Comet 3I/ATLAS, coming from another stellar system, could be between 10 and 12 billion years old. If this estimate is confirmed, it would mean that this object formed shortly after the birth of the Milky Way, thus becoming a privileged witness to the first moments of our galaxy.
First spotted in 2025, this comet is moving at the remarkable speed of 36 miles per second (58 kilometers per second) relative to the Sun. This velocity, the highest ever measured for such an object, likely betrays acceleration due to multiple gravitational encounters during its long journey.
Observations of comet 3I/ATLAS with the Gemini South Observatory.
Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Image processing: J. Miller & M. Rodriguez, T.A. Rector, M. Zamani
To refine its age, scientists used the James Webb Space Telescope and its infrared spectrometer. The analysis measured the ratios between carbon-12 and carbon-13, as well as the deuterium enrichment in the comet's water. These isotopes serve as temporal markers, as their abundance changes with the history and evolution of the galaxy. The obtained data show a particular chemical signature, very different from that usually observed in our nearby cosmic environment.
The results show that 3I/ATLAS contains very little carbon-13 compared to carbon-12, a sign that it formed before this isotope accumulated in the interstellar medium. Based on galactic evolution models, this corresponds to a remote period, 10 to 12 billion years ago, shortly after the start of star formation in the Milky Way. The deuterium enrichment corroborates this antiquity, indicating a cold environment poor in heavy elements.
Thus, this comet offers a glimpse into the conditions that prevailed during the genesis of the first planetary systems. Its composition rich in carbon and water indicates that the elements necessary for life were present very early in the Universe. 3I/ATLAS is a remnant, a time capsule delivering clues about primitive organic chemistry.
Although its exact origin remains difficult to trace due to gravitational perturbations, its great age points to a birth in the thick disk of the Milky Way. The star that gave birth to it may even have disappeared since, making this comet a relic of a bygone era. Research continues to complete its chemical inventory and trace its journey.
3I/ATLAS traveling in front of a starry background.
Credit: ESA Isotopes as cosmic clocks
Isotopes are atoms of the same element that differ in their number of neutrons. For example, carbon-12 has six protons and six neutrons, while carbon-13 has seven. This slight difference influences their chemical behavior and their presence in space over time.
In the galaxy, some isotopes like carbon-13 gradually accumulate thanks to stellar processes. Stars at the end of their life, particularly during explosions called novae, produce large quantities of this isotope. A low proportion of carbon-13 compared to carbon-12 therefore signals that the object formed before these events enriched the interstellar medium.
For comet 3I/ATLAS, the exceptionally high carbon-12/carbon-13 ratio allows astronomers to date its formation. By comparing with galactic evolution models, they estimate that it was born 10 to 12 billion years ago, a period when carbon-13 was still rare.
Deuterium, an isotope of hydrogen, provides complementary information. Its enrichment in the comet's water reveals very cold formation conditions, typical of primitive interstellar clouds. Together, these isotopic markers help reconstruct the history of objects and their original environment.