Interstellar Comet 3I/ATLAS Came From an Extremely Cold, Lonely Region of the Milky Way Galaxy, New Research Suggests
Astronomers analyzed the vapors coming off the comet when it neared the sun and found it had a lot of “heavy” water. That hints it grew up in a frigid planetary system, possibly before its home star formed
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Our time with our interstellar visitor, comet 3I/ATLAS, is almost up. It’s set to leave the solar system soon, after zipping by Jupiter last month.
As we say adieu to the mysterious guest, astronomers are starting to puzzle together where this icy space rock came from. In a study published April 23 in the journal Nature Astronomy, a team reports it may have grown up in an extremely cold planetary system, possibly before its star fully formed, that’s quite different from the one we call home.
3I/ATLAS was first spotted on July 1 by a NASA-funded telescope in Chile, part of the Asteroid Terrestrial-impact Last Alert System (ATLAS). At the time, it was zooming at about 137,000 miles per hour. Its trajectory hinted it wasn’t following a closed orbit around the sun, and tracing its path backward suggested it came from the center of the Milky Way galaxy.
The comet is just our third known visitor from another planetary system, after object 1I/ʻOumuamua and comet 2I/Borisov. Following months of observations by several spacecraft and ground-based instruments, researchers have discovered that 3I/ATLAS’s icy core, or nucleus, is probably between 1,400 feet and 3.5 miles wide, and that its surrounding cloud of gas and dust has strange chemistry that hints at its origins.
Did you know? You can track 3I/ATLAS’s journey
NASA’s “Eyes on the Solar System” app lets you follow along on our guest’s travels through the solar system.
To dig deeper into the comet’s backstory, scientists behind the new study examined the water vapor released when the icy rock made its closest approach to the sun last fall. Data from the Atacama Large Millimeter/submillimeter Array telescope in Chile revealed that our guest contains a surprising amount of “heavy” water.
All water contains one oxygen atom linked to two hydrogen atoms. On Earth, most hydrogen atoms in water don’t have a neutron, a neutral particle, in their core. But 3I/ATLAS has a high ratio of water with neutron-bearing hydrogen atoms—called “deuterated” water—to that without the neutral particles, the team found. The interstellar comet’s deuterated-to-standard water ratio was about 40 times that in the Earth’s ocean, and roughly 30 times that of a typical comet that formed in the solar system.
The unusual characteristic suggests 3I/ATLAS originated somewhere much colder than our neck of the galaxy, from before that planetary system’s star had even taken shape, study co-author Teresa Paneque-Carreño, an astronomer at the University of Michigan, tells the Associated Press’ Marcia Dunn. Unlike our sun, which was probably surrounded by several newborn stars during its early years, 3I/ATLAS’s stellar host may have formed in a lonelier—and therefore chillier—environment, she adds.
On a broader level, “this is proof that whatever the conditions were that led to the creation of our solar system are not ubiquitous throughout space,” Paneque-Carreño says in a statement. “That may sound obvious, but it’s one of those things that you need to prove.”
Darryl Seligman, an astronomer at Michigan State University who wasn’t involved in the study, praises the team’s work. “That is a very, very difficult measurement to make,” he tells Scientific American’s Lee Billings and Joseph Howlett. “It’s almost unprecedented for solar system comets, and now they’ve done it for an interstellar comet.”
Researchers still haven’t pinpointed exactly where 3I/ATLAS came from. But the findings have revealed new cosmic mysteries—and highlight what we don’t know about the universe.
“Either the solar system is weird and unique or planet formation in other stars is not quite understood,” Seligman tells Scientific American. “Those are really two different ways of saying the same thing.”
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