Interstellar Objects Like Comet 3I/ATLAS May Act As Planetary Seeds

Comet 3I/ATLAS’s look within the inside Solar System in July 2025 triggered a wave of curiosity. Not solely within the comet itself, however in interstellar objects (ISO) basically. So far we solely know of three ISOs, and it is solely pure to marvel about their origins, and the way widespread they’re. But scientists, being naturally curious, produce other questions, too. What would occur if an ISO was captured by a younger photo voltaic system?

At the latest Joint Meeting of the Europlanet Science Congress and the American Astronomical Society’s Division for Planetary Science (EPSC-DPS2025), a researcher requested what function comets like these might play once they’re captured by different stars with protoplanetary disks. It’s doable that ISOs like 3I/ATLAS may reply a longstanding query about planet formation.

Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany introduced analysis exhibiting that comets like 3I/ATLAS may act as seeds for the formation of big planet.

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

There are two broad understandings of planet formation: the core accretion mannequin and the gravitational instability mode.

The core accretion principle is a bottom-up mannequin. It hypothesizes that planet formation begins on a really small scale with mud particles sticking collectively in a protoplanetary disk. Eventually there are pebbles, then rocks, then boulders, then planetesimals. If all goes nicely, the method varieties planets like Mercury, Venus, Earth, and Mars.

The gravitational instability mannequin is a top-down mannequin that is just like how we expect stars kind. It posits that areas within the disk grow to be dense with matter and finally collapse to kind a planetary core. From there, gravity dominates and the core accretes increasingly matter till a planet is shaped.

The core accretion principle is extra relevant to rocky planets, whereas the gravitational instability mannequin is extra relevant to massive planets like Jupiter. Recent analysis means that these mechanisms do not function in isolation from each other, however can work together to create planets.

Each of those theories, nevertheless, has unanswered questions. The core accretion principle, in accordance with simulations, cannot create something bigger than about one meter. Boulders bounce off one another or are shattered throughout collisions. Pfalzner says that ISOs can clarify 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 mentioned in a press release.

An artist’s illustration of Oumuamua, the primary ISO found. It got here by way of our Solar System in 2017. Image Credit: NASA

We solely know of three ISOs as a result of we have solely been capable of detect them for a brief time period. The first one, Oumuamua, was found in 2017, and within the eight years since then, we have discovered two extra. Looking again over the Solar System’s roughly 5 billion yr age, it is simple to see how massive numbers of ISOs have seemingly travelled by way of our Solar System.

But not all of them essentially got here and went. When photo voltaic programs are younger, they’re dense with mud. In these environments, ISOs usually tend to be captured. Pfalzner’s analysis reveals {that a} photo voltaic system may probably seize tens of millions of ISOs about 100 meters in diameter. Those captured objects could possibly be the seeds for the formation of planets.

The Hubble Space Telescope captured this picture of interstellar comet 2I/Borisov in 2019. It was solely the second ISO ever detected. Image Credit: By NASA, ESA, and D. Jewitt (UCLA) – Public Domain

Pfalzner’s findings additionally handle one other particular problem in exoplanet science. Jupiter-mass fuel giants are uncommon round low-mass stars. They’re way more widespread round stars like ours. But the issue is that planet-forming disks round stars just like the Sun aren’t long-lived. After about two million years, the star’s wind and radiation dissipate the disk. Observations present that stars older than about 10 million years don’t have any protoplanetary disks. So which means there’s solely a few million years for a large planet to kind earlier than the disk is gone. That’s not a lot time.

But if Pfalzner is correct, then ISOs can act because the seeds for large planets, giving them a kickstart that permits them to kind earlier than the protoplanetary disk is gone.

“Higher-mass stars are more efficient in capturing interstellar objects in their discs,” mentioned 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 round a younger star. Observations present that these disks might not final lengthy sufficient for large planets to kind. Image Credit: ESO/L. Calçada

ISOs appearing as planetary seeds is not the one potential resolution the planet formation time scale drawback. The pebble accretion model has gained traction lately as a result of it could clarify how big planets may kind extra shortly than thought. It posits that fuel drag within the disk slows down pebbles in order that once they collide they have an inclination to stay collectively. It may cut back the time it takes for fuel big cores to kind to as little as a million years.

It’s additionally doable that inside areas of a protoplanetary disk persist for longer than thought, giving big planets extra time to kind. Astronomers know that the large planets in our Solar System additionally migrated, including one other factor to the massive image. It’s doable that no ISOs are wanted.

Nature would not all the time select A or B. There could also be a number of pathways to massive planets, and ISOs could possibly be considered one of them. It’s totally doable that Jupiter, Saturn, or one of many different big planets solely exist due to an historical ISO from a distant star system that was captured by the younger Sun.