How can a comet from another star system have a tail pointing toward the Sun? This counter-intuitive phenomenon, observed for the first time on an interstellar visitor, intrigues astronomers. On its approach to our star, comet 3I/ATLAS actually revealed an anti-solar tail, whereas comet tails usually trail away under the effect of the solar wind.
Not only is the presence of this Sun-facing tail rare, but researchers have discovered much more. By studying the comet over several nights, they identified jet structures within this unusual tail. These jets displayed a regular oscillating motion, suggesting a dance to the rhythm of the rotation of the comet's icy nucleus.
Center image: The interstellar comet 3I/ATLAS captured by the Hubble Space Telescope in November, with a characteristic blue glow.
Credit: NASA, ESA, STScI, D. Jewitt (UCLA); image processing: J. DePasquale (STScI)/ Serra-Ricart, Licandro, and Alarcon For these observations, astronomers used the Two-meter Twin Telescope in the Canary Islands, which allowed them to track the comet's evolution. The jets were visible for seven nights in August, and their oscillation every 7 hours and 45 minutes allowed them to deduce the nucleus's rotation. It would spin on its axis in about 15 and a half hours, a faster rate than previously estimated.
3I/ATLAS is only the third known interstellar object after 'Oumuamua and 2I/Borisov, which makes any observation of its behavior particularly informative. Unlike its predecessors, this is the first time such outgassing with oscillating jets has been observed in a comet from elsewhere. This offers an opportunity to study the physics of a body formed in another planetary system.
The results were shared on the preprint site
arXiv, offering a detailed glimpse of the mechanisms at play. As the comet now moves away from the Solar System, the data collected continue to fuel analyses. Every detail, like the transformation of the coma into an anti-solar tail, helps to understand how these distant visitors interact with our stellar environment.
The study of 3I/ATLAS thus opens a window onto the diversity of comets in the Galaxy. Although the object will soon leave our neighborhood, the knowledge gained will remain. Future similar discoveries could allow comparison of these behaviors with those of comets native to our Solar System.
What are interstellar comets?
Interstellar comets are icy objects that travel between stars, originating from planetary systems other than our own. Unlike comets in the Solar System, they were ejected from their birthplace by gravitational perturbations, perhaps due to giant planets or stellar encounters. Their hyperbolic trajectory brings them through our system temporarily, offering a direct glimpse of materials formed elsewhere.
Identifying these visitors relies on their speed and orbit, which indicate they are not gravitationally bound to the Sun. So far, only three have been confirmed, including 3I/ATLAS. Their rarity makes every observation precious, as they can reveal differences in composition or structure compared to local comets.
Studying these comets helps scientists test models about the formation of planetary systems. By analyzing their nucleus, coma, and tail, we can infer the conditions that prevailed in their original system. This allows us to better understand whether the processes at work in our Solar System are universal or distinct across galactic environments.
Future space missions or telescope improvements could increase the number of detections. Each new interstellar comet provides clues about the frequency of such objects and their role in distributing materials across the Galaxy, thus shedding light on the history of stellar and planetary formation.