The interstellar object 3I/ATLAS intrigues astronomers worldwide
3I/ATLAS is the third interstellar object ever detected in our solar system. Its observation has led to unprecedented global scientific mobilization and several international publications within a few days. The TRAPPIST telescopes at the University of Liège contributed the first data collected on this object from elsewhere.
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uly 1, 2025, marks a new important date in modern astronomy: the observation of the third interstellar object to pass through our solar system. Named 3I/ATLAS, it was detected by the ATLAS (Asteroid Terrestrial-impact Last Alert System), consisting of four robotic telescopes installed in Hawaii, Chile, and South Africa. This object joins the short but fascinating list of objects from other planetary systems to visit us after the strange 1I/ʻOumuamua in 2017 and comet 2I/Borisov in 2019.
"An interstellar object is a celestial body—like a comet or an asteroid—that is not part of our solar system and is not gravitationally bound to our Sun," explains Emmanuel Jehin, astronomer and head of the COMETA research group at ULiège. "These cosmic travelers are ejected from their home system by gravitational interactions or other mechanisms that are still poorly understood, before wandering freely in interstellar space and eventually passing through our solar system”
A global discovery, a collective effort
It was from one of the ATLAS consortium telescopes, located in Chile, that the alert was sent worldwide, based on the special orbital characteristic of the object. A vast international community of researchers, including the Michigan State University (MSU) team led by Darryl Seligman, quickly mobilized. Within hours of the detection, telescopes around the world were focused on 3I/ATLAS.
Among the many telescopes mobilized is the network of two TRAPPIST telescopes, one of which is installed in Chile at the La Silla Observatory (ESO) and the second in Morocco (Oukaïmeden). "Early on the morning of July 2nd, I saw an alert in my emails about the discovery of a possible new interstellar object! I immediately stopped the ongoing observations running on TRAPPIST-South in Chile (still at night with a 6-hour time difference from Europe) to prioritize observing this newcomer. It was indeed there, moving rapidly in the Milky Way in a very dense field of stars.” These initial observations were important for improving the object's orbit and confirming its interstellar origin as closely as possible, which was done the very next day by the NASA.
In the following days, the world's largest telescopes turned their attention to this still-enigmatic object to begin its detailed study, and NASA's JWST and HST space telescopes are scheduled to follow. "Interstellar objects are rare and precious witnesses to the diversity of worlds beyond our solar system. Observing them is an exceptional scientific opportunity, and we are proud that TRAPPIST is contributing to it," continues Emmanuel Jehin.
Furthermore, as part of an international collaboration led by Cyrielle Opitom, graduate student of the University of Liège, now astrophysicist at the University of Edinburgh, astronomers launched their program on ESO's Very Large Telescope (VLT) at Paranal with the MUSE instrument to determine the true nature of 3I/ATLAS: a comet or an asteroid?
What we know about 3I/ATLAS
Although 3I/ATLAS is still very distant, 465 million km from Earth, the first data reveal several interesting characteristics. The new interstellar object is currently moving at the incredible speed of 61 kilometers per second (approximately 219,600 km/h) relative to the Sun. It is following a hyperbolic trajectory, confirming its extrasolar origin, and will never return after leaving our system.
Its unusual brightness suggests that it could be an active comet, although the presence of characteristic gas emissions such as water (H2O), carbon monoxide (CO), or carbon dioxide (CO₂) has not yet been confirmed. "However, thanks to high-resolution data from the VLT's MUSE instrument, we have clearly detected a compact coma and a small dust tail surrounding the object", explains Cyrielle Opitom. "A coma that should grow and become increasingly brighter as 3I/ATLAS approaches the Sun. We will monitor it with our VLT program in the coming weeks and months to determine its detailed chemical composition and learn more about the system from which it originates." 3I/ATLAS will be at its closest to the Sun in October and to Earth in December, offering a limited but crucial observation window.
This is only the beginning of the study, as many questions remain unanswered: How big is the object? How will it behave as it approaches the Sun? What is its composition? Is it similar to solar system comets? Where does it come from? How many years has it been traveling in the Galaxy? This discovery, beyond its scientific importance, highlights the essential role of public funding in fundamental research. As Darryl Seligman puts it: "What we do doesn't generate immediate financial returns. But it's human curiosity, the curiosity that questions our origins, our cosmic solitude, and our place in the universe that justifies this research." The discovery and observations of 3I/ATLAS add a new chapter to our understanding of the universe and provide us with new information on the formation and evolution of our system and other neighbors.
Scientific references
Darryl Z. Seligman et al. , Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS, Submitted to AAS Journals https://arxiv.org/abs/2507.02757
Cyrielle Opitom, Colin Snodgrass, Emmanuel Jehin et al., Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum, Submitted to A&A https://arxiv.org/abs/2507.05226
Contacts
Emmanuel Jehin
ULiège staff and students who also took part in this study :
Marin Ferrais, Mohamed Amine Miftah, Mathieu Vander Donckt , Said Hmiddouch, Damien Hutsemeckers , Jean Manfroid
Nomenclature
Interstellar objects are named according to a classification system established by the International Astronomical Union. The prefix "I" indicates that the object is interstellar, followed by a number indicating its chronological position in the history of discoveries. Thus, 1I/ʻOumuamua is the first interstellar object identified, 2I/Borisov the second, and 3I/ATLAS the third. The name following the number is generally linked to the detection team or system that led to its discovery. In the case of 3I/ATLAS, the name refers to the ATLAS (Asteroid Terrestrial-impact Last Alert System) system that detected it.
The first of its kind, 1I/ʻOumuamua, was a slight exception to this rule. Although detected by the Pan-STARRS telescope in Hawaii, scientists named it ʻOumuamua, which means "messenger from afar" in Hawaiian. It had puzzled scientists with its very elongated shape and reddish color, likely due to prolonged exposure to cosmic rays.
2I/Borisov, meanwhile, was spotted by amateur astronomer Gennady Borisov from Crimea. This second object resembled a much more conventional comet, displaying behavior similar to that of comets in our own system.
