A Remarkably Ancient Visitor: 3I/ATLAS

A new study published in Nature suggests that the interstellar comet 3I/ATLAS may be as old as 12 billion years. Using JWST observations, supported by measurements from the Atacama Large Millimeter/submillimeter Array (ALMA), researchers found that the comet’s chemistry points to formation in a cold, metal-poor environment, very different from the one that produced comets in our own Solar System.

This result is especially remarkable because 3I/ATLAS is only the third confirmed interstellar object ever discovered, meaning it did not form around the Sun, but came from another planetary system. It is also only the second interstellar object observed to be active.

As it warmed near the Sun, ices preserved from its home system began to sublimate, releasing gas into a surrounding coma. That gas gave astronomers an extraordinary way to measure the composition of material that formed around another star.

3I’s Unique Chemical Fingerprint

Like all active comets, 3I/ATLAS released gas and dust as it traveled through the inner Solar System, but its coma changed over time. Different ices became more or less important as the comet warmed, passed perihelion, and cooled again.

Early observations showed a coma dominated by carbon dioxide. Around perihelion, when 3I/ATLAS passed closest to the Sun, water appears to have become dominant. The data for this new study were collected about two months after perihelion; the comet was outbound, and carbon monoxide had become the dominant gas.

But this is only part of the story.

Reading the Chemistry of Another Solar System

To look deeper, the research team studied isotopes, which are different forms of the same element. For example, ordinary hydrogen (H) and deuterium (D) are both hydrogen, but deuterium is heavier because it contains an extra neutron. Carbon has isotopes too, including carbon-12 (¹²C) and carbon-13 (¹³C).

Since different environments can leave behind different isotope ratios, scientists use these ratios like chemical fingerprints. Comets can preserve information about the temperatures, radiation conditions, and chemical environment in which the ices formed.

Using JWST’s NIRSpec instrument to spread the light coming from 3I’s coma into a high-resolution spectrum, the researchers measured isotopes of water, carbon dioxide, and carbon monoxide in the coma of 3I/ATLAS. From these measurements, they found unusually high ¹²C/¹³C ratios in CO₂ and CO, as well as a water D/H ratio more than an order of magnitude higher than values measured in known Solar System comets. These measurements make 3I/ATLAS chemically distinct from the comets we know.

What 3I's Isotopes Reveal About Its Past

In planetary systems, cometary carbon isotope ratios tend to reflect the material that was present when the host star formed. That makes 3I/ATLAS especially interesting. Its ¹²C/¹³C ratios are substantially higher than those measured in Solar System comets, strongly indicating that it did not originate in our own Solar System. However, the high carbon isotope ratios are unusual even beyond our Solar System.

To understand why that matters, we have to think about how a galaxy changes over time. As generations of stars are born, evolve, and die, they return newly processed material to the interstellar medium. Over time, this process enriches the Galaxy with heavier elements and with more ¹³C, causing the overall ¹²C/¹³C ratio of interstellar material to decrease. That means a very high ¹²C/¹³C ratio can point to material from an earlier, less chemically evolved stage of the Galaxy.

In the case of 3I/ATLAS, the authors argue that its carbon isotopic composition is most consistent with formation in a relatively old, metal-poor environment. When compared with models of Galactic chemical evolution, the measurements suggest that 3I/ATLAS could have formed as long as 10-12 billion years ago.

A Window into the Early Galaxy

This would make 3I/ATLAS dramatically older than our own Solar System.

Our Sun formed about 4.6 billion years ago from material that had already been enriched by earlier generations of stars. If 3I/ATLAS formed 10 to 12 billion years ago, it would come from a much earlier chapter of Galactic history, when the material available to form stars and planets was less chemically evolved.

The water tells a complementary story.

While the carbon isotope ratios give clues about the environment on galactic timescales, the D/H ratio gives clues about the environment’s temperature and processing.

Deuterium forms slightly more stable bonds than ordinary hydrogen. In warm environments, that tiny difference is overwhelmed by thermal energy, so hydrogen and deuterium are more easily shuffled around. But in very cold environments, there is not enough energy to undo some deuterium-enriching reactions, so deuterium can become preferentially locked into molecules, including water ice.

Radiation and cosmic rays can strengthen this effect. When ultraviolet radiation or cosmic rays pass through cold gas and dust, they ionize molecules, creating reactive particles that drive the chemistry forward. This process can make it easier for deuterium to be incorporated into water-forming chemistry.

This kind of enhanced ionization is expected in a lower-metallicity environment shaped by early, intense star formation. That cold chemical fingerprint can be altered later if ice is warmed, vaporized, mixed through a young planetary disk, and refrozen with lower-D/H water. The very high D/H ratio in 3I/ATLAS therefore suggests that much of its water formed at extremely low temperatures and avoided later heating or reprocessing.

Together, the carbon and hydrogen isotopes suggest that 3I/ATLAS may preserve ices from an environment much colder, older, and chemically different from the one that produced Solar System comets.

The Search Is Just Beginning

Pinpointing the origin of an interstellar comet is nearly impossible. By the time we observe one, its path has already been altered by countless gravitational interactions. The older the object, the harder it is to trace backward. That is why JWST observations like these are so powerful, because 3I’s chemistry can still preserve clues about the environment where it formed.

Of course, remote observations are not the same as holding a piece of 3I/ATLAS in a laboratory. A physical sample could provide more definitive measurements of mineralogy, isotopic composition, structure, and formation history.

But scientists have not yet collected material from any confirmed interstellar object, and doing so would be extremely difficult. Interstellar objects move fast, and any small fragments entering Earth’s atmosphere would be likely to burn up before they could be recovered. For now, astronomy is our best tool.

3I/ATLAS has already given us an unprecedented opportunity, as it was discovered early enough for detailed follow-up, giving astronomers the longest and most detailed look yet at an interstellar object. But with only three confirmed interstellar objects, the sample is still far too small to determine what is typical for these objects.

The next decade may change that. With telescopes like the Vera C. Rubin Observatory beginning its Legacy Survey of Space and Time, astronomers expect to discover many more fast-moving visitors from beyond the Solar System, potentially at least 1 per year. JWST and other observatories may soon have many more opportunities to study them in detail.

Someday, we may not only watch an interstellar object pass by. ESA’s Comet Interceptor mission, planned for launch later this decade, is designed to wait in space for a suitable target of opportunity. That target could be a dynamically new comet from our own Solar System, or, if we are lucky, an interstellar visitor. Either way, 3I/ATLAS shows why these objects are worth chasing: they are rare messengers from planetary systems beyond our own, carrying chemical records we are only beginning to learn how to read.