Remember the interstellar comet? It was formed in the cold corner of the galaxy at -240 °C

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Favorable -243

The keyč discovered that it lies in the first analysis of so-called semi-heavy water, also known as deuterated water, in some interstellar object. Observations made by the ALMA (Atacama Large Millimeter/submillimeter Array) telescope in Chile have shown that 3I/ATLAS contains at least 30 times more deuterated water than comets from our Solar System. In this variant of the water molecule (HDO), one hydrogen atom is replaced by deuterium, a heavier isotope of hydrogen that contains an additional neutron.

Such a high ratio is a direct window into the conditions under which the comet’s parent star system was formed. Scientists explain that the chemical processes that lead to an increase in the proportion of deuterated water require extremely low temperatures, lower than 30 Kelvin, which is about -243 degrees Celsius.

Once set during formation, that chemical composition was preserved throughout its interstellar journey. The research was led by PhD student Luis E. Salazar Manzano from the University of Michigan, who emphasized the validity of the findings.

​- Our new observations show that the conditions that led to the formation of our solar system were significantly different from the way planetary systems developed in other parts of our galaxy – said Salazar Manzano.

A guest from the distant past

Besides coming from an extremely cold environment, 3I/ATLAS is probably incredibly old. Scientists estimate that its age could be up to 11 billion years, which makes it more than twice as old as our Sun, which is 4.6 billion years old. The trajectory of the comet, traced back through the gravitational field of the Milky Way, suggests that it originated in the so-called “thick disk” of the galaxy. That part of the galaxy is mostly inhabited by older stars, which further supports the theory of the ancient origin of comets.

By comparison, the first two confirmed interstellar visitors, 1I/ʻOumuamua and 2I/Borisov, are thought to be significantly younger and originate from the “thin disc” of the galaxy, home to younger stars like the Sun. Because of this, 3I/ATLAS has been described as the “elder” of interstellar travelers, bringing us fossil records from the very early ages of the Milky Way.

Comet 3I/ATLAS was discovered on July 1, 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Čile. Its hyperbolic trajectory and extremely high speed immediately confirmed that it does not belong to our Sun system. It passed closest to the Sun on October 29, 2025 at a distance of about 1.36 astronomical units, which is between the orbits of Earth and Mars.

The key measurements of water composition are due to the unique capability of the ALMA radio telescope. Unlike most optical telescopes, ALMA can direct its observations towards the Sun, which allowed scientists to study the comet just six days after its closest approach to the Sun.

​- Most instruments can’t aim at the Sun, but radio telescopes like ALMA can. We were able to observe the comet just a few days after perihelion. This gave us insight into these molecules in a way that is not possible. other instruments – explained Teresa Paneque-Carreño, the main researcher of the program that made these observations possible.

An unusual composition and a view into other worlds

In addition to water, observations by the James Webb Space Telescope (JWST) revealed that 3I/ATLAS is unusually rich in carbon dioxide (CO2), and contains water ice, carbon monoxide and methane. The high proportion of CO2 is consistent with the hypothesis of formation in a very cold environment, far from the parent star, beyond the so-called “freezing line” of CO2.