Opening 1I/Oumuamua, 2I/Borisov and 3I/ATLAS showed that a significant number of interstellar objects inhabit interstellar space. Their ubiquity means that such objects are also found in protoplanetary disks, which are the reservoir for planet formation. There, interstellar objects could give rise to the formation of giant exoplanets, breaking the 1 m (3.3 ft) barrier present in the standard model of planet formation.
Interstellar comet 3I/ATLAS is visible in this color image taken by the CaSSIS instrument on board ESA's Trace Gas Orbiter on October 3, 2025. Image credit: ESA/TGO/CaSSIS.
Interstellar objects are bodies similar to asteroids and comets that have been ejected from their home system and now wander through interstellar space, occasionally colliding with other star systems.
Since 2017, astronomers have discovered three interstellar objects passing through our solar system: 1I/'Oumuamua, 2I/Borisov, and most recently 3I/ATLAS.
“However, interstellar objects may turn out to be more influential than they seem at first glance,” said Professor Susanne Pfalzner, an astronomer at Forschungszentrum Jülich.
“Interstellar objects can give rise to the formation of planets, especially around higher-mass stars.”
Planets form in dusty disks around young stars through a process of accretion, which, according to the theory, involves smaller particles coming together to form slightly larger objects, and so on, until planet-sized bodies are assembled.
However, theorists are struggling to explain how anything larger than one meter is formed by the accretion of a tumultuous planet-forming disk around a young star – in computer simulations, the boulders either bounce off each other or break apart on impact rather than sticking together.
Interstellar objects could potentially circumvent this problem. The team's models show how the planet-forming dust disk around each young star could gravitationally capture millions of interstellar objects the size of 1I/'Oumuamua, which is estimated to be about 100 m (328 ft) long.
“Interstellar space will deliver ready-made seeds for the formation of the next generation of planets,” said Professor Pfalzner.
If interstellar objects can act as planet seeds, this also solves another mystery.
Gas giant planets such as Jupiter are rare around the smallest and coolest stars, which astronomers call M dwarfs. They are more common around more massive stars like the Sun.
The problem, however, is that the disks that form planets around stars like the Sun have a lifetime of about two million years before dissipating, and it is very difficult to form gas giant planets in such a short period of time.
However, if captured interstellar objects are present as seeds on which more material can accumulate, this speeds up the planet formation process, and giant planets can form within the lifetime of the disk.
“Stars with higher mass are more efficient at capturing interstellar objects in their disks,” Professor Pfalzner said.
“The formation of seeded interstellar planets should therefore be more efficient around these stars, providing a faster way to form giant planets.”
“And their rapid formation is exactly what we observed.”
Professor Pfalzner introduced her results in September 2025 at EPSC-DPS2025 Joint Meeting In Helsinki, Finland.
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S. Pfalzner. 2025. Interstellar objects serve as seeds for planet formation, predominantly around high-mass stars. EPSC Abstracts 18:EPSC-DPS2025-1927; doi: 10.5194/epsc-dps2025-1927
