2025 was the year of 3I/ATLAS, the third interstellar visitor that telescopes have been able to capture prowling the Earth. From the beginning it was considered that it could possibly be much older than the other two, Oumuamua and 2I/Borisov. However, there were many unknowns about its origin. Now, thanks to a recently published study by scientists at the University of Michigan, we have much more data on the matter.
A visitor from a cold and distant place. According to observations made from the ALMA Observatoryin the Atacama Desert (Chile), 3I/ATLAS formed in a cold and isolated corner of our galaxy, before definitively integrating into its own solar system.
The key is in deuterium. The observations that led to this recent study were made between October and December 2025. Several telescopes focused their attention on our interstellar visitor to observe the water on its surface and observed something striking: it had very high levels of deuterium. This is an isotope of hydrogen, somewhat heavier than the most abundant on Earth. In astronomy the deuterium/hydrogen ratio is used to estimate the age of objectssince it has been observed that the more primitive they are, the more deuterium they have. But this proportion also helps to know the temperature at which they were formed.
A reaction without turning back. In the cold gas clouds in which stars form, the most abundant molecules They are hydrogen, followed by carbon monoxide (CO). Hydrogen participates in something known as deuteron-proton exchange reactions, where hydrogen protons and deuterium isotopes react with each other, forming something known as deuterated hydrogen. This is the reaction: H3++HD⇌H2D++H2
CO can compete with this reaction, making it less efficient. However, when it is very cold, the CO freezes into dust grains and cannot react. On the other hand, the deuteration reaction can occur in both directions (hence the two arrows), but if it is cold there is not enough energy to produce it backwards. All the hydrogen with deuterium that is formed accumulates.
a lonely star. The fact that 3I/ATLAS occurred in such a cold environment may indicate that it possibly originated around a solitary star. If there had been other stars forming around it, it would be much hotter.
Very far away now. Today, that interstellar visitor is near Jupiter, preparing to leave our solar system. It can only be observed with a few instruments. Luckily, he came to our neighborhood long enough to give us a lot of valuable information.
Why is it useful information?. We already have a lot of data about 3I/ATLAS. For example, its core measures between 440 meters and 5.6 kilometers. Also that it moves at 220,000 km/h. Now, in addition, we know that it originated in an extremely cold environment, about 11,000 million years ago. This helps us understand the conditions of primitive planetary formation much better. As they always say, to know where we are going, it is very important to understand where we come from. There’s nothing like a visitor from a place far, far away to give us the pieces we need to understand it.
Image |NSF/AUI/NSF NRAO/M.Weiss
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