New Anomalies in the Isotope Abundances of 3I/ATLAS
The chemical interactions of atoms are dictated by the number of electrons they possess. The electron cloud around the atomic nucleus balances the charge of the nucleus, which is proportional to the number of protons in it. Since the charge of the electron equals to minus that of the proton, the number of electrons in a neutral atom equals the number of protons and dictate the chemical behavior of the atom. However, atomic nuclei can also contain neutrons which are electrically neutral. Stable nuclei often have comparable number of protons and neutrons but they can have variants with a surplus or a deficit of a few neutrons. Isotopes are atoms with nuclei that have identical number of protons but different number of neutrons. The relative abundance of different isotopes of the same element depends on local production channels, such as exploding stars of different masses, the distance to the nearest historic merger of neutron stars or bombardment of nuclei by energetic cosmic-rays.
The Solar System formed out of a cloud of gas that was uniformly enriched by the same local processes. As a result of this specific origin, the isotopes found on Earth, other Solar System planets, asteroids or comets, have similar isotope ratios and serve as fingerprints of Solar System materials. Whereas the relative abundances of elements can be modified by chemical reactions which select some of them relative to others, the isotopes abundance ratio of a specific element can only be modified by nuclear processes which require temperatures in excess of ten million degrees, not found on planets, asteroids or comets.
Two new papers (posted here and here) report today about anomalous isotope abundances in the material that makes the interstellar object 3I/ATLAS.
The first paper, led by Martin Cordiner, reports that isotope measurements of 3I/ATLAS with the Webb telescope reveal a composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium — an isotope of hydrogen (one proton) whose nucleus contains a proton and a neutron, at a level of D/H = (0.95 ± 0.06) percent, which is more than ten times higher than in known comets. 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 nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at freezing temperatures below 30 degrees Kelvin in a relatively metal-poor environment, early in the history of the Milky Way galaxy. Astronomers refer to elements heavier than hydrogen and helium (both of which are relics of the Big-Bang), as `metals’. When interpreted in terms of models for chemical evolution, the carbon isotopic composition implies that 3I/ATLAS formed 10–12 billion years ago. Hence, 3I/ATLAS is interpreted in this paper as a fragment of an ancient planetary system with a low metallicity.
The second paper, led by Cyrielle Opitom, reports the measurement of carbon and nitrogen isotope ratios in 3I/ATLAS from observations of the cyanide (CN) molecule, based on observations with the Very Large Telescope in Chile. The data implies a 12C/13C ratio of 147(+87/-40) and a 14N/15N ratio of 343(+454/-124). The 14N/15N ratio is more than twice above the value of about 150 usually measured for solar system comets. The 12C/13C is marginally higher than the values usually measured for solar system comets and in the interstellar medium. Similarly to the first paper, the authors here conclude that their measurements might indicate an origin from an old, low-metallicity star.
However, both papers do not realize that a low-metallicity origin for 3I/ATLAS generates untenable tension with its inferred mass and abundance. Analysis of the latest data from the Hubble Space Telescope on 3I/ATLAS (reported here), suggests a nucleus radius of about 1.3 kilometers and a number density of about 0.007 per AU cubed (where AU is the Earth-Sun separation). This implies 30 trillion objects and a total mass of 100 Earth masses within the volume of the Oort cloud out to 100,000 AU around the Sun — which is roughly half way to the nearest star, Proxima Centauri.
Only a tenth of all stars in the Milky-Way’s thick disk has a metallicity that is 10 times below the solar value (as discussed here). By restricting the source population of 3I/ATLAS to these low-metallicity stars, I find that each of these low-metallicity stars must produce 1,000 Earth masses in objects the size of 3I/ATLAS. Most of the material in the gas plume around 3I/ATLAS is made of carbon or oxygen based molecules (as reported here), suggesting that the object is made of heavy elements, considered as metals.
This mass budget calculation requires the production of 0.003 solar masses in 3I/ATLAS-like objects per star. However, solar mass stars with a tenth of the solar metallicity have only 0.002 solar masses in heavy elements within them. In addition, their planetary systems which serve as the natural birth sites of interstellar objects, are expected to originate from debris disks that contain at least ten times less mass than the host star. On top of that, one expects a mass spectrum of ejected interstellar objects with at least ten times more mass in objects with masses that are orders of magnitude different from that of 3I/ATLAS (as discussed here).
This calculation implies that low-metallicity stars miss the required mass budget by at least three orders of magnitude and cannot account for the interstellar population of 3I/ATLAS-like objects even if they ejected all their heavy elements to interstellar space.
The more data we get about 3I/ATLAS, the more puzzling it looks. As Forrest Gump said in the 1994 film: “Life is like a box of chocolates, you never know what you’re gonna get.”
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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