A rare comet from outside the solar system is providing astronomers with a rare window into what an "alien" planetary system born in a completely different environment from the solar system looks like. The object, named 3I/ATLAS, was discovered less than a year ago as it traveled through our solar system. While scientists still don't know where exactly it originated, a new study led by the University of Michigan shows that the comet most likely formed in an unusually cold region of the universe.
The research was published in the latest issue of "Nature Astronomy". The core finding is that the water contained in 3I/ATLAS is rich in a special kind of water molecules called "heavy water", which is characterized by an abnormally high content of deuterium (heavy hydrogen). The research received funding from the U.S. National Science Foundation and the Chilean National Agency for Scientific Research and Development, among others. The research team pointed out that through this special isotope ratio, scientists can conduct unprecedented chemical "reverse deductions" on the formation conditions of other planetary systems in the Milky Way.
Water molecules are made up of two hydrogen atoms and one oxygen atom and have the chemical formula H₂O. In ordinary water, hydrogen atoms contain only one proton; deuterium, a heavier isotope of hydrogen, has both protons and neutrons in the nucleus. The researchers found that a significant proportion of the water molecules in 3I/ATLAS were "deuterated water," which is heavy water containing deuterium. Heavy water is found on Earth and in comets throughout the solar system, but the amount in 3I/ATLAS far exceeds previous observations.
"The ratio of deuterium in heavy water to ordinary hydrogen in this comet is higher than what we see in other planetary systems and planetary comets," said Luis Salazar Manzano, first author of the paper and a doctoral student in the Department of Astronomy at the University of Michigan. The study shows that the deuterium/hydrogen ratio of the water in 3I/ATLAS is about 30 times greater than that measured for any comet in the solar system, and about 40 times greater than that found in Earth's oceans. This extreme ratio suggests that it formed in an extremely cold environment and received far lower radiation levels than the local environment when the solar system was young.
The scientific research team pointed out that chemical ratios such as deuterium/hydrogen in water are important "fingerprints" for understanding the conditions for the formation of comets and planets. After comparing the chemical signatures of 3I/ATLAS to solar system objects, the researchers concluded that the comet was likely born in an interstellar region where temperatures were cooler and radiation was weaker. "This proves that the conditions that contributed to the formation of our solar system are not a universal template in the universe." Teresa Paneque-Carreño, one of the co-leaders of the paper and an assistant professor of astronomy at the University of Michigan, pointed out, "This may sound obvious, but many seemingly 'obvious' judgments in science must be confirmed one by one with data."
To conduct such a detailed chemical analysis of an interstellar visitor, luck is also crucial. First, 3I/ATLAS must be discovered early enough into the solar system so that astronomers can seize the observation time of major telescopes in time. After completing the initial confirmation, Salazar Manzano and his team successfully applied for observation time at the MDM Observatory in Arizona, USA, where they captured some of the earliest signs of the gas released by the comet.
He then collaborated with Panek-Carreño, who specializes in molecular composition studies using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile. ALMA is sensitive enough to distinguish the weak signal differences between ordinary water and deuterated heavy water, allowing the team to directly calculate the ratio between the two. Scientists emphasize that this is the first time such fine water isotope analysis has been successfully carried out on an interstellar object.
“Being at the University of Michigan and having access to these facilities is key to making this work possible,” Salazar Manzano said. He said that team members have rich experience and complementary strengths in multiple fields, which allows them to efficiently integrate different observational data and give physical explanations that can withstand testing.
So far, the astronomical community has only confirmed three interstellar visitors in the solar system, one of which is 3I/ATLAS. Researchers believe that this study shows that as long as observation conditions permit, chemical "physical examination" of interstellar objects is completely feasible in the future, which will open a new window for understanding the formation process of various planetary systems in the Milky Way. As a new generation of sky survey facilities come online, the discovery of such interstellar visitors is expected to increase significantly.
Panek-Carreño also emphasized that the importance of protecting dark night skies and reducing light pollution is growing day by day. Only with a sufficiently clear and dark sky background can astronomers continue to capture these weak signals from deep space.