Astronomers have observed the aftermath of two massive collisions in a young star system—events so rare they were expected to occur only once every 100,000 years.
The collisions were detected around Fomalhaut, a nearby star long studied for its unique debris disk, and occurred within just two decades of each other. The unexpected frequency is forcing astronomers to reconsider how often violent impacts may shape planetary systems during their formative stages, raising new questions about how planets and moons assemble from cosmic debris.
“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,” said first author Paul Kalas, adjunct professor of astronomy at the University of California, Berkeley. “We do not directly see the two objects that crashed into each other, but we can spot the aftermath of this enormous impact.”
Those collisions would cause the dust surrounding Fomalhaut to twinkle like Christmas lights over tens of thousands of years. From its location in the southern constellation Piscis Austrinus, Fomalhaut is one of the brightest stars in our night sky, due to its proximity and luminosity 16 times that of the Sun.
Kalas’s interest in Fomalhaut goes back to 1993, when he first began looking for a dusty disk around the young star, 25 light-years from Earth. At only 440 million years old, compared to the Sun’s 4.6 billion years, Fomalhaut may provide clues to our own solar system’s evolution.
Finding the Bright Spot
Kalas’s early interest did yield some results. In 2008, he reported identifying a bright spot near the star’s disk in imagery collected by NASA’s Hubble Space Telescope, which he named Fomalhaut b. Part of his rationale for believing the bright spot was a planet was that the disk had a sharp inner edge, which he interpreted as evidence of carving by planets, although he allowed that it might be a dust cloud.
By 2014, Fomalhaut b was no longer visible, but Kalas noted another spot, designated Fomalhaut cs1, in its vicinity. In imagery from 2023, a new place, Fomalhaut cs2, appeared; it was imaged again last year and became the focus of the new paper. Since his initial viewing of the bright spot, however, it has been identified as dust, likely a cloud produced by planetesimal collisions within the dusty disk.
“This is a new phenomenon, a point source that appears in a planetary system and then over 10 years or more slowly disappears,” Kalas said. “It’s masquerading as a planet because planets also look like tiny dots orbiting nearby stars.”
Creating Dust Clouds
The objects colliding around Fomalhaut are enormous, as deduced from their brightness. To produce such luminosity, they would need to be about 37 miles wide, which is 4 times the size of the asteroid that ended the age of the dinosaurs. At this size, they fit into the category of planetesimals, still smaller than dwarf planets, but they are the large asteroid-sized pieces that can eventually form planets.
“Fomalhaut is much younger than the solar system, but when our solar system was 440 million years old, it was littered with planetesimals crashing into each other,” Kalas said. “That’s the time period that we are seeing, when small worlds are being cratered with these violent collisions or even being destroyed and reassembled into different objects.”
“It’s like looking back in time in a sense, to that violent period of our solar system when it was less than a billion years old,” he said.
Research on these Fomalhaut dust clouds is already planned for the future. Kalas has been awarded time on the James Webb Space Telescope over the next three years to follow the cloud’s evolution. Future missions to image exoplanets directly have Kalas excited that those images may reveal more than just planets.
“These collisions that produce dust clouds happen in every planetary system,” Kalas concluded. “Once we start probing stars with sensitive future telescopes such as the Habitable Worlds Observatory, which aims to directly image an Earth-like exoplanet, we have to be cautious because these faint points of light orbiting a star may not be planets.”
The paper, “A Second Planetesimal Collision in the Fomalhaut System,” appeared in The Science on December 18, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.