Scientists examining the interstellar comet known as 3I/ATLAS say this cosmic traveler is extraordinarily old — likely formed somewhere between 10 and 12 billion years ago in an ancient planetary system far from our own.
The comet, which measures roughly 1.6 miles (2.6 km) across, is believed to be the oldest known object ever to pass through our solar system. That conclusion comes from Martin Cordiner, a planetary scientist and astrochemist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the research. The study was published Monday in the journal Nature.
3I/ATLAS is only the third interstellar object ever observed traveling through our solar system. By studying its chemical composition, researchers were able to piece together clues about the physical and chemical environment where it originally formed.
According to the research team, the comet appears to have originated in a far colder environment — approximately minus-405 degrees Fahrenheit (minus-243 degrees Celsius) — compared to the conditions that gave rise to Earth and other bodies in our solar system about 4.5 billion years ago. The comet has since traveled an enormous distance after being somehow ejected from its original planetary system.
“We have never before seen an object like 3I/ATLAS,” Cordiner said.
Researchers used the James Webb Space Telescope to measure isotope ratios — essentially different versions of chemical elements like hydrogen and carbon — present on the comet. The hydrogen isotopes shed light on the temperature and radiation levels where 3I/ATLAS formed, while the carbon isotopes offered insight into the interstellar gas cloud that gave birth to it and its home planetary system.
One striking finding: the comet’s water contained roughly 30 times more deuterium — a type of hydrogen isotope — than comets found within our own solar system. Its carbon isotope ratios also differed significantly from anything observed in solar system objects or in nearby interstellar clouds and planet-forming disks around young stars.
Cordiner described 3I/ATLAS as most likely a leftover fragment from the process of planet formation around a distant star.
“Our James Webb Space Telescope observations tell us that the planet-forming environment of 3I/ATLAS’s host system was distinct from our own solar system. It was likely colder, and less metal rich, while being more heavily irradiated by UV and cosmic rays,” Cordiner said.
Despite its cold and distant origins, the comet is loaded with organic molecules containing carbon, hydrogen, nitrogen, oxygen, and sulfur. Cordiner said this “shows that despite a cold and distant origin, the volatile elements for life as we know it were abundant in this distant planet-forming disk.”
The carbon composition suggests 3I/ATLAS may have formed as far back as 12 billion years ago, during a period of intense star formation in its region of space. Given that the universe is estimated to have begun with the Big Bang roughly 13.8 billion years ago, 3I/ATLAS would have formed when the cosmos was only about 13% of its current age.
While researchers believe the comet originated within the Milky Way, its age means they cannot entirely rule out that it came from another galaxy. Cordiner noted that “it could take as little as a billion years for a fast interstellar object to get here from our nearest galactic neighbors, the Magellanic Clouds.”
The comet was likely flung out of its home system through gravitational interactions with planets, though some kind of collision has not been ruled out either.
The two previously observed interstellar objects were comets 1I/’Oumuamua, detected in 2017, and 2I/Borisov, discovered in 2019. 3I/ATLAS is now approaching Saturn’s orbit, with scientists expecting it to pass beyond Pluto’s orbit in 2029 and exit the outer boundary of our solar system around 2035.
Despite some earlier speculation that the object could be an alien spacecraft, researchers are confident it is a naturally occurring comet. “While good scientists always remain open to updating their understanding, we take great care to weigh the evidence for each hypothesis,” Cordiner said. “In this case, the evidence was clear from a very early stage that we were looking at a comet-like object, and over time that interpretation has been confirmed by subsequent observations.”
