Comets have played a fascinating role in the history of astronomy. Since ancient times, many civilizations regarded them as omens or spirits that foretold good or bad news for kings, queens, and emperors. However, over the past few centuries, astronomers have closely studied them to uncover the scientific nature of these “visitors” to the inner Solar System. Today, we know that these ghostly phenomena in the sky consist of dirty balls of ice and rock hurtling through space and scattering dust and gases.
It turns out that comets also serve a purpose in the history of the Solar System. Each one holds within its ice and dust a treasure trove of clues about conditions in our Solar System, especially during the time they formed in the protosolar nebula about 4.5 billion (or more) years ago. And if our “native” comets possess such properties, just imagine what “alien” comets from other planetary systems could reveal about distant corners of the galaxy!
A perfect example is comet 3I/ATLAS, an interstellar object that passed through the inner Solar System between Earth and Mars in 2025. It approached Earth to within less than 1.8 astronomical units and developed a dense coma (a cloud of gas and dust). Astronomers used the James Webb Space Telescope (JWST) and its highly sensitive infrared spectrograph NIRSpec to examine the chemical makeup of this comet and discovered it is enriched with deuterium. In fact, it contains more than 30 times the amount of deuterium found in comets from our Solar System. This tells astronomers a great deal about the conditions under which the comet formed in its own system, its age, and even a bit about our own Solar System.
“This was a unique opportunity to study an ancient object from a distant galaxy, likely existing before our Sun and Solar System,” said astrochemist Martin Cordiner from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “On one hand, we gain direct insight into that distant time and place, and on the other, we learn something about how unusual our own Solar System might be.”
When approaching a heat source, any comet begins to sublimate and release gases. Comet 3I/ATLAS was no exception. Its passage between Earth and Mars heated its ice, leading to the formation of a gas coma. The NIRSpec instrument on JWST captured the light spectra emitted by the coma, and this data was used to determine the ratio of carbon to deuterium within it.
Deuterium is a useful element for study. It is an isotope of hydrogen that does not withstand heating well. Most of the deuterium we see in nature was produced during the Big Bang. It can also form in stars, but fusion reactions quickly break it down. Therefore, it prefers cold environments. Under prolonged exposure to heat, it gets converted into hydrogen, which makes up water. If you take a water sample here on Earth (or from any “local” comet in our system), you will find a specific deuterium-to-hydrogen ratio. The more hydrogen present, the less deuterium remains. This ratio is valuable for determining the conditions under which a comet formed.
No one has yet precisely identified the home system of comet 3I/ATLAS, but its high deuterium-to-hydrogen ratio suggests it most likely formed in a very cold system at an early stage of the Milky Way’s history. Based on its trajectory, some astronomers have proposed that its parent system might have been located in the thin or thick disk of the Milky Way. Additionally, they suspect it formed at least 10 billion years ago, or even earlier. This was a time when star formation was actively progressing in the galaxy, which determined the comet’s birthplace around a now-ancient star. Its journey through interstellar space did not expose it to intense heating, so it still retains the high deuterium ratio characteristic of its “birth” period. In other words, for most of its existence, it has remained in deep freeze.
Deuterium is not the only indicator of age and chemical composition that research teams used with JWST to study the alien comet. Carbon isotopes also shed light on the comet’s past. NIRSpec revealed only traces of carbon-13 compared to the lighter carbon-12. This, too, points to a very ancient origin for 3I/ATLAS. This is because stellar systems become enriched with carbon-13 over time, as generations of stars are born and die in the galaxy. When stars die, they release their carbon (and other elements) into space, and eventually this material is absorbed by new generations of stars (and planets). That is why our system, around our Sun, shows a higher level of carbon-13, having formed relatively recently, 4.5 billion years ago.
JWST was not the only telescope studying this remarkable comet. The European Southern Observatory’s Very Large Telescope (ESO) also examined the comet and detected a compound of carbon and nitrogen called cyanide. This is a prebiotic compound involved in the formation of life, and it suggests that conditions capable of fostering the emergence of life might have existed in the place where this comet formed. According to team member Stefanie Milam from NASA’s Goddard Center, discovering such chemicals elsewhere in the galaxy is a rare find.
“For us scientists, detecting these rare isotopes is an exciting event, but the more important aspect is exploring the possibilities of prebiotic chemistry elsewhere in the galaxy,” Milam said. “So far, we only know of one place in the vast cosmos where chemical components led to the emergence of life—our Solar System, our Earth. Analyzing these interstellar objects is a significant step toward understanding how common or rare the conditions for the evolution of life are in the universe.”
About the Author
