ORBITAL TODAY 5 October 2025
A team of astronomers studying interstellar comet 3I/ATLAS have made a startling discovery; its light behaves unlike that from any comet ever recorded. In their preprint paper, submitted to The Astrophysical Journal Letters, the researchers report that the comet shows an “extremely deep and narrow negative polarization branch,” a phenomenon that sets it apart from every known interstellar or Solar System comet.
“This polarimetric behaviour is significantly different from all known comets (either interstellar or bound to our Solar System),” the team wrote. “It does not fit into either the high or low-polarization comet categories, nor that of the unique category occupied by just Hale-Bopp and 2I.”
This finding, based on polarimetric observations (measurements of how sunlight scatters off the comet’s dust particles), suggests that 3I/ATLAS may belong to an entirely new class of interstellar bodies.
What Polarization Can Reveal
Polarimetry allows scientists to infer the composition, shape, and structure of a comet’s dust. Light waves reflected by dust can become polarised, either positively or negatively, depending on their scattering angle.
“Light is a wave consisting of a coupled oscillating electric and magnetic fields which are orthogonal to each other in a plane that is perpendicular to the direction of propagation of the wave (in vacuum),” explained Harvard astronomer Avi Loeb, who was not involved in the study.
“The polarization of the wave corresponds to different modes of field oscillations. For linear polarization, the electric and magnetic fields oscillate along a single direction, whereas for circular polarization, the fields rotate at a constant rate in their plane as the wave travels, either in the right-hand or in the left-hand direction.”
The team measured a minimum polarization of −2.77 ± 0.11%, a value that falls far outside typical cometary ranges. Such extreme “negative” polarization means the light waves reflected by the comet align parallel to the scattering plane, a striking deviation from normal cometary behaviour.
More Like a Trans-Neptunian Object Than a Comet?
Interestingly, 3I/ATLAS’s optical characteristics resemble those of trans-Neptunian objects (TNOs) and Centaurs, icy bodies orbiting far beyond Neptune.
“While 3I’s inversion angle resembles that of rare F-type asteroids and cometary nuclei, the depth of its negative polarization branch is almost twice as large,” the researchers note. “Extrapolation to very small phase angles indicate a steep polarization slope between phase angles 0–2°, reminiscent of certain small TNOs and Centaurs.”
These results, they say, line up with other independent findings showing water-ice features and a reddish spectral slope, hinting at a possible kinship with the outer Solar System’s frozen worlds.
The Comet Behind the Data
The mysterious object was first spotted on 1 July 2025, racing through the Solar System at nearly twice the speed of earlier interstellar visitors such as ‘Oumuamua and Comet Borisov. Early estimates placed its rocky nucleus at roughly 5.6 kilometres wide and its mass at more than 33 billion tons.
As it neared the Sun, astronomers observed a green-tinged coma forming around it, proof that volatile materials were boiling off its surface. Shortly afterwards, the comet faded from view, possibly after being struck by a coronal mass ejection, and will not be visible again until later this year.
A Glimpse Into a New Type of Interstellar Body
Despite limited visibility, imaging confirmed a diffuse coma surrounding 3I/ATLAS. No strong polarimetric features could yet be spatially resolved, but the evidence so far hints at a comet unlike any other.
“These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies,” the authors concluded.
As astronomers await its return, 3I/ATLAS remains one of the most puzzling objects ever to enter our Solar System, a fleeting messenger from another star system that refuses to play by the usual rules of light and dust.
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