ALMA shows birthplace conditions for Interstellar Comet 3I/Atlas (19459000)
The water in our Solar System is unique in its chemical composition: It is deuterated. Deuterium is an isotope heavy of hydrogen. The enrichment occurs at low temperatures, such as the ones found in the prestellar cloud, star-formation sites, and the diffuse zones of distant protoplanetary discs.
The evolution of stars is unique because not all are created in the same nursery.
Deuterium-to-hydrogen (D/H), the ratio of water on a planet, reveals this difference in environment. It also acts as an indication for the degree-of freedom birth certificate.
Astronomers used ALMA observations to study the interstellar 3I/ATLAS comet and measure the ratio between deuterium and regular hydrogen within its water.
The unusual signal appeared only a few days after comet 3I/ATLAS’ closest approach to Sun. ALMA was able to collect data in a very narrow range of the Sun, something that most optical telescopes are unable to do.
The comet 3I/ATLAS had a chemical composition that was very unique. It contained 30 times as much semi-heavy (HDO), than any comets in the Solar System. This telltale chemistry sign provides an unfiltered view of the ultra-cold, freezing environment from which this comet was formed, and encodes in ice memories much harsher than what formed our planet’s backyard.
Luis E. Salazar Manzano, a PhD student at the University of Michigan stated that.
Our new observations reveal that conditions leading to the formation and evolution of the Solar System were very different than how planet systems developed in other parts of the Galaxy.
In the Solar System, semi-heavy (HDO), or heavy water (H2O) has a high subsolar ratio. One molecule of HDO is equal to 10,000 molecules normal water. This ratio in 3I/ATLAS is over 40 times larger than what we find on Earth’s oceans.
Also, it is important to remember that ALMA didn’t directly detect the comets H2O (normal water). Instead of measuring the water directly, they used HDO tracer and methanol signal to estimate it. The complex model allowed them to calculate the D/H and showed ALMA’s ability to gather chemical data from faint signals.
This ratio is remarkably high, and suggests that our Solar System originated in an environment that was incredibly cold and chemically distinct.
Discovering unexpected organic molecules within interstellar Comet 3I/ATLAS
Salazar Manzano said, The chemical reactions that enhance deuterated waters are sensitive to temperatures and require an environment colder than 30 Kelvin or minus 406 degrees Fahrenheit. This ratio was established when the comet formed its home system and it has remained unchanged throughout its interstellar voyage.”
ALMA played a crucial role in the discovery.
Paneque-Carreno noted: Most instruments cannot point towards the Sun. But radio telescopes such as ALMA are able to. Within days of perihelion we were able observe the comet as it popped out behind the Sun.
It allowed us to constrain these molecules in a way that is not possible with other instruments.”
Journal Reference
- Salazar Manzano, L.E., Paneque-Carreno, T., Cordiner, M.A. et. al. Water D/H is used in 3I/ATLAS to probe the formation conditions of another planet system.
Nat Astron (2026). DOI: 10.1038/s41550-026-02850-5
