The Mass-Budget Discrepancy of 3I/ATLAS
In a new paper (accessible here), I show that the recently inferred radius and interstellar number density of 3I/ATLAS-like objects, imply a local mass density that is larger by orders of magnitude than the available reservoir of heavy elements locked in low metallicity stars. This association was suggested by recent isotope abundance measurements. Either the inferred radius or number density are overestimated or the association with metal-poor stars is incorrect.
The interstellar object 3I/ATLAS offers new insights into the mass reservoir of planetary systems across the Milky-Way galaxy. The latest data from the Hubble Space Telescope (reported here), was used to derive a nucleus radius of R_n = 1.3 ± 0.2 km and an interstellar number density of n ∼ 7 × 10^{−3} au^{−3}, where au is the Earth-Sun separation.
For a typical nucleus density of ρ_n ≈ 0.5 g/cm^3, the inferred radius implies a nucleus mass of m_n ≈ (4π[R_n]^3ρ_n/3) = 4.6×10^{15} grams. Hence, the local interstellar mass density of the population of 3I/ATLAS-like objects is,
ρ_{3I} ≈n×m_n =10^{−26} g/cm^3
Two recent papers (posted here and here) reported anomalous isotope abundances in the material that makes 3I/ATLAS. Based on Webb telescope observations, Cordiner et al. (2026) had found an isotope composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium at a level of D/H = (0.95 ± 0.06) percent, which is an order of magnitude higher than in known comets, suggesting a metal-poor origin. In addition, the 12C/13C isotope ratios (141–191 for CO2 and 123–172 for CO) exceeds typical values found in the Solar System, as well as in nearby proto-planetary disks. Chemical evolution models imply that the carbon isotopic composition originated 10–12 billion years ago. A similar conclusion was reached by Opitom et al. (2026), who reported measurements of carbon and nitrogen isotope ratios in 3I/ATLAS from observations of the cyanide (CN) molecule by the Very Large Telecope. This data suggests a 12C/13C ratio of 147 (+87/−40) and a 14N/15N ratio of 343(+454/-124), more than twice above the value of ∼ 150 usually measured for Solar System comets.
Below, I show that a low-metallicity origin for 3I/ATLAS generates untenable tension with the inferred mass budget of the 3I/ATLAS population of interstellar objects.
The Galactic orbit of 3I/ATLAS suggest a likely origin in the disk of the Milky-Way galaxy. The composition of the coma of 3I/ATLAS in terms of carbon, oxygen and nitrogen — based molecules, implies that most of its mass is associated with heavy elements.
For reference, the Galactic mass density of stars in the neighborhood of the Sun is,
ρ_⋆ ≈ 0.04M_⊙ pc^{−3} = 2.7 × 10^{−24} g/cm^3
Only a tenth of all stars in the Milky-Ways disk have metallicities below a tenth of the solar value. Considering those metal-poor stars as the suggested source population of 3I/ATLAS and adopting their metal mass fraction to be ∼ 2 × 10^{−3}, we find the corresponding local mass density of heavy elements in them to be,
ρ_z ≈2×10^{−3}×0.1×ρ_⋆ =5.4×10^{−28} g/cm^3
Since ρ_z ∼ 0.05ρ_{3I}, we conclude that the total mass density of heavy elements locked in low-metallicity stars is more than an order of magnitude below the required mass density in interstellar objects like 3I/ATLAS.
Planetary systems — which serve as the natural birth sites of interstellar objects — originate from debris disks that contain at least ten times less mass than the host star. In addition, one expects a mass spectrum of ejected interstellar objects to contain at least ten times more mass in objects with masses that are orders of magnitude different from that of 3I/ATLAS. When these additional factors are included, we find that low-metallicity stars miss the required mass budget by at least 3 orders of magnitude. They cannot account for the interstellar population of 3I/ATLAS-like objects unless they are capable of ejecting to interstellar space more than a thousand times the heavy-element content of their planetary disks.
In conclusion, either the inferred radius or number density of the population of 3I/ATLAS-like objects are overestimated or their association with metal-poor stars is incorrect.
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.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
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https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
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