We live at an exciting time. The first large interstellar objects were discovered only over the past decade: the interstellar meteor IM1 in 2014, the anomalous near-Earth object `Oumuamua in 2017 and the interstellar comet Borisov in 2019. A fundamental unknown is the likely source for each of these unusual objects from outside the solar system.
To shed new light on this unknown, I offered a summer project to Shokhruz Kakharov, an undergraduate student at Harvard College. My idea was to calculate the trajectories of these interstellar objects back in time in the gravitational potential of the Milky-Way galaxy and figure out where they came from. The Galactic region that their orbits sampled in the past would constrain the properties of their sources. For example, if they originated near a star, one could constrain the age of the star and the physical process that could have produced each of these interstellar objects.
We initiated the past trajectories of these interstellar objects by reversing their measured velocities relative to the Local Standard of Rest — the frame of reference obtained by averaging the random motions of local stars near the Sun. This frame is circling the center of the Milky-Way at a speed of about 240 kilometers per second, ten thousand times faster than the speed-limit on a highway.
Using a computer code, Shokhruz numerically integrated the trajectories of interstellar objects back in time in the gravitational potential of the Milky-Way. For simplicity, we ignored transient gravitational features such as spiral arms and the Galactic bar. This is a reasonable approximation for orbits in the outer part of the Galactic disk.
By integrating the orbits of these interstellar objects back in time, we were able to constrain the spatial region of their putative sources within the Milky Way. These constraints limit the potential birthplaces of different interstellar objects and provide insights into the Galactic environment from which they originated.
Stars near the Sun follow an exponential distribution above and below the midplane of the Galactic disk, with a scale-height that increases with age. We used the vertical excursion of each interstellar object from the Milky-Way disk midplane to constrain the likelihood function for its possible age. Our approach was simple. Given the maximum vertical excursion of each object from the midplane of the Milky-Way disk, we calculated the age distribution of stars that could have given birth to them within that region to infer the probability distribution for the object’s age. Any dynamical effect on the stellar scale-height by gravitational perturbations would also affect interstellar objects as well, since both populations are collisionless. Hence, our age constraints apply directly to the full age of the interstellar objects irrespective of their travel time.
We discovered a small vertical extent of `Oumuamua’s past trajectory out of the Milky-Way midplane, about six time smaller than that of the Sun. This suggests that `Oumuamua originated near the midplane of the thin disk of young stars. This fact implies a likely age younger than 1–2 billion years. Cosmologically speaking, `Oumuamua is an infant, younger by an order of magnitude relative to the age of the Universe. It is even much younger than the Sun, which is a late bloomer in cosmic history.
The past evolution of the distance of the interstellar object `Oumuamua from the Sun follows a period of about 2.2 billion years. `Oumuamua was on the other side of the Milky Way disk relative to the Sun about 1.1 billion years ago.
The maximal excursion of the comet Borisov is similar to that of the Sun, suggesting a similar age. The meteor IM1 exhibits larger vertical excursions, suggesting an older source.
We also applied the same code to calculate the future trajectories of the interstellar probes launched by NASA decades ago, Voyager 1 & 2 and Pioneer 10 & 11.
We have found that human-made interstellar probes, like Voyager 1 or Pioneer 10, will arrive at the opposite side of the Milky Way disk relative to the Sun in about 2 billion years and will return to the vicinity of the Sun in 4 billion years. This future “return near home” will occur long before the Sun will evolve to become a red giant star in about 7.6 billion years.
The radial and vertical extent of Voyager 1’s trajectory relative to the Galactic plane resembles the corresponding ranges for the Sun.
After the Sun will die, it will leave behind a compact metallic sphere, roughly the size of the Earth and containing 60% of the current mass of the Sun. Such a remnant is called a white dwarf. We know this fate for the same reason that we realize that we are doomed to die after visiting a graveyard. There are numerous white dwarfs from sun-like stars which died by now and are buried in the Milky-Way galaxy.
Based on the measured age of these white dwarfs, one can infer the star formation history of the Milky-Way. The procedure is similar to inferring historic birth rates from dated death certificates. Most of the Milky-Way stars formed billions of years before the Sun, with a peak in the star formation rate at about 10 billion years ago. If civilizations like ours were born around that peak and launched Voyager-like probes more than 2 billion years ago, these probes could have made it by now to the vicinity of the Sun from the other side of the Milky-Way disk.
This is why it is worth checking whether the anomalous shape and non-gravitational acceleration of `Oumuamua or the anomalous material strength and speed of IM1 might be indicative of a technological origin. While some regard this idea controversial and heretic, it sounds like common sense to me. But what can I say … I am just a simple-minded, curious farm-boy, not as high-brow as some editors of Scientific American might be — those who prefer not to confuse their readers with common sense.
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. His new book, titled “Interstellar”, was published in August 2023.
