The Hubble Space Telescope has captured a rare collision in a nearby planetary system. The image was obtained after astronomers directly imaged two separate collisions between rocky objects in the Fomalhaut star system.
The reason this has attracted interest in the astronomical field is because these rare, observable collisions provide unprecedented insights into the processes of planet formation.
NASA’s Hubble Space Telescope (HST) captured the dramatic aftermath of colliding space rocks within a nearby planetary system. Fomalhaut is the brightest star in the southern constellation of Piscis Austrinus, the Southern Fish, and one of the brightest stars in the night sky.
When astronomers initially spotted a bright object in the sky, they assumed it was a dust-covered exoplanet, reflecting starlight. But when the “exoplanet” disappeared and a new bright object appeared, the international team of astrophysicists — led by Northwestern University’s Jason Wang — realised these were not planets at all.
Two distinct, violent collisions generated two luminous clouds of debris in the same planetary system. The discovery offers a unique real-time glimpse into the mechanisms of planet formation and the composition of materials that coalesce to form new worlds.
According to Wang: “Spotting a new light source in the dust belt around a star was surprising. We did not expect that at all. Our primary hypothesis is that we saw two collisions of planetesimals — small rocky objects, like asteroids — over the last two decades. Collisions of planetesimals are extremely rare events, and this marks the first time we have seen one outside our solar system.”
Emphasising the importance of the study, Wang says: “Studying planetesimal collisions is important for understanding how planets form. It also can tell us about the structure of asteroids, which is important information for planetary defence programs like the Double Asteroid Redirection Test (DART).”
Smash-up
For several years, astronomers have puzzled over a bright object called Fomalhaut b, an exoplanet candidate residing just outside the star Fomalhaut. Located 25 light-years from Earth Fomalhaut is more massive than the sun and encircled by an intricate system of dusty debris belts.
Since discovering Fomalhaut b in 2008, astronomers have struggled to determine whether it is, indeed, an actual planet or a large expanding cloud of dust. In 2023, researchers used the HST to further examine the strange light source. Surprisingly, it was no longer there. But another bright point of light emerged in a slightly different location within the same system.
Colliding planetesimals
The disappearance of Fomalhaut b supports the hypothesis that it was a dissipating dust cloud, likely produced by a collision. The appearance of a second point of light (called Fomalhaut cs2) further supports the theory that neither are planets, but the dusty remnants of dramatic smash-ups between planetesimals — the rocky building blocks of planets.
The location and brightness of Fomalhaut cs2 bear striking similarities to the initial observations of Fomalhaut cs1 two decades prior. By imaging the system, the team was able to calculate how frequent such planetesimal collisions occur.
Similar events could become easier to detect in the future. As next-generation telescopes, including the Giant Magellan Telescope, aim to directly image habitable-zone planets around nearby stars, understanding and distinguishing these transient collision clouds from genuine exoplanets will be important.
The findings have been published in the journal Science, with the paper titled “A second planetesimal collision in the Fomalhaut system”.
