Are interstellar invaders like comet 3I/ATLAS seeding the universe with exoplanets?

Astronomers have a new theory about planetary formation, and it revolves around one mysterious concept: interstellar visitors. No, not little green men, but rather deep-space wanderers like the peculiar comet 3I/ATLAS discovered this summer.

A new study presented by professor Susanne Pfalzner of Forschungszentrum Jülich at the Joint Meeting of the Europlanet Science Congress and the Division of Planetary Sciences last week suggests such interstellar objects could serve as "seeds" for exoplanet growth around young stars.

Planetary formation is believed to occur through a process called accretion — which involves small particles in dusty, gas-rich disks around young stars colliding and sticking together, gradually growing to the size of planets. But there's a bit of a blip in the story. Collisions between boulder-size objects should tend to cause them to bounce or shatter rather than merge.

Pfalzner's models show that interstellar objects — bodies ejected from other star systems — could be captured by these planet-forming disks. These objects could "seed" the disks, sweeping past the growth barrier by providing substantial mass onto which more material can accrete.

"Interstellar objects may be able to jump-start planet formation, in particular around higher-mass stars," Pfalzner said in a statement, noting that simulations predict millions of interstellar bodies could be captured per disk.

This discovery might also solve another mystery. Jupiter-like giant gas planets are most commonly found around more massive stars rather than smaller ones. But the protoplanetary disks around these massive stars only last around 2 million years before dispersing — and that's not quite enough time to create gas giants. But the arrival of interstellar objects into a massive star's disk might speed up the process.

"Higher-mass stars are more efficient in capturing interstellar objects in their disks," 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."

This summer's discovery of 3I/ATLAS — only the third confirmed interstellar object ever observed passing through our solar system, after 1I/'Oumuamua in 2017 and 2I/Borisov in 201— adds credence to this theory. Its detection suggests such objects may be far more common than previously thought, increasing the plausibility that young stars frequently acquire these alien building blocks.