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Press Releases
Press Releases
Press Release
Interstellar Comet 3I/ATLAS Shows Never-Before-Seen Light Behaviour
Interstellar Comet 3I/ATLAS Shows Never-Before-Seen Light Behaviour
Abstract
Abstract
Astronomers have discovered that interstellar comet 3I/ATLAS (C/2025 N1) reflects sunlight in a completely unprecedented way. Using the Very Large Telescope (ESO), the Nordic Optical Telescope (ORM), and the 2-meter RCC telescope (Rozhen Observatory), an international team found that the comet’s dust produces the deepest negative polarisation ever observed — a distinctive light-scattering signature not seen in any comet or asteroid.
Astronomers have discovered that interstellar comet 3I/ATLAS (C/2025 N1) reflects sunlight in a completely unprecedented way. Using the Very Large Telescope (ESO), the Nordic Optical Telescope (ORM), and the 2-meter RCC telescope (Rozhen Observatory), an international team found that the comet’s dust produces the deepest negative polarisation ever observed — a distinctive light-scattering signature not seen in any comet or asteroid.
The results, published in The Astrophysical Journal Letters, suggest that 3I/ATLAS’s dust is a mixture of dark and icy grains, possibly resembling materials found on distant trans-Neptunian objects. Discovered in 2025, 3I/ATLAS is only the third known interstellar object, after 1I/ʻOumuamua and 2I/Borisov.
The results, published in The Astrophysical Journal Letters, suggest that 3I/ATLAS’s dust is a mixture of dark and icy grains, possibly resembling materials found on distant trans-Neptunian objects. Discovered in 2025, 3I/ATLAS is only the third known interstellar object, after 1I/ʻOumuamua and 2I/Borisov.
Researchers believe this comet may represent an entirely new class of interstellar body, revealing how planetary systems beyond our own may form and evolve. Further observations near its closest approach to the Sun will test these extraordinary findings.
Researchers believe this comet may represent an entirely new class of interstellar body, revealing how planetary systems beyond our own may form and evolve. Further observations near its closest approach to the Sun will test these extraordinary findings.
Press Release
Astronomers Discover Unprecedented Polarisation Behaviour in Third Interstellar Comet 3I/ATLAS
An international team of astronomers has revealed that the newly discovered interstellar comet 3I/ATLAS (C/2025 N1) displays the most extreme light polarisation ever measured for any comet or asteroid—a finding that could redefine our understanding of small bodies originating beyond our solar system.
Using some of the world’s most powerful telescopes, including the Very Large Telescope (ESO) in Chile, the Nordic Optical Telescope (ORM) in La Palma, and the 2-meter RCC telescope at Rozhen Observatory in Bulgaria, researchers observed how sunlight scattered by 3I/ATLAS’s dust and ice created an unusually strong “negative polarisation” signal.
“The polarisation signature of 3I/ATLAS is unlike anything we’ve ever seen,” said Dr. Zuri Gray of the University of Helsinki, lead author of the study. “It shows a deep and narrow negative polarisation branch—stronger than that of any known comet—pointing to dust and ice properties we’ve never encountered before.”
A Mysterious Messenger from Another Star
3I/ATLAS is only the third interstellar object (ISO) ever detected, after 1I/ʻOumuamua and 2I/Borisov. Unlike its predecessors, which resembled an asteroid and a typical comet respectively, 3I/ATLAS appears to represent a new and distinct type of interstellar body.
Polarimetric data revealed that sunlight reflected by 3I/ATLAS becomes strongly polarised in a direction parallel to the scattering plane—a phenomenon known as negative polarisation—reaching an extraordinary −2.7% at a phase angle of just 7°, with an inversion angle at 17°. No other comet or asteroid exhibits such a combination of parameters.
Hints of Water Ice and Exotic Dust
Spectroscopic observations from other teams had already shown that 3I/ATLAS is unusually red in colour and contains water ice and carbon dioxide, even at large distances from the Sun. The new polarimetric measurements support these findings, suggesting that the comet’s dust is composed of a mixture of dark and icy grains, possibly similar to those found on distant trans-Neptunian objects.
“This object seems to bridge the gap between icy bodies from the outer solar system and completely interstellar material from another star,” said Dr. Stefano Bagnulo of Armagh Observatory and Planetarium, a co-author of the study. “Its properties challenge our existing models of cometary dust and formation environments.”
A Glimpse into Galactic Diversity
Because interstellar visitors like 3I/ATLAS appear only once before leaving the solar system forever, astronomers have a limited window to study them. The team’s findings demonstrate that the diversity of interstellar bodies is far greater than previously thought, with 3I displaying traits unseen in any known comet, asteroid, or interstellar visitor.
“3I/ATLAS expands the known range of what interstellar objects can be,” said Dr. Galin Borisov of the Bulgarian Academy of Sciences, a co-author of the study. “It may represent an entirely new class of small bodies shaped by conditions in other planetary systems.”
Next Steps
The team plans to continue monitoring 3I/ATLAS as it approaches and passes perihelion, when it will be closest to the Sun and brightest. Further observations could reveal how its coma and dust properties evolve—and whether the extraordinary polarisation behaviour persists.
Publication:
Gray, Z. et al. (2025). “Extreme Negative Polarization of New Interstellar Comet 3I/ATLAS.” The Astrophysical Journal Letters, 992, L29.
DOI: 10.3847/2041-8213/ae0c08
Contacts:
Dr. Zuri Gray — University of Helsinki, Finland
Dr. Stefano Bagnulo — Armagh Observatory & Planetarium, Northern Ireland, UK
Dr. Galin Borisov — Institute of Astronomy and National Astronomical Observatory, Bulgaria
The contribution from the Bulgarian side is supported by a grant number KΠ-06-H88/5 (Dec 2024) “Physical properties and chemical composition of asteroids and comets – a key to increasing our knowledge of the Solar System origin and evolution” by the Bulgarian National Science Fund.
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