Fomalhaut, a celestial body ranking as the 18th most brilliant star in the night sky, is accompanied by a closely situated entity, Fomalhaut b. This entity was previously hypothesized to be either a nascent planet enveloped in dust or a particulate cloud resulting from the impact of two planetesimals. Such cosmic collisions are seldom witnessed directly, though their resultant detritus can be captured in observational imagery. Contemporary analyses employing the NASA/ESA Hubble Space Telescope have revealed the emergence of a secondary point source orbiting Fomalhaut in 2023. This newly detected feature bears a striking resemblance to Fomalhaut b as it appeared approximately two decades prior. Astronomers affiliated with the University of California, Berkeley, including Paul Kalas and his research associates, posit that this additional observed source represents a cloud of cosmic dust generated by a recent collision between 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 classifies as an A-type star, positioned a mere 25 light-years distant within the Piscis Austrinus constellation.
The appellation Fomalhaut originates from the Arabic term for this star, Fum al Hut, which translates to ‘the Fish’s Mouth.’
This star possesses double the Sun’s mass and shines with twenty times its luminosity, being encircled by a substantial ring of dust and particulate matter.
In the year 2008, observational data acquired by the Hubble telescope led to the identification of a potential planet in proximity to Fomalhaut, marking the first instance of a stellar system with a possible exoplanet detected utilizing visible light spectrum analysis.
That celestial object, designated Fomalhaut b, is now understood to be a dust cloud that previously mimicked the appearance of a planet, a consequence of planetary body collisions.
While conducting a search for Fomalhaut b within recent Hubble observational records, Dr. Kalas and his collaborating researchers were taken by surprise to discover a second luminous point in a comparable orbital position around the star.
This newly identified celestial phenomenon has been termed circumstellar source 2 (cs2), with the initial object now referred to as cs1.
“It is truly unprecedented for me to observe a point of light manifest from obscurity within an exoplanetary system,” stated Dr. Kalas.
“Its absence in all our prior Hubble imagery signifies that we have directly witnessed a cataclysmic collision between two substantial celestial bodies, resulting in a vast debris cloud, a spectacle unlike any currently present in our own Solar System. It is truly astonishing!”
The rationale behind the observed proximity of these two distinct debris clouds remains an open question.
Should collisions between asteroids and planetesimals occur randomly, cs1 and cs2 would be anticipated to appear at disparate, unassociated locations purely by chance.
However, their spatial arrangement is notably close, situated along the inner perimeter of Fomalhaut’s extensive debris disk.
An additional enigma pertains to the remarkably short temporal interval during which scientists have detected these two events.
“Prevailing theoretical models posited a collision event occurring once every 100,000 years, or even more infrequently. Yet, within a span of two decades, we have documented two such occurrences,” Dr. Kalas remarked.
“Imagine viewing a time-lapse rendition of the past 3,000 years, accelerated such that each year is represented by a mere fraction of a second; consider the sheer number of luminous flashes one might perceive across that duration.”
“The planetary system surrounding Fomalhaut would be scintillating with such impact events.”
Collisions play an indispensable role in the evolutionary trajectory of planetary systems; however, their infrequency and the challenges associated with their study render them elusive subjects of research.
“The compelling aspect of this particular observation lies in its capacity to enable researchers to ascertain both the dimensions of the colliding celestial bodies and their sheer numbers within the disk, data that is exceedingly difficult to acquire through alternative methodologies,” commented Dr. Mark Wyatt, an astronomer at the University of Cambridge.
“Our calculations indicate that the planetesimals responsible for the creation of cs1 and cs2 were approximately 30 kilometers in diameter, and we infer that an estimated 300 million such objects are currently in orbit within the Fomalhaut system.”
“This system serves as a natural experimental environment for investigating the behavior of planetesimals during collisional events, which in turn provides insights into their constituent materials and their formative processes.”
The findings of this research are published this week in the esteemed journal Science.
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Paul Kalas et al. 2025. A second planetesimal collision in the Fomalhaut system. Science, published online December 18, 2025; doi: 10.1126/science.adu6266
