Press enter or click to view image in full sizePolarization versus phase angle of 3I/ATLAS compared to other objects, including Solar System comets,such as 9P/Tempel 1, 22P/Kopff, 47P/Ashbrook-Jackson, 67P/Churyumov-Gerasimenko, as well as the interstellar comet, 2I/Borisov. (Credit: Z. Gray et al. 2025)
A new study of 3I/ATLAS (reported here) establishes this interstellar object as uniquely distinct in its polarization properties from solar system bodies as well as the interstellar comet 2I/Borisov.
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). 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 level of polarization displayed by an object which scatters sunlight like 3I/ATLAS, reflects asymmetries in its global shape or its corpuscular composition. The polarization depends on the phase angle at which the 3I/ATLAS is observed, namely the angle between the Sun-3I/ATLAS axis and the 3I/ATLAS-observer axis.
This semester, I am teaching these fundamental principles about radiation and matter in “Radiative Processes in Astrophysics,” the only mandatory course for PhD students in the Harvard Astronomy department.
3I/ATLAS is characterized by an extremely deep and narrow negative polarization of −2.77 percent at a phase angle of 6.41 degrees with displays a low inversion angle, where the polarization changes sign, of 17.05 degrees. This polarimetric behavior is significantly different from all known comets, either interstellar or those bound to the Solar System, not fitting into either the high or low-polarization comet categories. The combination of low inversion angle and extreme negative polarization is unprecedented among comets and asteroids, marking 3I/ATLAS the first object known with such polarimetric behavior and representing a previously unobserved population.
This polarization anomaly highlights the fact that 3I/ATLAS is different from previous interstellar objects. The first interstellar object 1I/`Oumuamua displayed no signs of gas or dust around it but nevertheless exhibited non-gravitational acceleration. The second interstellar object 2I/Borisov behaved like a familiar comet.
In addition to this new anomaly, 3I/ATLAS displayed a number of other anomalies that make it unique, leaving the option of a technological origin for it as a viable alternative:
1. The retrograde trajectory of 3I/ATLAS is aligned with the ecliptic plane of the planets around the Sun to within 5 degrees, with a chance probability of 1 in 500 (as discussed here). This brings 3I/ATLAS within several tens of millions of kilometers from Mars, Venus and Jupiter.
2. The estimated diameter of the nucleus of 3I/ATLAS is up to 46 kilometers (as derived here). On October 3, the HiRISE camera on the Mars Reconnaissance orbiter will be able to image 3I/ATLAS with a pixel resolution of 30 kilometers, providing a tight limit on the diameter of its nucleus.
3. When 3I/ATLAS was at a heliocentric distance of 3–4.5 times the Earth-Sun separation, it featured a glow which extended towards the Sun and not in the opposite direction. This feature of an anti-tail was never observed before for comets (as discussed here).
4. The plume of gas around 3I/ATLAS is composed primarily of carbon dioxide — CO2 (87% by mass) with traces of carbon monoxide — CO (9%), and water — H2O (most of the remaining 4%), as derived here. It also features nickel without iron (as known to exist in industrially-produced nickel alloys) and cyanide, both rising sharply with decreasing heliocentric distance (as reported here).
Given these unprecedented anomalies, we must stay open-minded regarding the nature of 3I/ATLAS until we get better data.
Once the HiRISE camera gets a close-up image of 3I/ATLAS on October 3, 2025, its brightest pixel will tightly constrain the nucleus diameter. At that time, I will update the rank I assigned to 3I/ATLAS on the so-called “Loeb Scale,” defined in two recent papers (available here and here). Science is exciting because it resembles a detective story. Stay tuned.
ABOUT THE AUTHOR
Press enter or click to view image in full size(Image Credit: Chris Michel, National Academy of Sciences, 2023)
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.