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The anomalous interstellar object Oumuamua was too small to be detected by our best telescopes, but it was deduced to be a highly elongated object because it varied dramatically in brightness as it rotated every 8 hours.
We live in an exciting time. The first large interstellar objects were discovered only in the last decade: the interstellar meteor IM1 in 2014, the near-Earth anomalous object Oumuamua in 2017, and the interstellar comet Borisov in 2019. A fundamental unknown is the likely source of each of these unusual objects from outside the solar system.
To shed new light on this unknown, I proposed a summer project to Shokhruz Kakharov, an undergraduate student at Harvard University. My idea was to calculate the trajectories of these interstellar objects in time in the gravitational potential of the Milky Way 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 initialized the past trajectories of these interstellar objects by inverting their measured velocities relative to the Local Standard of Rest, the reference frame obtained by averaging the random motions of local stars near the Sun. This frame is orbiting 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 computer code, Shokhruz numerically integrated the trajectories of the interstellar objects over time into 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.
simulation Of Asteroid Oumuamua
By integrating the orbits of these interstellar objects over time, we were able to constrain the spatial region of their possible sources within the Milky Way. These constraints limit the possible birthplaces of the different interstellar objects and provide information about the galactic environment in which they originated.
Stars near the Sun follow an exponential distribution above and below the midplane of the galactic disk, with scale height increasing with age. We used each interstellar object’s vertical excursion from the midplane of the Milky Way disk to constrain the probability function for its possible age. Our approach was simple. Given each object’s maximum vertical excursion from the midplane of the Milky Way disk, we calculated the age distribution of stars that could have spawned them within that region to infer the probability distribution for the object’s age.
Any dynamical effects on stellar scale height from gravitational perturbations would also affect interstellar objects, since both populations are collision-free. Thus, our age constraints apply directly to the full age of interstellar objects regardless of their travel time.
We discovered a small vertical extension of Oumuamua’s past path outside the Milky Way’s midplane, about six times 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 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 maximum excursion of Comet Borisov is similar to that of the Sun, suggesting a similar age. Meteor IM1 exhibits larger vertical excursions, suggesting an older source. We also applied the same code to calculate the future trajectories of NASA’s interstellar probes launched decades ago, Voyager 1 and 2 and Pioneer 10 and 11.
The maximum excursion of Comet Borisov is similar to that of the Sun, suggesting a similar age. Meteor IM1 exhibits larger vertical excursions, suggesting an older source. We also applied the same code to calculate the future trajectories of NASA’s interstellar probes launched decades ago, Voyager 1 and 2 and Pioneer 10 and 11.
The radial and vertical extent of Voyager 1’s path relative to the galactic plane resembles the ranges corresponding to the Sun.
When the Sun dies, it will leave behind a compact metallic sphere, roughly the size of Earth and containing 60% of the Sun’s current mass. This remnant is called a white dwarf. We know this fate for the same reason we realize we are doomed to die after visiting a cemetery. There are numerous white dwarfs of Sun-like stars that have already died and are buried in the Milky Way.
From the measured ages of these white dwarfs, one can infer the star formation history of the Milky Way. The procedure is similar to inferring historical birth rates from dated death certificates. Most stars in the Milky Way formed billions of years before the Sun, with a peak in star formation rate about 10 billion years ago. If civilizations like ours were born around that peak and launched Voyager-type probes more than 2 billion years ago, these probes could have already reached the vicinity of the Sun from the other side of the Milky Way disk.
So it is worth checking whether the anomalous shape and non-gravitational acceleration of Oumuamua or the anomalous strength and speed of the material from IM1 could be indicative of a technological origin. Although some consider this idea controversial and heretical, it seems common sense to me. But what can I say? I am just a curious farm boy, not as intellectual as some of the editors of Scientific American might be, those who prefer not to confuse their readers with common sense.
ABOUT THE AUTHOR
Avi Loeb is the Director of the Galileo Project, Founding Director of the Black Hole Initiative at Harvard University, Director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and former Chair of the Department of Astronomy at Harvard University (2011-2020). He is a former member of the President’s Council of Advisors on Science and Technology and former Chair of the Board on Physics and Astronomy of the National Academies. He is the author of the best-selling book “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and co-author of the textbook “Life in the Cosmos,” both published in 2021. His new book, titled “Interstellar,” was published in August 2023.
