Interstellar objects such as 3I/ATLAS, when captured in the planet-forming discs of young stars, may serve as the building blocks of giant planets, offering a solution to a long-standing problem in planet formation theory.
These wandering asteroid- and comet-like bodies are ejected from their home systems and drift through interstellar space, occasionally crossing into other star systems. by Robert Schreiber
Berlin, Germany (SPX) Sep 25, 2025
Interstellar objects such as 3I/ATLAS, when captured in the planet-forming discs of young stars, may serve as the building blocks of giant planets, offering a solution to a long-standing problem in planet formation theory.
These wandering asteroid- and comet-like bodies are ejected from their home systems and drift through interstellar space, occasionally crossing into other star systems. Since 2017, three such objects have been observed entering our Solar System: 1I/'Oumuamua, 2I/Borisov, and most recently 3I/ATLAS, discovered in mid-2025.
Professor Susanne Pfalzner of Forschungszentrum Julich in Germany presented new findings at the EPSC-DPS2025 Joint Meeting in Helsinki, highlighting their potential importance. "Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars," she said.
Conventional models propose that planets form by the gradual accretion of dust and rock in circumstellar discs. Yet, simulations struggle to show how particles grow beyond metre-sized boulders, which typically rebound or fragment on collision rather than sticking together.
Interstellar visitors could bypass this bottleneck. Pfalzner's models suggest that planet-forming discs might gravitationally capture millions of such objects, each comparable in size to 'Oumuamua, estimated at about 100 metres in length. These ready-made seeds could then gather additional material, accelerating planetary growth.
This mechanism also sheds light on another puzzle: why gas giants like Jupiter are common around Sun-like stars but rare around small, cool M dwarfs. Planet-forming discs last only about two million years, a timeframe too short for gas giants to form via traditional accretion alone. With captured interstellar seeds, however, the process speeds up enough to allow giant planets to emerge before the disc dissipates.
"Higher-mass stars are more efficient in capturing interstellar objects in their discs," Pfalzner explained. "Therefore, interstellar object-seeded planet formation should be more efficient around these stars, providing a fast way to form giant planets. And, their fast formation is exactly what we have observed."
Next, Pfalzner plans to model the likelihood of these captured objects developing into planetary bodies, and whether their distribution within discs is even or concentrated into hotspots that could favor planet formation.
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