Thirteen times. That is how much the deuterium proportion in 3I/ATLAS's methane exceeds that of comet 67P/Churyumov-Gerasimenko, the only comet in our own system with a "secure" measurement of this kind. And 67P isn't the only one left behind: according to the study published in Nature Astronomy, no body in the Solar System measured so far holds as much. If you don't keep the periodic table in your head: deuterium is hydrogen with an extra neutron, so heavier, and the ratio between it and ordinary hydrogen works like a fingerprint of the place where the molecule formed. #### Properly cold At around -263 °C, gases freeze onto dust grains, and deuterium, being heavier, sticks to the ice preferentially. The colder the cradle and the more cosmic rays hitting it, the more deuterium piles up. The team, led by scientists from NASA's Solar System Exploration Division, reads 3I/ATLAS's value as a sign of ice formed somewhere frigid, poor in heavy elements and heavily irradiated. It could have been an interstellar cloud, or the inside of a disk around a star still forming. These nurseries are opaque, which is why astrochemistry leans so hard on models: from the outside, you barely see what goes on in there. Here, at least, we have a piece that came from there and dropped by our doorstep. #### Measured from afar The James Webb looked at the comet in late December 2025, when it was about 2.5 times the Earth-Sun distance from the Sun, crossing the asteroid belt zone between Mars and Jupiter. At that distance the ice starts to sublimate and the coma, the comet's atmosphere, swells enough for the telescope's infrared to catch what's inside. What they read was the molecular emission of that atmosphere, and the methane ratio came out of it. Compare 67P, where it took a probe, Rosetta, measuring on the spot. Here what we have is light, caught from afar, and a telescope doing the job of a lab. That's not little. It's not the same thing either. #### The number the headlines round off Time to hit the brakes. The deuterium ratio is a measured fact. The age is not: it's a reading. The low carbon-13 suggests an origin in a universe not yet much enriched by generations of stars and supernovae, and from that comes the hypothesis that 3I/ATLAS formed more than seven billion years before the Sun lit up, perhaps even before the very star it was thrown out of. None of these numbers seems to come from direct dating. It's a chain of inferences, and each link is reasonable. The source I read puts ten billion years in the headline and seven billion years before the Sun in the body. They're not incompatible calculations, but they show how elastic the number is. If the hypothesis holds, the consequence has weight. The comet would have taken shape before many of the atoms that make up the Sun and planets were forged inside massive stars and scattered when those stars died. It's ice from a time when the Universe hadn't yet finished preparing the ingredients we're made of. #### Thirteen times relative to what? The weak link, as I read it, is the first one. The "thirteen times" compares against a single comet. One. 67P is the only other local case with a secure value, which means that what we consider normal in the Solar System, for this indicator, rests on one comparison point. If tomorrow a comet of ours turns up with a high value, the astonishment shrinks. And 3I/ATLAS is only the third interstellar visitor identified, after 1I/'Oumuamua and 2I/Borisov: if the next visitor comes out similar, it stops being an anecdote and becomes a population. There's also context that weighs in: it's not the first analysis to describe unparalleled chemistry in this comet. When strange results pile up, the temptation is to stitch them into one coherent story. It's worth resisting a little. This is one observation, of one molecule. It doesn't mean it's wrong. It means the question "how old is this?" still has more air than ground. What I'd like to see next is the same ratio measured in other molecules. Water, for instance, has decades of measurements in comets of our system, and a 3I/ATLAS value there would give a comparison with far more points than methane allows. If both indicators tell the same story, the thesis gains body. If not, the story gets more interesting, but a lot less tidy. Discovered on 1 July 2025. Sixteen months later, we're already arguing over a seven-billion-year head start on the Sun. I like the hurry. I just wouldn't bet dinner on the final number.