This artist’s concept shows the violent collision of two massive objects in orbit around the star Fomalhaut. (Credit: NASA, ESA, STScI, Ralf Crawford (STScI))
Astronomers finally have the answer to a 20 year mystery.
In A Nutshell
- Hubble Space Telescope captured evidence of two separate planetesimal collisions in the Fomalhaut system over 20 years, the first time such events have been directly observed outside our solar system
- The mysterious “planet” Fomalhaut b turned out to be an expanding dust cloud from a collision, then disappeared as a second collision created a new bright spot in nearly the same location
- Each crash involved rocky bodies roughly 30 kilometers across, generating dust clouds with masses billions of times greater than asteroid breakups observed in our solar system
- These rare observations show planet formation in action, revealing how rocky debris grinds down over hundreds of millions of years to build stable planetary systems like our own
Astronomers just witnessed the aftermath of something rarely seen: a fresh dust cloud from a cosmic collision around a nearby star, and then another one appearing in nearly the same spot.
Twenty years ago, the Hubble Space Telescope spotted a mysterious point of light near the star Fomalhaut, about 25 light-years from Earth. Scientists named it Fomalhaut b and debated whether it was a dusty planet or something else entirely. Now, observations from 2023 reveal a second bright spot has appeared in nearly the same location, strongly supporting the collision explanation over the planet hypothesis.
Both objects are dust clouds generated by massive collisions between planetesimals, rocky bodies tens of kilometers across orbiting in Fomalhaut’s debris belt. The findings appear in the journal Science.
“Spotting a new light source in the dust belt around a star was surprising. We did not expect that at all,” said Jason Wang, assistant professor of physics and astronomy at Northwestern University. “Our primary hypothesis is that we saw two collisions of planetesimals — small rocky objects, like asteroids — over the last two decades.”
A Vanishing Act Solves a Two-Decade Mystery
Fomalhaut is a young star surrounded by a ring of rocky debris that resembles our solar system’s Kuiper Belt, just far more active. The first collision that created Fomalhaut cs1 (circumstellar source 1) happened before 2004, producing a cloud with about 10²⁰ grams of dust in total, with the tiniest grains doing most of the shining.
When researchers revisited Fomalhaut in September 2023, they found something unexpected. A new dust cloud, designated Fomalhaut cs2, had materialized at the inner edge of the debris belt. Meanwhile, the original cs1 could no longer be clearly detected, likely having faded and expanded beyond visibility as radiation from the star dispersed it into space.
“This is certainly the first time I’ve ever seen a point of light appear out of nowhere in an exoplanetary system,” said lead author Paul Kalas, an astronomer at the University of California, Berkeley. “It’s absent in all of our previous Hubble images, which means that we just witnessed a violent collision between two massive objects and a huge debris cloud unlike anything in our own solar system today.”
Both dust clouds appeared within 8 degrees of each other on the debris ring. The research team calculated the probability of finding a second source this close to the first at only 10 percent.
Thomas Müller (MPIA))
Cosmic Demolition on a Massive Scale
Watching planetesimal collisions unfold in real time is extraordinarily rare. In our own solar system, astronomers have documented only a handful of similar events involving much smaller asteroid disruptions. The Fomalhaut collisions involve bodies potentially 30 kilometers in radius, roughly the size of large asteroids, producing dust masses nine orders of magnitude greater than anything observed near Earth.
“Collisions of planetesimals are extremely rare events, and this marks the first time we have seen one outside our solar system,” Wang said. “Studying planetesimal collisions is important for understanding how planets form.”
The research team estimates that maintaining Fomalhaut’s dusty ring requires about 22 million collision events like cs1 over the star’s 440-million-year lifetime. That translates to roughly one major impact every 20 years, though most collisions produce debris too faint for current telescopes to detect.
Each collision releases approximately 4 percent of the impacting bodies’ mass as tiny dust grains smaller than 3 micrometers. Radiation from the central star then pushes these particles outward at accelerating speeds, causing the clouds to expand and eventually fade below detection thresholds.
How Dust Clouds Disappear Into Space
The original cs1 cloud appeared to follow a normal orbit between 2004 and 2012 before suddenly accelerating outward. Researchers believe this happened when the expanding cloud became optically thin, allowing radiation to push on all the dust simultaneously rather than just the star-facing surface.
By 2013, cs1 was moving outward at nearly 12 kilometers per second—more than three times faster than its initial velocity. If that acceleration continued for another decade, the cloud would have traveled roughly 50 astronomical units farther from the star, becoming too faint to see.
Why Two Crashes Happened So Close Together
The spatial clustering of these collisions raises questions about whether they’re truly random. One possibility involves planetesimals scattered from an inner dust belt that Fomalhaut also hosts. Infrared observations have detected a misaligned intermediate belt centered at 94 astronomical units. However, the researchers note this belt’s geometry doesn’t align well with where the two dust clouds appeared, about 70 degrees away from the predicted intersection point.
Another explanation involves gravitational resonances with undiscovered planets. If Earth-mass worlds orbit within the debris field, they could trap planetesimals in specific orbital patterns, concentrating impacts in certain locations and creating collision hotspots.
The Fomalhaut system provides a window into the chaotic early histories of planetary systems. Our own solar system went through similar violent phases billions of years ago, grinding down larger bodies into the dust and debris we observe today. Catching these collisions as they happen around other stars helps astronomers understand how planetary systems evolve from dusty disks into stable configurations.
The research team plans to continue observing the system using NASA’s James Webb Space Telescope, which can reveal the size and composition of dust grains, including whether the clouds contain water and ice.
Animation: NASA, ESA, STScI, Ralf Crawford (STScI))
Paper Summary
Limitations
The observations face several constraints. The 2024 follow-up imaging had substantially lower sensitivity than 2023, making it difficult to confirm whether a detected candidate source is actually cs2. The field of view and residual noise obscure more than half of the belt edge, limiting the ability to search for additional collision events. Calculations about collision frequencies and planetesimal sizes depend on assumed power-law size distributions that haven’t been directly measured for the Fomalhaut system.
Funding and Disclosures
This research was supported by grant HST-GO-17139 from the Space Telescope Science Institute under NASA contract NAS5-26555. Additional funding came from UKRI/STFC grants, the European Union’s Horizon Europe program, the National Science Foundation of China, and Xiamen University. One author holds affiliations with commercial entities Unistellar and SkyMapper Inc.
Publication Details
“Glutamate indicators with increased sensitivity and tailored deactivation rates” by Abhi Aggarwal, Adrian Negrean, Yang Chen, Rishyashring Iyer, Jeremy P. Hasseman, and Kaspar Podgorski. Published in Nature Methods, December 23, 2025. DOI: 10.1038/s41592-025-02965-z. Affiliations include Allen Institute for Neural Dynamics, Janelia Research Campus, Technical University of Munich, and University of California San Diego.
