For the first time in human history, three objects from outside the solar system: 1I/‘Oumuamua, 2I/Borisov and 3I/ATLAS, were discovered in space over the past decade. Where did these interstellar objects originate and how old are they?
Most stars in the Milky-Way galaxy reside in a disk. Young stars are made out of gas in a thin cold disk that rotates around the Galactic center. These constitute the thin disk of stars with a scale-height (disk thickness as measured from its mid-plane) of about a thousand light-years. Older stars formed in this gas disk billions of years earlier. During their lifespan, they were scattered by gravitational perturbations from passing star clusters, infalling dwarf galaxies or spiral arms in the disk. As a result, they currently populate a thicker disk with a scale-height that increases with stellar age and extends up to a few thousand light-years. The Sun was born 4.6 billion years ago in the last third of cosmic history. It represents a young adult, intermediate between the oldest stars from 13.7 billion years ago and the youngest stars which are less than a billion years in age.
To find out where the three interstellar objects came from, I asked my excellent student Shokhruz Kakharov, to follow their trajectories back in time in the gravitational field of the Milky-Way galaxy. Our results were submitted for publication in a new paper, available here.
We initiated the trajectories based on the measured velocities of the three interstellar objects relative to the so-called Local Standard of Rest (LSR). This is the Milky-Way frame of reference that averages over the random motions of the stars in the vicinity of the Sun. Since the scale-height of stars in the Milky-Way disk increases with age, we used the vertical excursion of each interstellar object from the disk mid-plane to constrain the likely age of these interstellar objects.
One of the anomalies of 1I/`Oumuamua was that it started nearly at rest in the Galactic LSR frame of reference before entering the Solar system. As a result, integrating 1I/‘Oumuamua’s past trajectory suggests that it originated near the mid-plane of the thin Galactic disk of stars, with a likely age that is younger than 1–2 Gyr. Simply put, 1I/`Oumuamua is a kid in our cosmic block.
However, we have found that the excursion of the comet 2I/Borisov is similar to that of the Sun, suggesting a similar age. Simply put, 2I/Borisov is a young adult in our cosmic block.
Finally, the interstellar object 3I/ATLAS exhibits a larger vertical excursion when its trajectory is integrated back in time, suggesting that it originated from an older population in the thick Galactic disk, compared to 1I/’Oumuamua or 2I/Borisov. Simply put, 3I/ATLAS is among the elders in our cosmic block.
Our constraints apply to the full age of these interstellar objects, because they respond — just like the underlying population of stars — to gravitational perturbations that pump up their scale-height over time.
Our constraints on the ages of the various interstellar objects represent upper limits because the velocity dispersion of the interstellar objects includes both the velocity dispersion of their parent stars and the dispersion in their characteristic ejection speed away from their birth system.
How quickly did it take these objects to migrate from the opposite side of the Milky-Way disk relative to the Sun? For 1I/`Oumuamua, the time is about a billion years; for 2I/Borisov, it is 1.7 billion years, and for 3I/ATLAS, it is 0.8 billion years. Through their orbit around the Milky-Way, all three objects travel a few thousand light-years closer to the Galactic center than the Sun does.
In summary, 1I/`Oumuamua is anomalous relative to 2I/Borisov and 3I/ATLAS, not only because it is much younger than they are, but also because it had an extreme disk-like shape, it exhibited a non-gravitational acceleration and it did not show any evidence for cometary activity like 2I/Borisov or 3I/ATLAS.
Thomas Kuhn’s book, titled “The Structure of Scientific Revolutions,” argues that science operates within paradigms encompassing comprehensive worldviews that define legitimate problems, methodologies, and solutions within the scientific community. These paradigms shape not only what questions scientists ask, but also what they can perceive as meaningful data. Contemporary astronomy operates under an implicit paradigm that systematically excludes non-naturalistic explanations of interstellar objects, creating a disciplinary worldview where all interstellar objects are assumed to result from natural processes. Astronomers interpret anomalous objects like 1I/`Oumuamua as puzzles that must be solved within this natural framework, not as potential evidence requiring paradigmatic revision. Future data from the new Rubin Observatory will test whether objects like 1I/`Oumuamua, constitute messengers from afar that convey our next scientific revolution.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.