The Near Transits of the Inner Planets Across the Face of the Sun from the Vantage Point of 3I/ATLAS
When a planet transits in front of the disk of its host star, it blocks some of the starlight and reduces slightly the brightness of the star. The tiny fraction of starlight passing through the atmosphere of the planet allows us to measure the atmospheric composition.
In particular, the transit of the Earth in front of the Sun produces a dimming of only 0.008 percent for a distant observer that is aligned with the Sun-Earth axis. Nevertheless, a high-quality spectrograph on a purposely designed space telescope could identify during the Earth’s transit of the Sun the spectral fingerprints of oxygen and methane molecules as biological signatures of terrestrial life, as well as tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F) as technological-signatures of industrial pollution.
In 2014, I tasked my undergraduate student then, Henry Lin — who currently serves as an assistant professor at the Princeton Physics department, to calculate the detectability of the spectral fingerprints of industrial pollution in exoplanet atmospheres. Our paper, published here, identified detectable spectroscopic features of chlorofluorocarbons (CFCs) in planetary atmospheres.
An interested observer passing through the ecliptic plane of the Solar system could employ the transit method to monitor the evolving biological and technological conditions on Earth in real time. We must keep this possibility in mind while monitoring anomalous interstellar objects as candidates for technological probes which were sent into the solar system by extraterrestrial civilizations. The ideal path for transit-focused probes are retrograde trajectories (opposite to the motion of the Earth) that are aligned with the ecliptic plane of the Earth around the Sun, where they will be able to monitor as many transits of Earth or other solar-system planets throughout their journeys.
As it turns out, we have right now a suspicious interstellar visitor which follows a retrograde orbit which is aligned to within 4.9 degrees with the ecliptic plane. Even though this interstellar object 3I/ATLAS displays many anomalies (as listed here), the official view is that 3I/ATLAS is a comet of natural origin. However, if it happened to be a technological probe equipped with a spectrograph — it could have used its journey to observe transits of the Sun by all the inner solar system planets: Mars + Mercury, Venus and Earth on October 2, 2025; November 4, 2025; and January 22, 2026, respectively. Unfortunately, the actual orbital inclinations by several degrees of 3I/ATLAS of the planets relative to the ecliptic plane, make all these transits a near miss from the precise vantage point of 3I/ATLAS as it crosses the inner solar system, except for Venus where an actual transit was actually observed by 3I/ATLAS on November 4, 2024.
If 3I/ATLAS had been perfectly aligned with the ecliptic plane, then it would have detected biological markers on Earth such as oxygen and methane, when it entered the outskirts of the Oort Cloud at 100,000 times the Earth-Sun separation (AU) some 8,000 years ago. More recently, in 1950 — when Enrico Fermi had asked the question: “Where is everybody?”, 3I/ATLAS had been at a distance of about 1,000 AU where it could have detected industrial pollution in the Earth’s atmosphere — indicating that our planet went through an industrial revolution. In 1977, when the “Wow! Signal” was detected, 3I/ATLAS was at a distance of about 600 AU from Earth. And on January 22, 2026 when the Earth will nearly aligned with the Sun from its vantage point, 3I/ATLAS will be at a small distance of only 2.35 AU from Earth.
The NSF-DOE Rubin Observatory in Chile is expected to harvest several dozens of interstellar objects over the next decade. Given 3I/ATLAS, we should check if any of these interstellar objects align precisely with transits of planets as they approach the Sun.
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.
