Objects from deep-space like the interstellar comet 3I/ATLAS could 'jump-start' planet formation around massive stars, according to a study.
If true, that means these interstellar interlopers could solve a problem that current theories about planet formation have been unable to explain.
More on 3I/ATLAS
Interstellar objects could even help explain how it's possible for Jupiter-sized planets to form around stars like our Sun.
Interstellar objects explained
Comet 3I/ATLAS is currently passing through our Solar System, having originated from elsewhere in the Galaxy.
Unlike comets that originated within our Solar System, and which orbit the Sun, 3I/ATLAS will soon exit and be gone forever.
Scientists have only ever observed three such interstellar objects: 1I/ʻOumuamua, 2I/Borisov and 3I/ATLAS.
That means these deep-space visitors – when we do find them – provide a rare but enticing chance to learn more about primordial space rocks that formed around distant stars.
They can tell scientists a lot about what the conditions are like around those distant stars, including what planet-forming ingredients are present across the Galaxy.
Solving a planet formation mystery
One study suggests objects like 3I/ATLAS, if captured by the gravitational pull of young stars, could become the seeds of giant planets.
Young stars are surrounded by rotating disks of gas and dust; the remaining ingredients from which the star formed.
Over time, these particles of dust stick together, then pull in more particles via their gravitational pull, and grow and grow to eventually become fully-fledged planets in orbit around the star.
This is how Earth and the other planets of our Solar System formed around our host star, the Sun.
But astronomers say there's a sticking point in our understanding of planet formation, and that's how any object larger than a metre can form through this 'accretion' process.
The dusty disks around young stars are chaotic places. In computer simulations that show what conditions would be like in these disks, large boulders collide with each other and bounce away, or shatter, rather than sticking together.
If that's the case, how is it possible that planets could form from tiny seeds under such destructive conditions?
One solution to this problem could be interstellar objects like 3I/ATLAS.
Interstellar comets to the rescue?
Interstellar objects may be more influential than they seem, says Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany
"Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars," she says.
A computer model generated by Pfalzner shows dusty planet-forming disks around young stars could capture millions of interstellar objects the size of 1I/’Oumuamua.
1I/’Oumuamua was the first interstellar object ever discovered in our Solar System, in 2017, and was estimated to be around 100 metres long.
Perhaps, rather than planets having to grow and grow from tiny seeds, some planets could form from interstellar objects captured by a young star's gravity.
"Interstellar space would deliver ready-made seeds for the formation of the next generation of planets," says Pfalzner.
Solving the Jupiter mystery
If interstellar objects can act as the seeds of planets, it could solve another mystery.
Astronomer say gas giant planets, like Jupiter in our Solar System, are rare around smaller, cooler stars across the Galaxy. They're more often found around massive stars like our Sun.
However, planet-forming discs around Sun-like stars only last for about 2 million years, and planet formation theory can't quite explain how large gas giant planets could form in such a short time.
Interstellar objects could be the answer.
If captured by a young star, these ready-made planet 'seeds' could jump-start the gas giant formation process, thereby explaining how such large worlds can form around distant stars.
"Higher-mass stars are more efficient in capturing interstellar objects in their discs," says 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's next steps will be to create computer models that look into this further, studying how many of the millions of captured interstellar objects could lead to the formation of planets.
Pfalzner, S.: Interstellar objects function as seeds for planet formation predominantly around high-mass stars, was presented at the EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1927, https://doi.org/10.5194/epsc-dps2025-1927, 2025.
