How interstellar objects similar to 3I/ATLAS could jump-start planet formation around infant stars

(Press-News.org) Interstellar objects like 3I/ATLAS that have been captured in planet-forming discs around young stars could become the seeds of giant planets, bypassing a hurdle that theoretical models have previously been unable to explain.

Interstellar objects are asteroid- and comet-like bodies that have been ejected from their home system and now wander through interstellar space, occasionally encountering other star systems. Since 2017 astronomers have detected three interstellar objects passing through our Solar System: 1I/’Oumuamua, 2I/Borisov and most recently 3I/ATLAS, discovered in summer 2025.

However, interstellar objects may be more influential than they at first appear to be, says Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany, who presents her new findings on the subject at this week’s EPSC-DPS2025 Joint Meeting in Helsinki.

“Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars,” said Pfalzner.

Planets form in dusty discs around young stars through a process of accretion, which according to theory involves smaller particles come together to form slightly larger objects, and so on until planet-sized bodies have assembled. However, theorists struggle to explain how anything larger than a metre forms through accretion in the hurly-burly of a planet-forming disc around a young star – in computer simulations, boulders either bounce off each other or shatter when they collide rather than sticking together.

Interstellar objects can potentially bypass this problem. Pfalzner’s models show how the dusty planet-forming disc around each young star could gravitationally capture millions of interstellar objects the size of 1I/’Oumuamua, which was estimated to be around 100 metres long. 

“Interstellar space would deliver ready-made seeds for the formation of the next generation of planets,” said Pfalzner.  

If interstellar objects can act as the seeds of planets, it also solves another mystery. Gas giant planets like Jupiter are rare around the smallest, coolest stars, which astronomers refer to as ‘M dwarfs’. They are more commonly found around more massive stars similar to the Sun. The problem, though, is that planet-forming discs around Sun-like stars have a lifetime of about two million years before dissipating and it’s very challenging to form to form gas giant planets on such a short timescale. However, if captured interstellar objects are present as seeds onto which more material can accrete, it speeds the process of planet formation up and giant planets can form in the lifetime of the disc.

“Higher-mass stars are more efficient in capturing interstellar objects in their discs,” said Pfalzner. “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.”

Pfalzner says that her next steps are to model the success rate of these captured interstellar objects – investigating how many of the millions of captured interstellar objects are able to form planetary bodies, and whether they are captured evenly across a planet-forming disc, or whether they are concentrated in certain areas that could become hotspots for planet-birth.

END