A team of astronomers has attempted to uncover the mysterious origins of 3I/ATLAS, a recently discovered interstellar comet hurtling through our Solar System. Traveling nearly twice as fast as previous visitors like ‘Oumuamua and Comet Borisov, this cosmic wanderer has captivated scientists eager to understand where it came from and how long it has been adrift.
First detected on July 1, astronomers quickly recognized 3I/ATLAS as something extraordinary. It was moving through the Solar System at unprecedented speed, suggesting an interstellar origin. Estimates suggest it has a rocky nucleus roughly 5.6 kilometers (3.5 miles) across, weighing in at over 33 billion tons.
While the object briefly passed behind the Sun from Earth’s perspective, it remained visible to observatories and possibly even NASA’s Perseverance rover on Mars. The big question, however, remained: where did 3I/ATLAS come from?
Because gravity is time-reversal symmetric, scientists can retrace an object’s galactic trajectory using precise orbital data. Leveraging measurements from the European Space Agency’s Gaia mission, which maps stars and cosmic objects in three dimensions, the research team simulated the comet’s path 10 million years into the past.
Over that timespan, they tracked 93 close stellar encounters, with 62 considered statistically significant. However, none of these came close enough to explain the comet’s present-day orbit. As the researchers wrote: “The strongest perturber Gaia DR3 6863591389529611264 at 0.30 pc and with a relative velocity of 35 km s?1, imparted only a velocity change of |?v| ? 5 × 10?4 km s?1 to the orbit of 3I/ATLAS.”
In simpler terms, none of the nearby stars exerted enough gravitational influence to meaningfully alter 3I/ATLAS’s path or to suggest a likely birthplace.
By retracing its galactic motion, the team concluded that 3I/ATLAS most likely originated from the Milky Way’s thin disk, which contains many of the galaxy’s younger stars and planetary systems. This finding contrasts with earlier research that proposed a thick-disk origin, based on its relatively stable trajectory close to the galactic plane.
In their analysis, the authors note: “Together, all data indicate that while 3I/ATLAS follows a thin-disk orbit in the solar neighborhood, it may nonetheless be an old object, consistent with ejection from a primordial planetesimal disk in an early-formed system, or from an exo-Oort cloud.”
This implies that 3I/ATLAS may have been ejected from a distant, ancient planetary system billions of years ago, possibly during the chaotic early days of its parent star’s formation.
The team estimates that the comet could be as old as 10 billion years, making it a time capsule from the earliest era of planetary formation. Its unaltered composition and long interstellar voyage may hold clues about how the first solid bodies in the galaxy formed and how they evolve over cosmic timescales.
As Pérez Couto, the study’s lead author and a researcher at the Centre for Research in Information and Communication Technologies, explained: “What makes 3I/ATLAS unique is that it allows us to study the evolution of objects originating from other stellar systems, something we had only theorised about until now. Each observation is like opening a window into the Universe’s past.”
The findings are recently submitted to the Astrophysical Journal and posted on arXiv.
