Comet 3I/ATLAS’s appearance in the inner Solar System in July 2025 triggered a wave of interest. Not only in the comet itself, but in interstellar objects (ISO) in general. So far we only know of three ISOs, and it’s only natural to wonder about their origins, and how common they are. But scientists, being naturally curious, have other questions, too. What would happen if an ISO was captured by a young solar system?
At the recent Joint Meeting of the Europlanet Science Congress and the American Astronomical Society’s Division for Planetary Science (EPSC-DPS2025), a researcher asked what role comets like these may play when they’re captured by other stars with protoplanetary disks. It’s possible that ISOs like 3I/ATLAS could answer a longstanding question about planet formation.
Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany presented research showing that comets like 3I/ATLAS could act as seeds for the formation of giant planet.
“Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars,” said Pfalzner.
There are two broad understandings of planet formation: the core accretion model and the gravitational instability mode.
The core accretion theory is a bottom-up model. It hypothesizes that planet formation begins on a very small scale with dust particles sticking together in a protoplanetary disk. Eventually there are pebbles, then rocks, then boulders, then planetesimals. If all goes well, the process forms planets like Mercury, Venus, Earth, and Mars.
The gravitational instability model is a top-down model that’s similar to how we think stars form. It posits that regions in the disk become dense with matter and eventually collapse to form a planetary core. From there, gravity dominates and the core accretes more and more matter until a planet is formed.
The core accretion theory is more applicable to rocky planets, while the gravitational instability model is more applicable to giant planets like Jupiter. Recent research suggests that these mechanisms don’t operate in isolation from one another, but can work in combination to create planets.
Each of these theories, however, has unanswered questions. The core accretion theory, according to simulations, can’t create anything larger than about one meter. Boulders bounce off each other or are shattered during collisions. Pfalzner says that ISOs can explain how objects leap over the one meter barrier.
“Interstellar objects may be able to jump start planet formation, in particular around higher-mass stars,” Pfalzner said in a press release.
An artist’s illustration of Oumuamua, the first ISO discovered. It came through our Solar System in 2017. Image Credit: NASA
We only know of three ISOs because we’ve only been able to detect them for a short period of time. The first one, Oumuamua, was discovered in 2017, and in the eight years since then, we’ve found two more. Looking back over the Solar System’s roughly five billion year age, it’s easy to see how large numbers of ISOs have likely travelled through our Solar System.
But not all of them necessarily came and went. When solar systems are young, they’re dense with dust. In these environments, ISOs are more likely to be captured. Pfalzner’s research shows that a solar system could potentially capture millions of ISOs about 100 meters in diameter. Those captured objects could be the seeds for the formation of planets.
The Hubble Space Telescope captured this image of interstellar comet 2I/Borisov in 2019. It was only the second ISO ever detected. Image Credit: By NASA, ESA, and D. Jewitt (UCLA) – https://imgsrc.hubblesite.org/hvi/uploads/image_file/image_attachment/31897/STSCI-H-p1953a-f-1106×1106.png, Public Domain
Pfalzner’s findings also address another specific issue in exoplanet science. Jupiter-mass gas giants are rare around low-mass stars. They’re far more common around stars like ours. But the problem is that planet-forming disks around stars like the Sun are not long-lived. After about two million years, the star’s wind and radiation dissipate the disk. Observations show that stars older than about 10 million years have no protoplanetary disks. So that means there’s only a couple of million years for a giant planet to form before the disk is gone. That’s not much time.
But if Pfalzner is right, then ISOs can act as the seeds for giant planets, giving them a kickstart that allows them to form before the protoplanetary disk is gone.
“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.”
An artist’s illustration of a planet-forming protoplanetary disk around a young star. Observations show that these disks may not last long enough for giant planets to form. Image Credit: ESO/L. Calçada
ISOs acting as planetary seeds isn’t the only potential solution the planet formation time scale problem. The pebble accretion model has gained traction in recent years because it can explain how giant planets could form more quickly than thought. It posits that gas drag in the disk slows down pebbles so that when they collide they tend to stick together. It could reduce the time it takes for gas giant cores to form to as little as one million years.
It’s also possible that inner regions of a protoplanetary disk persist for longer than thought, giving giant planets more time to form. Astronomers know that the giant planets in our Solar System also migrated, adding another element to the big picture. It’s possible that no ISOs are needed.
Nature doesn’t always choose A or B. There may be multiple pathways to giant planets, and ISOs could be one of them. It’s entirely possible that Jupiter, Saturn, or one of the other giant planets only exist because of an ancient ISO from a distant star system that was captured by the young Sun.