Introduction: A visitor from the galaxy’s edge
The interstellar comet 3I/ATLAS has captivated astronomers and space enthusiasts alike as it speeds through the inner solar system. First spotted in late June and confirmed by NASA in early July, this icy wanderer is not only the third known interstellar object to visit our cosmic neighborhood, but also the largest detected so far. Recent estimates place its diameter between 4.8 and 11.2 kilometers, underscoring its impressive scale and mystery.
Beyond its size and speed—about 210,000 km/h (130,000 mph)—3I/ATLAS carries a deeper question: where did it come from? A new study published on arXiv attempts to trace the comet’s journey back millions of years to shed light on its origin, potentially revealing a relic from the Milky Way’s early epochs.
Tracing a path through the Milky Way
The researchers modeled roughly the last 4.27 million years of 3I/ATLAS’s orbit, using data from the European Space Agency’s Gaia mission. Gaia offers precise measurements of stars’ motions, velocities, and sizes, enabling scientists to reconstruct how the comet could have interacted with the gravitational tugs of neighboring stars. By mapping 62 nearby stars that the interstellar object likely encountered, the team assessed whether any could have meaningfully altered its course.
The findings were striking: none of the stars in the solar neighborhood seem to have significantly bent the comet’s trajectory or altered its high velocity in a way that would explain a nearby origin. Only one nearby star—roughly 70% the mass of the Sun—appeared to exert any effect, and even that influence was minimal. Lead author Xabier Pérez-Couto of the Universidad de A Coruña emphasizes that these results point away from a local origin and toward a much older source.
The implication: a very old object from the Milky Way’s frontier
The study suggests 3I/ATLAS could be a relic from the borders between the Milky Way’s thin and thick disks. The thin disk hosts most of the galaxy’s younger stars and metal-rich material, while the thick disk contains older stars with lower metallicity, orbiting the galaxy in a more extended halo. If the comet originated near this frontier, it could be billions of years old—potentially around 10 billion years—which would make it far older than our Sun and a unique fossil from the galaxy’s earliest days.
Such a scenario would imply that 3I/ATLAS formed in a primordial planetary system, later ejected into interstellar space, and only recently happened to cross into our solar neighborhood. The possibility elevates the comet from a curiosity to a tangible time capsule, offering insights into the materials and conditions that prevailed when the Milky Way itself was still young.
Limitations and next steps
Despite the intriguing implications, the study is careful to acknowledge its limits. Tracing an object’s history across millions or billions of years is inherently uncertain, and focusing on nearby stars can only approximate a far longer journey. The true birthplace of 3I/ATLAS remains ambiguous until more data can be gathered.
Earth-based and space-based observatories, including assets orbiting the Sun, Mars, and Jupiter, are poised to study 3I/ATLAS in greater detail as it continues its pass through the inner solar system. Analyses of its composition, structure, and velocity could reveal clues about the building blocks of ancient planetary systems and the chemical evolution of the Milky Way. If confirmed as an ancient interstellar relic, 3I/ATLAS could redefine our understanding of how planetary material circulates through the galaxy and how common such primordial visitors might be.
Why this matters for our view of the cosmos
Each interstellar object that visits the solar system is a messenger from somewhere else in the galaxy. 3I/ATLAS, in particular, may carry signatures of the Milky Way’s early epochs, potentially offering a direct link to the era before our Sun formed. By studying its makeup and trajectory, scientists hope to unlock chapters of a cosmic history that otherwise remain hidden in older starlight and distant dust clouds.
Bottom line
While not a definitive origin story, the latest modeling of 3I/ATLAS strengthens the case for an ancient origin near the thin–thick disk boundary. If confirmed, this interstellar visitor would mark a rare, tangible link to the Milky Way’s formative years and the processes that shaped countless star systems long before our own solar system appeared.
