(Credit: shutterstock)
How can we distinguish between the population statistics of interstellar rocks and a fleet of spacecraft that target that inner Solar system?
The answer is obvious: by the spatial distribution of their trajectories relative to the Sun. Whereas rocks arrive on random trajectories initiated near their parent stars, spacecraft aiming to probe the habitable zone around the Sun would favor paths directed towards the Sun.
A few days after the discovery of the interstellar object 3I/ATLAS, I wrote a paper (accessible here) in which I argued that over the ATLAS survey timescale of a decade the reservoir of rocky material in interstellar space can only supply rocks smaller than a kilometer to the inner solar system. If 3I/ATLAS is much bigger than a kilometer, then it must belong to a population of large objects on plunging trajectories towards the Sun, possibly by technological design (as mentioned in the concluding sentence of my preprint here). As it turns out, the trajectory of 3I/ATLAS is also fine tuned to align with the orbital plane of the planets around the Sun, having a chance probability of a fifth of a percent (as discussed here).
The brightness of 3I/ATLAS at a wavelength of 1 micrometer implies a nucleus diameter of 46 kilometers (as calculated here) for a typical albedo of 4%. However, if 99% of the brightness stems from icy fragments around 3I/ATLAS, then the nucleus diameter is ten times smaller, of order 5 kilometers (as calculated here). Given the unknown size of 3I ATLAS and its unusually high speed of 60 kilometers per second, it is appropriate to consider how directional the trajectories of its parent population are as a function of its size above a kilometer.
This calculation is addressed in a new paper (accessible here) that I co-authored with the exceptional graduate student at Vanderbilt University, Oem Trivedi. The derived formalism is particularly timely, in anticipation of the harvest of dozens of interstellar objects over the next decade by the NSF-DOE Rubin Observatory in Chile.
The new paper builds upon the population-level consideration regarding the rarity of 3I/ATLAS (as discussed in my paper here) and quantifies the resulting dynamical constraints. It follows velocity distributions that would allow an object of a given large size to be delivered through the observed trajectory of 3I/ATLAS for both natural and artificial populations. By considering both the small radius and large radius interpretations of 3I/ATLAS, the paper demonstrates how the rarity of the trajectory of 3I/ATLAS places stringent constraints on the abundance of massive interstellar objects. This analysis provides a new perspective on the emerging census of interstellar objects and directly addresses the question of whether the detection of 3I/ATLAS is dynamically compatible with a natural origin, or whether alternative explanations of a technological origin must be considered.
The paper develops a dynamical framework to constrain the population of massive interstellar objects, like 3I/ATLAS, using three complementary approaches. First, it uses encounter rate analysis to demonstrate that the detection of interstellar objects with radii of tens of kilometers cannot be reconciled with a purely random background of large rocks, implying the need for significant flux enhancements.
By applying the so-called Eddington inversion method, the paper establishes how steep the density profile of the parent population of 3I/ATLAS must be towards the Sun. The discussion is then extended to the so-called Liouville mapping in energy-angular momentum space, which propagates the interstellar velocity distribution inward under explicit conservation laws, showing that the required size dependent enhancement of encounter rates can arise from anisotropies that favor low angular-momentum trajectories directed towards the Sun.
These results suggest that the detection of interstellar objects with a size above 10 kilometers is dynamically consistent with strong gravitational directionality towards the Sun. This directionality suggests new physical mechanisms like gravitational kicks from a nearby star system or a local structure in the Galactic environment to produce the required velocity anisotropies. The other possibility is an orbit design of interstellar objects of technological origin. Future discoveries by the Rubin Observatory will provide the crucial statistics needed to test these different possible origins.
This morning, I received a note about commercially available T-shirts inspired by my recent scientific research on 3I/ATLAS (advertised here) as well as the following email on the same theme of financial benefits:
“Hope you’re doing well. I wanted to let you know about an interesting market on Polymarket right now, regarding whether the U.S. government will officially confirm the existence of extraterrestrial life by the end of 2025. You can check it out here:
https://polymarket.com/event/will-the-us-confirm-that-aliens-exist-in-2025
The way the market works is simple: It resolves to “Yes” if the President, a Cabinet member, a member of the Joint Chiefs of Staff, or any U.S. federal agency makes a clear statement that extraterrestrial life or technology exists by December 31, 2025. If not, it’s a “No.”
…It’s all about timing, and the rules are pretty flexible — even just a tweet would work…
Let me know what you think. Appreciate any help!
Your fellow American,”
My reply was short and simple:
“Frankly, my main interest is not in making money but in the question of whether we have a cosmic neighbor and what its impact might be on the future of humanity. This is a much bigger context, within which money would not maintain the same meaning or be used for the same purposes by future generations.”
ABOUT THE AUTHOR
Press enter or click to view image in full size(Image Credit: Chris Michel, National Academy of Sciences, 2023)
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.