Astronomers have observed the aftermath of two major collisions in the debris disk surrounding Fomalhaut, a star located about 25 light-years from Earth. These observations mark the first time that collisions between large objects have been directly imaged in a solar system outside our own.
Paul Kalas, adjunct professor of astronomy at the University of California, Berkeley, and lead author of a new report on these findings, said, “We just witnessed the collision of two planetesimals and the dust cloud that gets spewed out of that violent event, which begins reflecting light from the host star. We do not directly see the two objects that crashed into each other, but we can spot the aftermath of this enormous impact.” He added that over thousands of years, such dust clouds would make Fomalhaut’s surroundings appear to “sparkle” with ongoing collisions.
The first collision was detected in 2004 and the second in 2023. Both events produced bright clouds of dust visible through NASA’s Hubble Space Telescope (HST). The size of the colliding bodies is estimated to be at least 60 kilometers across—significantly larger than the object believed to have caused the extinction of dinosaurs on Earth.
Kalas began observing Fomalhaut in 1993 to study debris left after planet formation. The star is considered young at about 440 million years old and offers insights into early planetary system development. In earlier observations, Kalas reported what appeared to be a planet—named Fomalhaut b—but now believes it was likely a dust cloud resulting from one such collision. “This is a new phenomenon, a point source that appears in a planetary system and then over 10 years or more slowly disappears,” he explained. “It’s masquerading as a planet because planets also look like tiny dots orbiting nearby stars.”
Mark Wyatt, professor of astronomy at the University of Cambridge and co-author on the paper published online December 18 in Science, said: “The Fomalhaut system is a natural laboratory to probe how planetesimals behave when undergoing collisions, which in turn tells us about what they are made of and how they formed. The exciting aspect of this observation is that it allows us to estimate both the size of the colliding bodies and how many of them there are in the disk, information which it is almost impossible to get by any other means.”
Wyatt estimates there are around 300 million similar-sized objects within Fomalhaut’s disk. Observations have also detected carbon monoxide gas near Fomalhaut, suggesting these objects are rich in volatiles and resemble icy comets found in our own solar system.
Fomalhaut itself lies within Piscis Austrinus constellation and shines with sixteen times more luminosity than our sun. Its debris belt sits over three times farther from its star than our Kuiper Belt does from ours.
The research team analyzed images taken by HST between 2004 and 2024 and concluded that both bright spots observed—now referred to as circumstellar sources cs1 (2004) and cs2 (2023)—are likely dust clouds generated by recent collisions between large planetesimals rather than exoplanets.
Kalas compared these events to NASA’s DART mission impact on Dimorphos but noted that Fomalhaut’s dust clouds are much larger. Ongoing observations using both HST and James Webb Space Telescope aim to monitor changes in these clouds’ brightness and structure.
Kalas advised caution for future missions attempting direct imaging of exoplanets: “These collisions that produce dust clouds happen in every planetary system. Once we start probing stars with sensitive future telescopes such as the Habitable Worlds Observatory…we have to be cautious because these faint points of light orbiting a star may not be planets.”
Other contributors include astronomers from UC Berkeley, Northwestern University, UCLA, European Southern Observatory, UC Santa Barbara, Xiamen University (China), University of Cambridge (UK), and University of Warwick (UK). The project received support from NASA.
For additional background on this research:
– A summary describing these findings can be found via Science magazine: https://www.science.org/doi/10.1126/science.adl1185
– NASA provides further context regarding asteroid collisions near Fomalhaut: https://www.nasa.gov/image-feature/goddard/2023/hubble-sees-asteroids-colliding-at-nearby-star-for-first-time/
– More details about Paul Kalas’s work are available on his website: https://w.astro.berkeley.edu/~kalas/
