Over the past decade, astronomers discovered the first interstellar objects in the form of `Oumuamua, Borisov and IM1. Their origin from interstellar space was flagged by their speed exceeding the escape speed from the Solar system. All of these interstellar objects are bound gravitationally to the Milky Way galaxy.
Are there objects in the Solar system that may have originated in intergalactic space? My first popular science book was titled “Extraterrestrial.” The second was “Interstellar”. Should the third book be titled “Intergalactic”?
The speed needed to escape from the Milky-Way galaxy at the location of the Sun is about 500 kilometers per second, equivalent to 0.17% of the speed of light. The Sun is circling around the Milky-Way center at about 240 kilometers per second. This implies that any object ejected by the Sun at more than 260 kilometers per second along its direction of motion would reach intergalactic space. Such a speed could be endowed to a piece of debris from an asteroid break-up within a few times the radius of the Sun, as the escape speed from the surface of the Sun is 618 kilometers per second.
The Sun formed by accretion of matter from a surrounding disk. If a small fraction of the rocky debris disk around the young Sun happened to be ejected at a speed of a few hundred kilometers per second, then the rocks ejected in the direction of motion of the Sun would have traversed by now the distance to the Andromeda galaxy of a few million light years.
Other processes could have supplied intergalactic rocks. Examples include debris disks around supermassive black holes at the centers of galaxies, the tidal tails of stars and rocks in mergers of galaxies, or ejection of rocks by tight pairs of stars or black holes that act like pinball machines in kicking material between them into space. In collaboration with my former postdoc, James Guillochon, I calculated in a paper published a decade ago that there should be a substantial population of intergalactic stars with speeds exceeding ten thousand kilometers per second all the way up to the speed of light.
An even more common ejection mechanism was contemplated in a 1988 paper by Jack Hills, whereby the debris disk of rocks around a star can be torn apart from the host star by tides as the star passes in the vicinity of the 4 million solar-mass black hole, Sgr A*, at the center of the Milky Way galaxy. The Hills mechanism would apply to almost all galaxies, as they generically harbor a supermassive black hole at their center.
But in addition to natural sources of intergalactic objects, there might also be technological ones. All five of the interstellar probes launched by NASA: Voyager 1 & 2, Pioneer 10 & 11 and New Horizon, are on their way to leave the Solar system at a speed of tens of kilometers per second. As in the case of `Oumuamua, Borisov and IM1, this speed will keep our interstellar probes bound to the Milky-Way. However, future propulsion methods such as light sails pushed by high-power lasers — as envisioned by the Breakthrough Starshot Project for which I chaired the Science Advisory Board, could even reach relativistic speeds that will carry them to intergalactic space. Among all intergalactic rocks, we might one day find parts of functional or broken intergalactic spacecraft.
Because galaxies are separated by distances that are much larger than their size, the abundance of intergalactic objects per unit volume is likely to be much smaller than that of interstellar objects. However, nature could surprise us and therefore we should search for them. Intergalactic objects might have been missed by previous asteroid searches if they only showed once in sky images as a result of their high speed.
My undergraduate student at Harvard, Shokhruz Kakharov, is currently using a computer code to calculate trajectories in the gravitational potential well of the Milky-Way galaxy with the goal of identifying objects which originated in intergalactic space. Because of the motion of the Sun around the Milky-Way center, the minimum speed of intergalactic objects depends on their arrival direction.
The distance traveled by intergalactic objects depends on their ejection speed from their parent galaxy. The Hubble-Lemaître Law of cosmic expansion states that the recession speed of distant galaxies equals the Hubble constant times their distance. As a result, an intergalactic object cannot reach us from a distance larger than its ejection speed divided by the Hubble constant. Larger distances require a travel time that exceeds the current age of the Universe. This maximum travel distance corresponds to about 20 million light years for a speed of 500 kilometers per second. A region of this size around the Milky-Way galaxy contains several hundreds of galaxies.
Here’s hoping that other technological civilizations were far more ambitious than we are and ventured to intergalactic space with their most sophisticated propulsion technologies. Gladly, most stars formed billions of years before the Sun, so that semi-relativistic probes from distant galaxies could have reached us by now. It would be rather disappointing to only find rocks in our cosmic backyard.
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.
