Unveiling the Truth: Is Our 2025 Interstellar Dating Partner Unique?
As we reflect on the year 2025 and welcome the New Year, it's time to acknowledge the most significant dating partner humanity had in 2025: the interstellar object 3I/ATLAS. With only three interstellar dates in our history, it's easy to perceive this encounter as unique and our partner as exceptional. However, I will demonstrate below that if 3I/ATLAS is a natural comet, it is just one of a vast population - totaling 10^{23} similar objects in the Milky-Way galaxy alone.
Interstellar objects are recognized by their high speed, exceeding the value needed to escape from the solar system. The latest interstellar visitor, 3I/ATLAS, was discovered on July 1, 2025, but it spent the last 8,000 years traveling through the region containing objects that are gravitationally bound to the Sun, the so-called solar system.
3I/ATLAS reached the midpoint of its trip through the Solar system on October 29, 2025, at a minimum distance of 203 million kilometers from the Sun, which is 1.36 times the Earth-Sun separation (AU).
Assuming 3I/ATLAS is a natural object on a randomly drawn trajectory, we can estimate how many objects of its type are traveling through the Solar system out to the edge of the Oort Cloud at a distance of 100,000 AU. Given that 3I/ATLAS was discovered at a distance of ~5 AU in a survey that lasted 5 years, I calculate that there should be a trillion objects like 3I/ATLAS in the solar system right now!
Since the edge of the Oort cloud is roughly halfway to the nearest star system, Alpha-Centauri, the above calculation also means that roughly a trillion objects like 3I/ATLAS must be produced per star in the Milky-Way galaxy. If 3I/ATLAS is a kilometer in diameter, then its mass is of order a billion tons or a quadrillion (10^{15}) grams. Altogether a trillion such objects per star amount to a total mass of a sixth (17%) of the mass of the Earth. Interstellar matter is primarily composed of hydrogen (74% by mass) and helium (24% by mass) as relics of the Big Bang, with only 2% of heavier elements cooked in stellar interiors after the Big Bang. Taking account of the fact that most of the mass of 3I/ATLAS is associated with the 2% fraction of heavy elements, each star system had to process of order 10 Earth masses of interstellar matter in order to produce a Galactic population of comets like 3I/ATLAS.
In total, the 100 billion stars in the Milky Way galaxy processed 3 million solar masses to make a population of interstellar comets like 3I/ATLAS over the past 10 billion years. If 3I/ATLAS is a natural comet - there are 10^{23} similar objects in the Milky-Way galaxy alone and 10^{34} of them in the observable volume of the Universe.
In that case, our interstellar dating partner was not unique or special by any means! Over the age of the Earth, there were a billion similar interstellar visits.
The inferred population of interstellar visitors could be much smaller if 3I/ATLAS targeted the inner solar system by technological design, because in that case our immediate environment represents a special region of interest. Such a visit would merit an emotional connection akin to meeting an attractive dating partner worthy of a long-term relationship.
How could we identify interstellar dating partners which are worthy of our attention in years to come?
The screening setup for interstellar blind dates should be composed of three layers. The first layer involves a survey telescope like the NSF-DOE Rubin Observatory in Chile. Such a finding machine needs a wide field of view to cover the sky every few nights and a large telescope aperture to be sensitive to the reflection of sunlight from small objects. Given that the Rubin Observatory surveys the southern sky, it would be prudent to construct a twin of the Rubin Observatory in the northern hemisphere.
Once a new interstellar object is discovered by these survey telescopes, it would be essential to get a high-resolution image of it in order to decipher its nature. In particular, a 100-meter-long optical interferometer on the Moon - where there is no atmospheric turbulence to mess up the wavefront of light, could achieve the angular resolution (equal to the ratio between the wavelength of light and the interferometer’s length) needed to resolve a kilometer-scale interstellar object at a distance of ~1AU. This second layer of screening could distinguish between a natural rock and a technological artifact. Concepts for a lunar interferometer were motivated recently for other purposes here and here - in the context of NASA’s Artemis Program.
The third layer of screening for interstellar visitors involves an interceptor space mission (as discussed here and here), that could either land on a natural rock of interest - to search its materials for the building blocks of life-as-we-know-it, or mitigate the threat to humanity - in case the object is a technological artifact. The decision of which option to follow will be based on the rank of the interstellar object on the Loeb Classification Scale (as discussed here, here, and here).
I hope to live long enough to see what these three layers of screening will find for our interstellar dating partners. After many encounters, we will have a good sense of what constitutes a special catch. Here’s hoping that we would find a match made in heaven!
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.
