In the past decade, astronomers have witnessed three interstellar objects (ISOs) passing through the Solar System. This included the enigmatic 'Oumuamua in 2017, the interstellar comet 2I/Borisov in 2019, and 3I/ATLAS in July 2025. This latest object also appears to be a comet based on recent observations that showed it was actively releasing water vapor as it neared the Sun. The detection of these objects, which were previously theorized but never observed, has piqued interest in the origins of ISOs, their dynamics, and where they may be headed once they leave the Solar System.
Since asteroids and comets are essentially material leftover from the formation of planets, studying ISOs could reveal what conditions are like in other star systems without having to send interstellar missions there. In a recent paper, Shokhruz Kakharov and Prof. Abraham Loeb calculated the trajectories of all three interstellar visitors to determine where they came and apply age constraints. Their results indicate these ISOs originated from different regions in the Milky Way's disk, and range in age from one to several billion years.
Kakharov is a graduate student at Harvard University's Astronomy Department whose work includes studies on interstellar objects, the trajectories of spacecraft like Voyager, direct imaging, and the flux of extragalactic dark matter. Prof. Loeb is the Frank B. Baird Jr. Professor of Professor of Science at Harvard University and the Director of the Institute for Theory and Computation (ITC) at the Harvard & Smithsonian Center for Astrophysics (CfA). The paper that details their findings appeared online and is being reviewed for publication in Astronomy & Astrophysics.
Artist's impression of Project Dragonfly, a study for an interstellar spacecraft. Credit: i4is
The discovery of 'Oumuamua kicked off a revolution in astronomy, confirming the existence of ISOs and inspiring efforts to study them closer. As Kakharov told Universe Today via email, they've also transformed our understanding of galactic dynamics and the formation of planetary systems:
Before 1I/'Oumuamua's discovery in 2017, we had no direct evidence that objects from other star systems could reach our solar system. These visitors provide unique samples of material from distant planetary systems, offering insights into the chemical composition and physical properties of exoplanetary material that we cannot obtain through remote observations alone. They also serve as natural probes of the interstellar medium and galactic dynamics, revealing the gravitational interactions that shape stellar populations over billions of years.
Since asteroids and comets are essentially material leftover from the formation of planetary systems, the study of ISOs enables the study of other star systems without having to mount interstellar missions. Currently, the only viable means for sending spacecraft to neighboring star systems involves gram-scale wafercraft and lightsails that are accelerated by direct energy arrays to a small fraction of the speed of light. Examples include Breakthrough Initiative's Starshot, and the Institute for Interstellar Studies' (i4is) Swarming Proxima Centauri concept.
While these mission concepts could reach the nearest star (Proxima Centauri) within a human lifetime, they would be very expensive to mount, and it would be decades before we learned what conditions are like in neighboring star systems. But as 'Oumuamua, 2I/Borisov, and 3I/ATLAS have demonstrated, ISOs pass through our Solar System regularly, each offering unique research opportunities. Determining where each ISO originated is the first step toward understanding the diversity and dynamics of stellar populations in the Milky Way. Said Kakharov:
Understanding ISO origins provides a deeper context for interpreting their physical and chemical properties. For example, knowing that 3I/ATLAS likely originated from an old stellar population suggests it may have experienced different evolutionary processes than younger objects. This information helps us understand the diversity of planetary system architectures and the conditions under which objects are ejected into interstellar space. Also, tracing their origins helps identify potential source regions and ejection mechanisms, whether through gravitational scattering, stellar evolution, or other dynamical processes.
For their purposes, Kakharov and Loeb ran a series of Monte Carlo numerical simulations using the GalPot galactic potential model, a software package designed to calculate the gravitational potential of a galaxy:
For each ISO, we generated 10,000 different possible trajectories by sampling from the observational uncertainties in their velocities and systematic uncertainties in the Solar motion relative to the Local Standard of Rest. We integrated each trajectory for 1 billion years in the Milky Way's gravitational potential to determine their maximum vertical excursions from the galactic plane. This statistical approach provides robust estimates of their orbital parameters and accounts for the significant uncertainties inherent in long-term orbital predictions.
From this, they were able to numerically integrate the trajectories of these three interstellar objects back in time and relate them to potential stellar populations. "Our analysis revealed that the three ISOs originate from distinct stellar populations with different ages and galactic locations," said Kakharov. Their results showed 3I/ATLAS is the oldest of the three, with a median age of 4.6 billion years, and originated from the Milky Way's thick disk. This component is thicker than the galaxy's thin disk (where our Sun resides) and is populated by older, lower metallicity stars.
1I/'Oumuamua is relatively young by comparison, about 1 billion years, and originated from the thin disk where new stars are still forming. 2I/Borisov falls between them in age, approximately 1.7 billion years old, and originated from the thin disk. "This diversity suggests that ISOs are ejected from planetary systems throughout the galaxy's history, not just from young, recently formed systems." These results also offer a preview of what's to come thanks to new observational facilities that will become operational in the coming years. Said Kakharov:
The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will dramatically increase ISO detection rates, potentially finding dozens of new interstellar objects per year. Future missions like the European Space Agency's Comet Interceptor could potentially help with an ISO for in-situ analysis. These facilities will enable statistical studies of ISO populations, allowing us to understand their frequency, distribution, and diversity across different stellar environments.
Further Reading: CfA
