Young star systems are a place of violent collisions. Rocks, comets, asteroids and larger objects carom off one another and coalesce, gradually turning the primordial dust and ice of a stellar nebula into planets and moons. The largest of these collisions, however, are expected to be rare over the hundreds of millions of years it takes to form a planetary system - perhaps one every 100,000 years.
Now, astronomers have seen the aftermath of two powerful collisions within a 20-year period around a nearby star called Fomalhaut. These are either lucky observations or a sign that collisions are more frequent than predicted during planet formation.
The events - the first was detected in 2004 and the second in 2023 - are the first collisions between large objects directly imaged in any solar system outside our own.
"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 Paul Kalas, adjunct professor of astronomy at the University of California, Berkeley, and first author of the report. "We do not directly see the two objects that crashed into each other, but we can spot the aftermath of this enormous impact."
Over tens of thousands of years, he said, the dust around Fomalhaut would be "sparkling with these collisions" - like twinkling holiday lights.
Kalas first started searching for a dusty disk around Fomalhaut in 1993, hoping to see for the first time the debris left over after planet formation. Only 25 light-years from Earth, the star is young - about 440 million years old - and a proxy for what our solar system looked like in its formative years. Thanks to NASA's Hubble Space Telescope (HST), he eventually found such a disk around the star and, in 2008, reported finding a bright spot near the disk that was likely a planet, the first to be imaged directly at visible wavelengths. He called it Fomalhaut b, per the naming convention for exoplanets.
That planet discovery has now turned to dust. What he thought was a planet was likely the dust cloud kicked up by the collision of planetesimals.
"This is a new phenomenon, a point source that appears in a planetary system and then over 10 years or more slowly disappears," he said. "It's masquerading as a planet because planets also look like tiny dots orbiting nearby stars."
Based on the brightness of both the 2004 and 2023 events, the colliding objects are at least 60 kilometers (37 miles) across - at least four times larger than the object that collided with Earth 66 million years ago and killed off the dinosaurs. Objects of this size are referred to as planetesimals - objects similar in size to many of the asteroids and comets in our solar system but much smaller than a dwarf planet like Pluto.
"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."
The 2023 Fomalhaut observations are discussed in a paper posted online Dec. 18 in the journal Science.
"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," said Kalas's colleague, Mark Wyatt, a theorist and professor of astronomy at the University of Cambridge in the United Kingdom. "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."
He estimates that there are about 300 million objects around Fomalhaut the size of the ones that collided to generate these bright clouds of dust. Previous observations of the star detected the presence of carbon monoxide gas, which indicates that these planetesimals are volatile-rich and therefore very similar in composition to the icy comets in our solar system, he said.
Dust clouds masquerading as exoplanets
Fomalhaut, located within the southern constellation Piscis Austrinus, is 16 times more luminous than our sun and one of the brightest stars in the sky. After Kalas began observing it with HST in 2004, he discovered a large belt of dusty debris at a distance of 133 astronomical units (AU) from the star, more than three times the distance from the star as the Kuiper Belt is from the sun in our solar system. An AU is the average distance between the Earth and the sun, or 93 million miles.
NASA, ESA, Paul Kalas/UC Berkeley
To Kalas, the belt's sharp inner edge suggested that it had been sculpted by planets. After a second observation in 2006, he concluded that a bright spot in the outer belt visible in both the 2004 and 2006 images was, in fact, a planet. He acknowledged at the time that it could be a very bright dust cloud caused by a collision in the disk, but the likelihood of that seemed very low.
Kalas was able to schedule four follow-up HST observations of Fomalhaut, in 2010, 2012, 2013 and 2014. In the last, however, Fomalhaut b was nowhere to be seen. Nine years later, after three failed attempts to image Fomalhaut with HST, he obtained a new image that revealed another bright spot not far from the first, which is now referred to as Fomalhaut cs1, for circumstellar source 1. Based on its location, however, the new spot, dubbed Fomalhaut cs2, could not be a reappearance of Fomalhaut cs1. Because of the nine-year hiatus between the observations in 2014 and 2023, it's unclear when Fomalhaut cs2 appeared.
In the new paper, Kalas and an international team of astronomers analyzed the 2023 image of Fomalhaut and a subsequent, though poor image obtained in 2024, and concluded that it could only be light reflected from a dust cloud produced by the collision of two planetesimals.
Kalas noted that at first, Fomalhaut cs1 moved like an exoplanet, but by 2013 its path had curved away from the star. This type of motion would be possible for very small particles being pushed outward by the radiation pressure of starlight. The appearance of cs2 supports the idea that cs1 was in fact a dust cloud.
Kalas compares these events to the dust cloud generated in 2022 when NASA's DART (Double Asteroid Redirection Test) mission slammed into the moonlet Dimorphos, which was orbiting the asteroid Didemos. The cloud around Fomalhaut is about a billion times larger, the team estimated.
Kalas has been awarded time over the next three years to use the James Webb Space Telescope's Near-Infrared Camera (NIRCam) and the HST to observe Fomalhaut and track the evolution of the cloud to see if it expands in size and determine its orbit. It is already 30% brighter than Fomalhaut cs1. Additional observations in August 2025 confirmed that cs2 is still visible.
In anticipation of future space missions to directly image exoplanets, Kalas cautioned astronomers to be on the lookout for dust clouds masquerading as planets.
"These collisions that produce dust clouds happen in every planetary system," he said. "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."
Other co-authors of the paper are UC Berkeley research astronomer Thomas Esposito; former UC Berkeley graduate students Jason Wang, now at Northwestern University in Illinois, and Michael Fitzgerald, now at UCLA; former UC Berkeley postdoctoral fellow Robert De Rosa, now at the European Southern Observatory in Chile; Maxwell Millar-Blanchaer of UC Santa Barbara; Bin Ren of Xiamen University in China; Maximilian Sommer of the University of Cambridge; and Grant Kennedy of the University of Warwick in the UK. The work was supported by NASA (NAS5-26555, GO-HST-17139).
