Press enter or click to view image in full size(Credit: Chris Graythen/Getty Images)
The speed of 3I/ATLAS before it entered the solar system was about 60 kilometers per second, about 600 times faster than the fastest racing car in the world. This high value was interpreted (here and here) as implying that it originated from the thick disk of old stars in the Milky-Way galaxy. Let me explain why.
Stars are born in giant molecular clouds that reside within the thin disk of cold gas that circles the Milky-Way center. Over the course of time, these stars get kicked around gravitationally by satellite galaxies, spiral arms or star clusters in the Milky-Way and acquire enhanced random motions. Their increased velocity dispersion makes them deviate from the plane of the thin disk in which they were born. The resulting thickness of their disk-like configuration increases with age. Therefore, if the high speed of 3I/ATLAS was inherited from its parent star, that star must have been old — of order 10 billion years old.
However, this reasoning ignores the possibility that 3I/ATLAS was launched with a high-speed relative to its parent star. In that case, its observed speed has little to do with the random velocity that its parent star had relative to the thin disk of the Milky-Way galaxy. 3I/ATLAS could have acquired its speed at birth rather than inherited it from its parent star.
A year ago, I published a paper with my former postdoc Morgan MacLeod (accessible here) that described a process which naturally yields a launch speed as high as 60 kilometers per second at birth places of interstellar objects. It involves the most common type of parent stars, called dwarf stars, which possess about a tenth of the mass of the Sun and have a radius which is a tenth of that of the Sun. As a result, the average density of these dwarf stars is a hundred times larger than that of the Sun. Coincidentally, the average density of the Sun is 1.4 gram per cubic centimeter, comparable to that of solids.
Albert Einstein related gravity to spacetime curvature which is sourced by the mass density of matter. Given that, gravity can rip apart a solid by the tidal force in the vicinity of a compact object whose density is higher than that of the solid. Since the Sun is not dense enough to rip apart solids, a rocky planet like the Earth will not be disrupted by the Sun’s gravity even if it grazes the Sun. Instead, a planet like the Earth would be engulfed by the Sun’s envelope and get evaporated by the immense solar heat.
However, by virtue of its high density a dwarf star can spaghettify a planet like that Earth and convert it to a stream of matter that ejects half of its mass to interstellar space. At what speed? Remarkably, at 60 kilometers per second! That such a high speed can be naturally obtained near the most common type of stars was a eureka moment for me when I first derived it on a piece of paper.
However, we must keep in mind that 3I/ATLAS is an outlier relative to the previous interstellar objects, 1I/`Oumuamua and 2I/Borisov, not only because of its high speed but also because the direction of its retrograde velocity is aligned with the ecliptic plane of the planets around the Sun. This anomalous occurrence has a chance of 1 in 500 for a random arrival direction.
And there are additional anomalies about 3I/ATLAS. The highest resolution image of 3I/ATLAS obtained by the Hubble Space Telescope (accessible here) exhibited glow towards the Sun rather than a trailing tail as characteristic of comets. This suggests that there is not much refractory dust shed by 3I/ATLAS because dust particles with a size comparable to the wavelength of sunlight would be pushed behind the object by solar radiation pressure and appear as a cometary tail. Indeed, recent spectroscopic observations (reported here) show that 3I/ATLAS is anomalously depleted in carbon-chain molecules. This implies that the observed reddening of the reflected sunlight originates from the solid surface of 3I/ATLAS, and that its diameter may be as large as 46 kilometers, based on its brightness at 1 micrometer measured by the SPHEREx space observatory (as reported here). A solid object of that size would be a million times more massive that 1I/`Oumuamua and 2I/Borisov, and cannot be supplied by the reservoir of interstellar rocks over the survey period of the ATLAS telescope (as I showed here).
Adding to these anomalies, spectroscopy by the Webb telescope (reported here) implies a plume of gas composed mainly carbon dioxide (95% by mass) rather than water vapor (5% by mass), and spectroscopy by the Very Large Telescope (reported here) revealed a dramatic rise in the mass loss rate of nickel without iron, a feature of industrial production of nickel alloys.
The combination of these anomalies lead me to assign a rank of 4 to 3I/ATLAS on the Loeb Scale (defined here) where a rank of 10 flags a technological object that poses a threat to Earth. If you cross the street and realize that a racing car has a 40% chance of posing an existential threat, you better keep your eyes on that car.
And speaking about racing cars, I was contacted yesterday by a car racer who wishes to feature my image on the hood of his car in a forthcoming NASCAR race. He wrote to me: “You are definitely a hot commodity now in the NASCAR world. A lot of car owners are interested in you. The whole `Avi Loeb NASCAR idea’ is picking up incredible steam and it’s right around the expected arrival date of 3I/ATLAS to perihelion on October 29, 2025.”
Irrespective of where 3I/ATLAS came from, let us all hope that this racing object will be kind to us spectators as it passes by along its interstellar NASCAR race.
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.