heating and cooling cycles of objects native to our Solar System. This means they retain more original chemical signatures from the star system they came from. Previous interstellar visitors—‘Oumuamua and 2I/Borisov—showed surprising differences; ‘Oumuamua resembled a rocky body, while 2I/Borisov contained carbon monoxide and complex ices.
3I/ATLAS adds another layer to this diversity. The comet’s coma is dominated by dust with a reddish tint, similar to some of the most primitive bodies in our own Kuiper Belt. The unexpected presence of nickel without iron suggests a unique chemical pathway, potentially involving low-energy processes like photon-stimulated desorption or the breakdown of complex organic molecules.
The Future of Interstellar Object Research
If the nickel emission continues without iron as the comet nears its closest approach to the Sun, it will be the first confirmed case of interstellar cometary metal emission being independent of traditional refractory release. This could reveal new insights into how chemistry, metallicity, and irradiation history affect planetesimal microphysics—the tiny particles that eventually form planets.
This research is setting the stage for future observations with next-generation telescopes like the Rubin Observatory and the Extremely Large Telescope, where rapid-response spectroscopy of interstellar objects will become standard practice. The study of 3I/ATLAS is not just a snapshot of a fleeting visitor, but a crucial step toward understanding the diversity of planetary systems beyond our own.
The findings were published in The Astrophysical Journal Letters on December 10, 2025.
