Nearly a year ago, astronomers around the world focused many telescopes on a bright point of light that passed through the solar system at high speed - comet 3I/ATLAS. This was the third interstellar object observed by humans in the solar system. In the following months of observations, researchers measured that the 2.6-kilometer-diameter object was traveling across the solar system at a speed of 221,000 kilometers per hour.
A core question remains unresolved: where did 3I/ATLAS come from, and more precisely, in what age of the universe was it born?
Artist's impression of comet 3I/ATLAS. Image source: NSF/AUI/NSF NRAO/M.Weiss
A study published in "Nature" on June 22 provides the answer-3I/ATLAS was formed 12 billion years ago. Using NASA's James Webb Space Telescope (JWST), the scientific research team accurately determined the chemical composition of the comet and determined that it was born in a star-forming region of the Milky Way in the early universe. The discovery provides a glimpse into the composition of other planetary systems and how they differ from our own.
Darryl Seligman of Michigan State University in the United States said that finely analyzing the chemical composition of interstellar objects is "something astronomers have dreamed of for many years." If similar observations can be carried out on more similar celestial bodies in the future, "it will completely rewrite our understanding of interstellar comets, and even the entire process of star and planet formation."
Dust and gas around newborn stars gather to form planets, and the remaining debris is thrown out of the parent star system at high speeds. 3I/ATLAS was first discovered by the "Asteroid Impact Final Warning System (ATLAS)". It is not the first interstellar interloper to pique the curiosity of scientists, but it is the largest and brightest to date. The previous two interstellar objects were 1I/Oumuamua discovered in 2017 and 2I/Borisov discovered in 2019. Both were weakly luminous and both were less than 1 kilometer in diameter.
Martin Cordiner, first and corresponding author of the paper and NASA's Goddard Space Flight Center, said that 3I/ATLAS is extremely bright, making it an ideal observation target. In December 2025, the Cordiner team used JWST to observe the comet for two consecutive days, collecting a total of 71 minutes of observation data. Telescopes break down the infrared light emitted by the comet's gaseous coma into thousands of spectra of different wavelengths, thereby deciphering its unique chemical "fingerprint."
"Before the observation, we had no way of predicting what results we would get." Cordiner said, but he soon realized that compared with conventional comets and asteroids in the solar system, 3I/ATLAS "is not slightly different, but has completely different composition characteristics."
After being heated by sunlight, 3I/ATLAS ejects water vapor, carbon monoxide, carbon dioxide, and even metal vapors such as nickel and iron. There are two isotope characteristics that completely reveal its ancient origin. Isotopes are atoms of the same element with the same number of protons and different numbers of neutrons.
First, the comet's carbon-12 to carbon-13 ratio is much higher than that of any celestial body in the solar system. In the universe, violent explosions of massive stars will continue to accumulate carbon 13. The extremely low carbon 13 content of 3I/ATLAS indicates that it was born in the early universe, when a large number of stars had not yet evolved to the stage of supernova explosions.
Second, this comet is rich in semi-heavy water, that is, some of the hydrogen atoms in the water molecules carry one extra neutron. This type of water molecules is more likely to be generated in the strong radiation environment prevalent in the low-temperature and massive star-forming regions of the early universe.
Michele Bannister of the University of Canterbury in New Zealand said that previously, researchers only relied on the orbit and speed of 3I/ATLAS to estimate its age between 3 billion and 11 billion years. Now with independent chemical isotope evidence, the conclusion that this celestial body originated from the ancient universe has been basically confirmed. "It is older than the solar system and is the oldest comet currently observed by humans."
Susanne Pfalzner of the Jülich Supercomputing Center in Germany added: "The identity of this comet as the 'Elder of the Universe' also proves that the basic materials for planets already appeared only 2 billion years after the Big Bang." She said that even the most powerful telescopes cannot directly observe comet-sized objects in ancient star systems. "These interstellar objects from afar are the only physical evidence to prove the true existence of this stage of evolution."
At present, humans have only discovered three interstellar objects, but Seligman believes that this is enough to show that "interstellar objects are extremely numerous in the Milky Way and will continue to break into the solar system." Scientific researchers predict that after the Vera C. Rubin Observatory launches a 10-year all-sky survey, it is expected to discover more than 50 interstellar visitors. NASA's near-Earth object survey mission, which NASA plans to launch as early as 2027, will also significantly improve mankind's ability to detect such interstellar visitors.
The current new observational conclusions of 3I/ATLAS provide astronomers with the most complete reference to date to judge the material composition and formation environment of other planetary systems.
"We always think that the solar system is unique in the Milky Way. It is also the only planetary system known to mankind that has habitable conditions. But with every additional interstellar object observed, we can know more clearly how likely it is that habitable planets will be born and life can be nurtured in other areas of the universe." Cordiner said.
Related paper information: https://doi.org/10.1038/s41586-026-10771-6