A Rare Comet Made History as the Third Known Interstellar Object to Fly Through Our Solar System. Studies Are Now Revealing the Mysterious Conditions in Which It Formed
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In the latest research, telescope observations of 3I/ATLAS examine its chemical structure and suggest that it took shape in a peripheral region of a relatively calm planetary system After a visit from beyond the solar system, 3I/ATLAS, the third interstellar object ever detected, is on its way out.
Distinct from anything that circles the sun, the icy body has now passed the orbit of Jupiter. Soon it will slip back into the void, never to be seen by humanity again. The object—a cometlike body enshrouded in gas and dust—was forged in the debris disk surrounding a distant star, providing an extraordinary opportunity for experts to study the building blocks of another planetary system.
Astronomers around the world have trained telescopes on this celestial visitor ever since its discovery on July 1, 2025. Both before and after its closest approach to the sun, when the heat of our star vaporized material from its surface, they have analyzed the comet’s composition. 3I/ATLAS is the third interstellar object discovered in our solar system.
The first, 1I/‘Oumuamua, is a cigar-shaped object discovered in 2017. 2I/Borisov was spotted in 2019, and it was the first interstellar object to be confirmed as a comet. After a year of observation and study, scientists now say that 3I likely originated in the cold outer reaches of its home star’s protoplanetary disk, a ring of debris that later coalesced into objects like planets, moons and asteroids.
The latest of this work, posted Sunday on arxiv.org as a preprint awaiting peer review, used the James Webb Space Telescope to look at the dust of 3I. Based on the structure of silicate compounds in the comet’s coma, the team suggests that 3I formed far away from the inner part of its protoplanetary disk. Along with other chemical clues from hydrogen and carbon isotopes, the new observations add to a growing body of research suggesting that 3I is from the outskirts of an ancient system.
“It’s making a really nice picture, all of these pieces of evidence, that we have an unusual object that formed really far from its host star,” says Karen Meech, an astronomer at the University of Hawaii. “I don’t know that we’ve seen such a creature in our solar system.” Most of the mass in rocky bodies across the galaxy—terrestrial planets, asteroids and comets—consists of silicates, or minerals of silicon and oxygen. These solid compounds can be either crystalline, meaning they have an ordered atomic structure, or amorphous, meaning the atoms don’t line up in any particular way.
Planets, moons and smaller rocky bodies arise from the protoplanetary disk that surrounds their central star. By modeling this formation process, scientists know that crystalline silicates take shape most readily in the inner part of the disk, closer to the star. In our own solar system, these crystalline silicates were then swept to the outer regions by great streams of gas and dust flowing outward from the sun.
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