Second Planetesimal Collision in Fomalhaut System Challenges Planetary Formation Models

The Fomalhaut stellar system, situated approximately 25 light-years away, has afforded astrophysicists an unprecedented, real-time view into planetary genesis. This opportunity arose from documenting a second catastrophic collision involving massive rocky bodies, known as planetesimals, occurring within a mere two decades. The findings, spearheaded by Paul Kalas of the University of California, Berkeley, were formally presented in the journal Science on December 18, 2025. The evidence gathered strongly suggests that the process of planet formation might be a far more tumultuous and frequent event than current theoretical frameworks account for.

The sequence of dramatic events began with an object previously designated Fomalhaut b, which was first observed in the mid-2000s. This object later transformed into an expanding cloud of debris, labeled Circumstellar Source 1 (CS1), after its disappearance was confirmed by 2014. Fast forward to 2023, and the Hubble Space Telescope captured a new, brilliant point of light, Circumstellar Source 2 (CS2). Researchers now identify CS2 as the aftermath of a separate, second planetesimal impact event. Scientists estimate that the bodies involved in these impacts were roughly 30 kilometers in diameter—a size exceeding that of Mars’s moon, Phobos.

The sheer frequency of these collisions has taken the scientific community by surprise. Theoretical calculations had previously indicated that such violent encounters should occur no more often than once every 100,000 years, or perhaps even less frequently, as noted by astrobiologist Jason Van. To witness two such events in such a short observational window throws a wrench into established timelines for planetary accretion.

Fomalhaut itself is a crucial natural laboratory for studying the dynamics of rocky world formation. It is an A-type star, boasting twice the mass of our Sun and shining 20 times brighter. Its relative youth—estimated at only 440 million years, compared to the Sun’s 4.6 billion years—makes it an excellent proxy for understanding the early history of our own Solar System. Earlier observations had already hinted that the planetesimals in this system are rich in volatile materials, suggesting a composition akin to icy comets.

The investigation into the Fomalhaut system is far from over. Subsequent observation slots have already been approved, utilizing the James Webb Space Telescope’s (JWST) Near-Infrared Camera (NIRCam). These planned observations aim to precisely characterize the size and chemical makeup of the dust grains making up CS2, with a specific focus on detecting any signatures of water or ice. Furthermore, the research team is keen to determine if any unseen, larger exoplanets might be exerting gravitational influence over these collision events.

Astronomer Mark Wyatt from the University of Cambridge emphasized the value of these direct observations. He pointed out that this unique vantage point allows researchers to estimate the size and total quantity of colliding bodies within the disk—data that is practically unattainable through other means. This discovery serves as a significant cautionary note for upcoming exoplanet detection missions relying on reflected light, as a dense cloud of dust can easily mimic the appearance of a planet for many years, leading to potential misidentification.