The interstellar comet 3I/ATLAS was born in a distant corner of our Galaxy, in a region that's vastly different from our own Solar System.
Scientists studying the comet say its chemistry reveals the region in which it was born was much colder than our own cosmic neighbourhood.
And while scientists still don't know exactly where 3I/ATLAS came from, they're slowly piecing together clues as to what its home star system must have been like when it formed there about 10 billion years ago, long before the formation of our Sun.
More on 3I/ATLAS
An interstellar chemistry lab from deep space
Comet 3I/ATLAS was discovered in our Solar System on 1 July 2025. Soon after it was detected, astronomers calculated that it was an interstellar comet.
That means it originated from beyond our Solar System.
Of all the comets that have ever been discovered, almost all formed within our Solar System, but 3I/ATLAS is one of only three known comets that formed in deep space.
It's passing through our Solar System and, once it disappears from view, it will be gone forever.
That's why scientists have been studying it with some of humanity's most powerful telescopes and spacecraft, to learn as much about it as they can while it's passing through our cosmic environs.
A visitor from a cold corner of the Galaxy
Research led by the University of Michigan in the US reveals that, wherever comet 3I/ATLAS originated, it was in an environment much colder than our own Solar System.
Comet 3I/ATLAS is thought to be at least 7 billion years old, and perhaps even as old as 10 billion years. For perspective, our own Solar System is about 4.5 billion years old.
Comets are frozen bodies of rock, dust and ice, and when they get close to our Sun, that ice begins to sublimate into a gaseous form, venting outwards from the comet and forming a large, fuzzy cloud around the head of the comet and generating a long tail streaking behind it.
The team behind this study say they've found that 3I/ATLAS is rich in a specific type of water that contains deuterium.
Water is made of two hydrogen atoms and one oxygen atom, which is why its chemical formula is H2O.
Typically, hydrogen atoms in water molecules have a single proton at their core.
But in 3I/ATLAS, a high ratio of its water molecules contain deuterium, which is a form of hydrogen with the standard proton, plus a neutron.
Scientists say these 'heavier' forms of water also exist on Earth, but in lower quantities than observed in 3I/ATLAS.
"Our observations show that the conditions that led to the formation of our Solar System are much different from how planetary systems evolved in different parts of our galaxy," says Luis Salazar Manzano, lead author of the study and a doctoral student in the U-M Department of Astronomy.
"The amount of deuterium with respect to ordinary hydrogen in water is higher than anything we've seen before in other planetary systems and planetary comets."
The team say the ratio is 30 times that of any comet in our solar system and 40 times the value found in the water in our oceans.
That offers scientists a glimpse into the conditions present when 3I/ATLAS formed in a distant region of the Galaxy.
They can then compare what we know about the birthplace of 3I/ATLAS with what we know our early Solar System was like, when planets and comets were still forming.
The team say their study shows 3I/ATLAS formed somewhere colder and with lower levels of radiation.
"This is proof that whatever the conditions were that led to the creation of our Solar System are not ubiquitous throughout space," says Teresa Paneque-Carreño, co-leader of the study and U-M assistant professor of astronomy.
"That may sound obvious, but it's one of those things that you need to prove."
How the discovery was made
The team used the MDM Observatory Observatory in Arizona, USA to observe gas emission from the comet.
Salazar Manzano then contacted Paneque-Carreño to work on the study, who introduced the world-renowned Atacama Large Millimeter/submillimeter Array observatory in Chile to take a closer look.
ALMA was able to detect the difference between deuterated and conventional water, so the team could compare the two.
The team say their study shows that it will be possible to carry out similar studies of future interstellar objects.
While 3I/ATLAS is something of a rarity at the moment, astronomers expect new observatories like the Vera C. Rubin Observatory could usher in a new era of interstellar object discoveries.
That could mean more known visitors from deep space, and even more opportunities to learn about what's going on far beyond our Solar System.
Read the full study via the journal Nature
