A global team of astronomers from Chile, Europe, the USA, Canada, and New Zealand has achieved an unprecedented level of detail in spectroscopic observations of an interstellar comet as it moves through our solar system. Utilizing spectroscopic data from two instruments on the ESO’s Very Large Telescope (VLT), researchers detected emissions of nickel atoms and cyan gas from the interstellar comet 3I/ATLAS, marking it as the third confirmed interstellar object recorded.
This image of interstellar comet 3I/ATLAS was taken with Hubble’s Wide Field Camera 3 (WFC3) on December 27, 2025. Image credit: NASA/ESA/CSA/Hubble.
The interstellar traveler, 3I/ATLAS, was first discovered on July 1, 2025, using the NASA-funded ATLAS (Asteroid Terrestrial Impact Last Alert System) telescope.
Also referred to as C/2025 N1 (ATLAS) and A11pl3Z, this celestial object approached from the constellation Sagittarius.
At its discovery, the comet was located 4.51 astronomical units (AU) from the Sun, with an eccentricity of 6.13.
“Understanding the volatile components of interstellar objects that pass through our solar system grants us unique insights into the chemical and physical processes occurring in distant stellar systems,” noted Dr. Rohan Rahatgaonkar of the Catholic University of Chile.
“Interstellar objects maintain remnants of the chemical and physical processes active in their protoplanetary disks during formation and may be altered by interstellar medium exposure.”
“When subjected to solar radiation, these cometary interstellar objects emit solids and gases due to their activity.”
During July and August, astronomers carried out high-resolution spectroscopic analyses as 3I/ATLAS approached between 4.4 to 2.85 AU from the Sun.
To acquire the comet’s spectrum, they employed the VLT’s X-Shooter and the Ultraviolet and Visible Echelle Spectrometer (UVES).
Observations revealed that the comet’s coma, the cloud of dust and gas enveloping its nucleus, is primarily made up of dust, with a consistent reddish optical continuum indicating organic-rich materials.
This reddish coloration resembles that of comets within our solar system and primitive Kuiper belt objects, suggesting shared physical processes across the planetary system.
3I/ATLAS spectrum showing Ni I emission over observations from VLT/X-Shooter and VLT/UVES. Image credit: Rahatgaonkar et al., doi: 10.3847/2041-8213/ae1cbc.
As 3I/ATLAS continued its journey towards the Sun, researchers identified emissions of various cyanide (CN) compounds and neutral nickel (Ni).
Interestingly, iron (Fe) was not detected, implying that nickel is efficiently released by comatose dust particles under solar radiation influence.
The rate of production for these emissions increases significantly as the comet nears the Sun, establishing a strong power-law relationship with its geocentric distance.
These observations indicate that the release of these atoms may stem from low-energy mechanisms, like photon-stimulated desorption or the breakdown of complex organics, rather than the direct sublimation of ice. This distinguishes this interstellar comet from many others within the solar system.
This spectral data not only acts as a snapshot of a transient visitor, but interstellar comets like 3I/ATLAS offer pristine samples from materials formed around other stars. Their limited processing from solar proximity preserves valuable clues about distant protoplanetary disks—the swirling clouds of gas and dust which eventually form planets.
Past interstellar discoveries, such as ‘Oumuamua in 2017 and 2I/Borisov in 2019, have exhibited surprising contrasts. ‘Oumuamua appeared inert, while 2I/Borisov showcased an abundance of carbon monoxide and complex ice.
The new insights from 3I/ATLAS contribute another intriguing layer to this expanding cosmic narrative. Its dusty constitution reveals molecular traits that challenge our understanding of typical comet behavior and introduce novel physics.
3I/ATLAS spectrum from the monitoring campaign spanning July 4 to August 21, 2025. Image credit: Rahatgaonkar et al., doi: 10.3847/2041-8213/ae1cbc.
“If 3I/ATLAS maintains the absence of iron while exhibiting nickel emissions during perihelion, it will set a precedent for observing interstellar comet metal emissions decoupled from traditional refractory trends,” the researchers stated.
“This observation suggests a distinct low-temperature organometallic pathway for nickel in exocomets and may provide fresh perspectives on how disk chemistry, metallicity, and irradiation history affect planetesimal microphysics.”
The parent star of 3I/ATLAS is presumed to be less metallic than other interstellar progenitor stars, yet more metallic than the Sun, indicating no inherent conflict between its estimated age and the presence of iron-peak elements like nickel.
“Regardless of which interpretation is accurate, 3I/ATLAS promises a critical experiment linking metal emissions with volatile activation and particle physics in interstellar bodies.”
“The findings discussed will elevate nickel from being a mere curiosity into a crucial marker for determining both parent chemistry and galactic origins, and set new standards for rapid-response spectroscopy of interstellar objects at the Rubin Observatory and ESO’s Very Large Telescope.”
For further details, see the published findings on December 10, 2025, in the Astrophysics Journal Letter.
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Rohan Rahat Gaonkar et al. 2025. Observations of interstellar comet 3I/ATLAS using a very large telescope: From quiescence to luminescence—Dramatic increases in Ni i emissions and initial CN outgassing at extensive heliocentric distances. APJL 995, L34; doi: 10.3847/2041-8213/ae1cbc
Source: www.sci.news
