Comet 3I/ATLAS’s look within the internal Photo voltaic System in July 2025 triggered a wave of curiosity. Not solely within the comet itself, however in interstellar objects (ISO) basically. Thus far we solely know of three ISOs, and it is solely pure to surprise about their origins, and the way widespread they’re. However scientists, being naturally curious, produce other questions, too. What would occur if an ISO was captured by a younger photo voltaic system?
On the latest Joint Assembly 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 could play after they’re captured by different stars with protoplanetary disks. It is doable that ISOs like 3I/ATLAS might reply a longstanding query about planet formation.
Professor Susanne Pfalzner of Forschungszentrum Jülich in Germany introduced analysis exhibiting that comets like 3I/ATLAS might act as seeds for the formation of large planet.
“Interstellar objects might be able to soar begin planet formation, specifically round higher-mass stars,” stated 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. Ultimately there are pebbles, then rocks, then boulders, then planetesimals. If all goes effectively, the method types planets like Mercury, Venus, Earth, and Mars.
The gravitational instability mannequin is a top-down mannequin that is much like how we expect stars type. It posits that areas within the disk turn into dense with matter and finally collapse to type a planetary core. From there, gravity dominates and the core accretes increasingly more 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. Current analysis means that these mechanisms do not function in isolation from each other, however can work together to create planets.
Every of those theories, nonetheless, has unanswered questions. The core accretion principle, in keeping 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 might be able to soar begin planet formation, specifically round higher-mass stars,” Pfalzner stated in a press release.
An artist’s illustration of Oumuamua, the primary ISO found. It got here by our Photo voltaic System in 2017. Picture Credit score: NASA
We solely know of three ISOs as a result of we have solely been in a position to detect them for a brief time period. The primary one, Oumuamua, was found in 2017, and within the eight years since then, we have discovered two extra. Trying again over the Photo voltaic System’s roughly 5 billion yr age, it is simple to see how giant numbers of ISOs have seemingly travelled by our Photo voltaic System.
However not all of them essentially got here and went. When photo voltaic techniques are younger, they’re dense with mud. In these environments, ISOs usually tend to be captured. Pfalzner’s analysis exhibits {that a} photo voltaic system might probably seize tens of millions of ISOs about 100 meters in diameter. These captured objects may very well be the seeds for the formation of planets.
The Hubble House Telescope captured this picture of interstellar comet 2I/Borisov in 2019. It was solely the second ISO ever detected. Picture Credit score: By NASA, ESA, and D. Jewitt (UCLA) – Public Area
Pfalzner’s findings additionally handle one other particular concern in exoplanet science. Jupiter-mass fuel giants are uncommon round low-mass stars. They’re way more widespread round stars like ours. However the issue is that planet-forming disks round stars just like the Solar will not be 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 haven’t any protoplanetary disks. So meaning there’s solely a few million years for an enormous planet to type earlier than the disk is gone. That is not a lot time.
But when Pfalzner is correct, then ISOs can act because the seeds for large planets, giving them a kickstart that enables them to type earlier than the protoplanetary disk is gone.
“Increased-mass stars are extra environment friendly in capturing interstellar objects of their discs,” stated Pfalzner. “Due to this fact, interstellar object-seeded planet formation needs to be extra environment friendly round these stars, offering a quick approach to type large planets. And, their quick formation is precisely what we’ve noticed.”
An artist’s illustration of a planet-forming protoplanetary disk round a younger star. Observations present that these disks could not final lengthy sufficient for large planets to type. Picture Credit score: ESO/L. Calçada
ISOs appearing as planetary seeds is not the one potential answer the planet formation time scale drawback. The pebble accretion model has gained traction in recent times as a result of it may well clarify how large planets might type extra rapidly than thought. It posits that fuel drag within the disk slows down pebbles in order that after they collide they have a tendency to stay collectively. It might scale back the time it takes for fuel large cores to type to as little as a million years.
It is also doable that internal areas of a protoplanetary disk persist for longer than thought, giving large planets extra time to type. Astronomers know that the large planets in our Photo voltaic System additionally migrated, including one other aspect to the massive image. It is 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 may very well be one among them. It is fully doable that Jupiter, Saturn, or one of many different large planets solely exist due to an historic ISO from a distant star system that was captured by the younger Solar.
