Exiled interstellar visitors like comet 3I/ATLAS are time capsules from the cosmos at large, yeeted from their exosolar homes by an unknown cataclysm and forced to drift through lonely space for billions of years, perhaps.
If they’re lucky, they may cross paths with another star, such as the Sun, and warm themselves at our solar hearth for a short while before speeding back out into darkness.
This is lucky for us, too, since we get an unprecedented look at a chunk of an alien Solar System, which may have formed long before ours with chemistry unlike anything glimpsed before.
In fact, the exosolar system that formed 3I/ATLAS may have assembled it more than 7 billion years before our Sun ignited into life – possibly even before 3I/ATLAS’s own star erupted.
In a new study in the journal Nature Astronomy, a research team led by NASA Solar System Exploration Division scientists analyzed the chemical cocktail emanating from the interstellar comet’s excited atmosphere.
They used the James Webb Space Telescope’s unmatched infrared capabilities to view the comet in late December 2025, when it was about 2.5 times as far from the Sun as we are, shooting through the asteroid belt between Mars and Jupiter.
At this distance, its atmosphere, or coma, starts puffing up substantially as solar radiation sublimates its icy volatile chemicals. Think of it like the comet sneezing when it sees the Sun, in an astrophysical version of the possibly fake-sounding photic sneeze reflex, which makes people sneeze in response to bright lights.
The researchers focused on its deuterium-to-hydrogen (D/H) ratio, which is highly revealing because it shows how much deuterium – a heavier version of hydrogen – an object has in proportion to the lighter, regular hydrogen.
In this instance, they checked this ratio in another molecule, methane, to see how much heavy methane the 3I/ATLAS retained since its frozen dust grains formed.
They found it held significantly more than any body in the Solar System, and 13 times more than comet 67P/Churyumov-Gerasimenko, the only other local comet to display a “secure” signature, which required in-situ measurements from the Rosetta spacecraft.
“The elevated D/H ratio for methane in 3I/ATLAS is consistent with the hypothesis that its ices formed in a cold, low-metallicity interstellar or protostellar environment under a high cosmic ray irradiation rate,” the researchers explain.
One possible origin is within the pancake-shaped planet-forming disk of dust and gas around a very young or forming star. These astro-environments are hard to study because they’re thickly opaque, hiding the cool chemical actions occurring within, so researchers may often rely on astrochemical models.
And this is why 3I/ATLAS is so precious: it’s a pristine sample ejected from the hidden depths of an ancient cosmic nursery, possibly around one of the Milky Way’s oldest stars, and delivered directly to our doorstep by the caprices of gravity.
Viewing its molecular emission, akin to it losing its lunch due to the cosmic carsickness of hurtling toward the Sun, allowed astronomers to prod various pertinent puzzles.
Its isotopic ratios hint that it may have been exposed to lots of radiation. And maybe born in a prestellar core: a dense region of super-cold gas (since its interior is protected from starlight) and the earliest stages of star formation, so frigid that it acts as a cryo-preserver.
At temperatures of around -263 degrees Celsius (-440 degrees Fahrenheit), gases freeze onto dust grains, favoring deuterium since it’s heavier than hydrogen, and producing distinctive D/H ratios in molecules like methane.
Additionally, low carbon-13 content of 3I/ATLAS suggests its incredible age, originating in an ancient Universe that had not yet been enriched by generations of stars fusing heavier elements and releasing them space-ward during their explosive supernova deaths.
So, overall, this work highlights the mind-boggling extent of cosmic timelines. This comet from who knows where could have been traveling for 7 billion years before our Solar System existed. It may have formed before many of the atoms that comprise our Sun and planets were even forged in the hearts of massive stars, which died (and left behind remnants, of course) to seed our existence.
The research has been published in Nature Astronomy.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
