Interstellar comet 3I/ATLAS ejected the building blocks of life as it passed Earth last year, according to new data from NASA’s SPHEREx space telescope.
SPHEREx observed the rare interstellar visitor from its orbital position as it circled the sun and made its closest approach to Earth in December, before the comet began its long journey back from the solar system.
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SPHEREx researchers published their findings in the February issue of AAS (American Astronomical Society) Research Notes, highlighting ongoing work that has not yet been peer-reviewed.
Comet 3I/ATLAS will become a celestial celebrity in 2025, making it only the third interstellar visitor ever detected and potentially the oldest comet ever observed. The origin of this comet has been the subject of considerable speculation, with the possibility of it being an alien spacecraft being frequently suggested. But almost all astronomers are convinced that 3I/ATLAS is a natural comet from another star system, and the new discovery won’t change that.
As the comet approaches the star, it heats up, causing the ice on its surface to sublimate and turn into gas. These newly released gases eject from the comet’s surface as a jet, enveloping the body of the comet in a cloud called a coma, and being swept behind the comet to form a long tail. Researchers can detect and study these gases, thereby determining the composition of comets.
“After approaching the Sun, comet 3I/ATLAS began to eject into space in earnest in December 2025, becoming noticeably brighter,” study lead author Carrie Risse, an astronomer at the Johns Hopkins Applied Physics Laboratory in Maryland, said in a NASA statement. “Even water ice quickly sublimated into gas in interplanetary space, and since comets are made up of about one-third of their volume as water ice, they were ejecting large amounts of new carbon-rich material that remained trapped in subsurface ice.”
“We are now observing the usual range of early solar system material that is typically ejected by comets, such as organic molecules, soot, and rock dust,” Risse added.
These organic molecules may be typical for comets, but they may still be noteworthy in their own ways. One popular theory is that comets could spread seeds of life to celestial bodies throughout the universe (possibly including Earth).
Astronomers first detected comet 3I/ATLAS in July 2025, when they discovered an unknown object orbiting at about 137,000 miles per hour (221,000 km per hour) in the orbit of Jupiter. Researchers believe the star has been traveling through space for billions of years, gaining momentum through the slingshot effect of gravity as it leaps over other stars and nebulae. The comet passed the Sun and reached its closest point to our star (perihelion) in late October, before coming within about 168 million miles (270 million kilometers) of our planet in December.
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SPHEREx (short for Space History, Reionization Era, and Ice Explorer Spectrophotometer) was launched into low Earth orbit in March 2025, and happened to acquire decent observations of the comet from December 8th to 15th.
The space telescope is designed to scan the entire night sky using 102 infrared color sensors, helping researchers better understand the gaseous universe around comets and searching for the components of life. The timing of the flyby of Comet 3I/ATLAS was fortunate for the researchers, as obtaining a view of an interstellar visitor so soon after launch was not in SPHEREx’s itinerary.
“Our unique space telescope is collecting unprecedented data from across the universe,” study co-lead author Yunsoo Bach, a researcher at the Korea Institute of Astronomy and Space Science, said in a statement. “But in this case, our galaxy delivered us a piece of a distant star system just months after launch, and SPHEREx was ready to observe it. Science is like that sometimes: being in the right place at the right time.”
3I/ATLAS is now hurtling toward Jupiter, giving NASA’s Juno spacecraft the last chance to study the planet up close before the comet leaves the solar system for good.
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