Interstellar Comet 3I/ATLAS Reveals Clues About Its Cold Birthplace Beyond Our Solar System

In late 2025, astronomers detected an interstellar comet, designated 3I/ATLAS, during its rapid transit through the solar system. The comet was first spotted in July 2025 when it was just inside Jupiter’s orbit. As of now, it is approximately halfway between Jupiter and Saturn and continues to move farther away.

The European Space Agency’s Jupiter Icy Moons Explorer (ESA JUICE) mission captured an image of 3I/ATLAS on 5 November 2025, when the comet was 64 million kilometers from the spacecraft. Credit: ESA/Juice/JANUS, CC BY-SA 3.0 IGO

Astronomers have closely monitored 3I/ATLAS throughout its journey, both inward toward the Sun and back outward, compiling the most detailed observations yet of an interstellar object. These include measurements of the gases sublimating from its surface, which form its coma and tail.

Groundbreaking Discovery: Deuterium-to-Hydrogen Ratio Suggests Cold Formation

In a first-of-its-kind study, researchers found that the ratio of deuterium to hydrogen in 3I/ATLAS’s outgassed water is 30–40 times higher than in solar system objects. This unusually high ratio indicates the comet likely formed in a much colder environment than our solar system.

“It is always hard to really pinpoint where these objects form. We know that they were formed in different parts of the galaxy, but it’s hard to connect what we measure with how they were formed. These types of measurements, such as the relative abundance of deuterium to hydrogen in water, are one of the best ways we have to actually [learn] about their forming conditions and their evolution.”

Understanding Water’s Role in Interstellar Objects

Water is pervasive across the universe, found in distant galaxies and star-forming nebulae. However, water exists in different forms: heavy water, semiheavy water, and regular H₂O. In the frigid molecular clouds where stars form, a chemical reaction favors the creation of gaseous deuterium (D), an isotope of hydrogen. This deuterium bonds with hydrogen and oxygen to produce semiheavy water (HDO).

By measuring the amount of semiheavy water relative to regular water in an object, scientists can determine its deuterium-to-hydrogen (D/H) ratio. This ratio provides insights into the physical conditions under which the water formed. Similar measurements have been conducted on baby stars, planet-forming disks, solar system comets, meteorites, and even Earth’s oceans.

“What is fundamentally important about ISOs is that they are physical leftovers of the process of forming another planetary system and they can give us clues to that process. The conditions in the stellar system in which 3I/ATLAS formed may have been quite different from the one in the solar system.”

The team observed 3I/ATLAS using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile during November 2025, when the comet was 335 million kilometers (208 million miles) from Earth.