On October 3, 2025, the interstellar object 3I/ATLAS passed within 29 million kilometers from Mars and on March 16, 2026 it will pass within 54 million kilometers from Jupiter. This encounter with multiple solar system planets requires two independent coincidences:
1. A near alignment by 4.89 degrees of the retrograde orbital plane of 3I/ATLAS with the ecliptic plane of the Earth around the Sun — having a probability of 0.002.
2. A fine-tuning of the arrival times at the planets’ orbital radii — with a net probability of 0.0002, given the circumferences of the orbits of Mars and Jupiter.
The combination of these two independent coincidences has a likelihood of less than a millionth. If the trajectory of 3I/ATLAS was designed by extraterrestrial intelligence, why is it inclined by about 5 degrees relative to the ecliptic plane?
The ecliptic plane is defined by the motion of Earth around the Sun. The orbital planes of Mars and Jupiter are inclined by 1.9 and 1.3 degrees relative to it. If the planetary system around the Sun was discovered through a transit survey of stars, then the extraterrestrial observers who could have discovered it are located within a conical disk aligned with the ecliptic plane, from where the solar system planets appear to cross the face of the Sun. What is the opening angle of this conical disk? The answer depends on the quality of the telescopes that extraterrestrial observers use.
If 3I/ATLAS is technological in origin, then the fact that its mass is larger than 33 billion tons (as deduced here), suggests that its senders have access to technologies far more advanced than humans possess — as humanity’s biggest rocket, Starship, weighs 6 million times less, only 5.5 thousand tons. Therefore, it would be reasonable to assume that the telescopes used by the senders of 3I/ATLAS are capable of noticing transits by rocky objects that are smaller than Earth.
The main asteroid belt is a toroidal (doughnut-shaped) region between the orbits of Mars and Jupiter, containing numerous asteroids and minor planets. The largest dwarf planet there is Ceres with a mean diameter of 940 kilometers, roughly the size of the state of Texas, and an orbital radius of about 2.8 times the Earth-Sun separation. The asteroid belt contained the building blocks of planets, so-called planetesimals, in the primordial solar nebula. In the region between Mars and Jupiter, the gravitational perturbations from Jupiter disrupted the coagulation of these building blocks into a new planet. Jupiter’s gravity added relative speed to colliding planetesimals and shattering most of them. As a result, the vast majority of the asteroid belt’s original mass was lost in the first 100 million years of the Solar System’s history. Nevertheless, the remaining debris could still be noticed from afar in transit surveys with advanced extraterrestrial telescopes. Analysis of the transits would show a characteristic orbital period of 3–6 Earth years.
What is the opening angle of the conical disk relative to the ecliptic plane, inside of which solar transits of main belt asteroids can be observed?
The thickness of the asteroids’ doughnut is of order 150 million kilometers and its diameter is about 800 million kilometers, implying a conical opening angle of about 10 degrees above and below the ecliptic plane of the Sun.
Given that, we should expect extraterrestrial civilizations in our cosmic neighborhood that discovered our planetary system through transit surveys to send interstellar probes on the shortest straight trajectories to intercept the path of the Sun with an orbital inclination of up to 10 degrees, comparable to the 5 degrees inclination of 3I/ATLAS.
So far, 3I/ATLAS displayed 7 anomalies (detailed here) in terms of its size, jet, composition, polarization, inclination, timing, and angular alignment with the “Wow! Signal”. Whether these anomalies reflect chance coincidences for an interstellar comet of a natural origin or intelligent planning, remains unknown as of now.
Counting our blessings without knowing their origin, we can take advantage of the proximity of 3I/ATLAS to Mars and Jupiter. Here’s hoping that imaging and spectroscopy (as detailed here) by 7 human-made spacecraft around Mars: MRO, Mars Express, TGO, MAVEN, Tianwen-1 and Hope, as well as by 2 human-made spacecraft related to Jupiter: Juice and Juno, will help us figure out the true origin and nature of 3I/ATLAS.
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.
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