Fomalhaut, the 18th brightest star visible in the night sky, orbits the compact source Fomalhaut b, which has previously been interpreted as either a dust-shrouded exoplanet or a dust cloud formed by the collision of two planetesimals. Such collisions are rarely observed, but their debris may appear in live images. New observations from the NASA/ESA Hubble Space Telescope show the appearance of a second point source around Fomalhaut in 2023, reminiscent of the appearance of Fomalhaut b twenty years earlier. UC Berkeley astronomer Paul Kalas and his colleagues interpret this additional source as a cloud of dust formed by the recent collision of two planetesimals.
This Hubble image shows the debris ring and dust clouds cs1 and cs2 around Fomalhaut. Image credit: NASA/ESA/P. Kalas, UC Berkeley/J. DePasquale, STScI.
Fomalhaut is an A-type star located just 25 light years away in the constellation Pisces.
The name Fomalhaut comes from the Arabic name for this star – Fum al-Khutwhich means “Fish mouth”.
The star is twice as massive as the Sun and 20 times brighter and is surrounded by a ring of dust and debris.
In 2008, astronomers used Hubble to detect a planet candidate around Fomalhaut, making it the first star system to have a possible planet detected using visible light.
This object, named Fomalhaut bnow appears as a cloud of dust masquerading as a planet, the result of a planetesimal collision.
While searching for Fomalhaut b in recent Hubble observations, Dr Kalas and his co-authors were surprised to find a second point of light in a similar location around the star.
They call this object circumstellar source 2 (cs2), and the first object is now known as cs1.
“This is certainly the first time I've seen a point of light appear out of nowhere in an exoplanet system,” Dr Kalas said.
“It's not in all of our previous Hubble images, which means we've just witnessed a violent collision between two massive objects and a huge cloud of debris unlike anything in our solar system today. Amazing!”
Why astronomers see both of these debris clouds so physically close to each other remains a mystery.
If asteroid and planetesimal collisions were random, CS1 and CS2 should randomly appear in unrelated locations.
However, they are curiously located next to each other in the inner part of the outer disk of Fomalhaut debris.
Another mystery is why scientists witnessed these two events in such a short period of time.
“Previous theory suggested that there should be one collision every 100,000 years or longer. Here, in 20 years, we saw two,” Dr Kalas said.
“If you had a movie of the last 3,000 years and it was sped up so that each year lasted a fraction of a second, imagine how many flashes you would see in that time.”
“The planetary system of Fomalhaut would glow from these collisions.”
Collisions are fundamental to the evolution of planetary systems, but they are rare and difficult to study.
“The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and the number of them in the disk – information that is almost impossible to obtain in any other way,” said Dr Mark Wyatt, an astronomer at the University of Cambridge.
“We estimate that the size of the planetesimals that were destroyed during the creation of CS1 and CS2 is only 30 km, and we conclude that there are 300 million such objects orbiting the Fomalhaut system.”
“This system provides a natural laboratory for studying how planetesimals behave in collisions, which in turn tells us what they are made of and how they formed.”
results appearing in this week's magazine Science.
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Paul Kalas etc.. 2025. Second collision of planetesimals in the Fomalhaut system. Sciencepublished online December 18, 2025; doi: 10.1126/science.adu6266
