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Hubble did not photograph two planets smashing together. It detected a new compact source of light near the star Fomalhaut, and astronomers interpret it as dust from a violent collision between planetesimals—smaller bodies that help build planets. Their model suggests the destroyed bodies were tens of kilometers across, but the impact itself was not directly seen.
What did Hubble actually see?
Fomalhaut, about 25 light-years from Earth, is surrounded by dusty belts. Hubble had previously found a compact source called Fomalhaut b, now designated cs1. The research team interprets cs1 not as reflected light from a planet, but as a dust cloud. While looking for it, the team found another compact source, cs2, in 2023. Earlier Hubble observations had not detected cs2, whose appearance resembles cs1’s earlier appearance. The researchers interpret both as clouds produced by collisions between planetesimals.
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That distinction matters: the telescope recorded light sources and their changing positions against the debris belt. It did not resolve the colliding bodies or capture the moment of impact. The collision is the explanation the team says best fits the sources and the behavior of the earlier cloud.
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NASA’s December 18, 2025 report quoted co-author Mark Wyatt of the University of Cambridge estimating that the planetesimals destroyed to create cs1 and cs2 were about 37 miles (60 kilometers) across. Wyatt also estimated that about 300 million such objects orbit in the Fomalhaut system. These are population and dust-model estimates, not objects Hubble directly measured.
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The paper’s estimates depend on assumptions about grain density, the distribution of grain sizes, and the roughly 20-year interval inferred between events. In one model using a size-distribution slope of 3.5, the authors estimate parent bodies for cs1-like events at roughly 30 kilometers in radius. Other tested slopes, 3 or 4, lead to substantially different estimates and are discussed by the authors as less plausible for different reasons. NASA’s 60-kilometer-across description and the paper’s conditional 30-kilometer-radius estimate are broadly similar in scale, but they are not identical measurements or necessarily the same model quantity.
Wyatt described why the dust can yield clues about objects that cannot be seen directly: “The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and how many of them there are in the disk, information which is almost impossible to get by any other means,” NASA Goddard reported.
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What evidence points to a dust cloud?
The sources appeared and changed
Cs2 was absent from earlier Hubble observations and appeared in the 2023 search. Cs1’s observed history likewise fits a changing cloud better than a steady point of reflected light from a planet, according to the team’s interpretation.
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Cs1 accelerated outward
The paper reports that cs1’s radial velocity component increased from 3.4 kilometers per second in 2010–2012 to 11.7 kilometers per second in the following year. The authors say this acceleration is consistent with stellar radiation pressure pushing sub-micron dust grains outward.
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The amount of dust is inferred, not weighed
Under the paper’s stated assumptions, cs1’s optical brightness requires an estimated dust cross-sectional area of about 5 × 1022 square centimeters and a total dust mass of about 1020 grams. Small grains dominate the brightness. The authors estimate that grains up to about 3 micrometers in radius account for roughly 10% of the mass and can be pushed out of the system by radiation pressure. These values come from a model; Hubble did not directly measure the cloud’s mass.
What remains uncertain?
- When cs2 formed: It could have appeared at any point in the nine years since the last Hubble observation in 2014. A September 2024 follow-up produced a candidate source with signal-to-noise around 3, but sensitivity was limited and the authors do not regard it as a conclusive identification.
- Whether cs1 and cs2 are connected: Their close placement does not prove a dynamical link. The paper estimates a 10% chance that a second source would appear by chance within the stated observed region and discusses other possible dynamical explanations.
- How often such events occur: The paper estimates a mean interval of about 20 years between cs1-like events in Fomalhaut. That is a system-specific inference, not a rate established for planetary systems generally.
Why the Fomalhaut result stands out
The team’s estimated interval is striking because earlier theory suggested one collision every 100,000 years or longer. As principal investigator Paul Kalas of the University of California, Berkeley, put it: “Previous theory suggested that there should be one collision every 100,000 years, or longer. Here, in 20 years, we’ve seen two,” NASA quoted him in its report. The comparison concerns Fomalhaut and the team’s interpretation; it does not mean large collisions happen at this pace everywhere.
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Other observations of dusty collisions involve different systems and evidence. For example, NASA’s account of HD 166191 describes Spitzer observations of infrared brightening and a transiting debris cloud attributed to a collision involving dwarf-planet-sized objects (NASA’s HD 166191 report). A separate proposed collision around ASASSN-21qj involved infrared brightening followed by a long dimming as a hot debris cloud crossed the star; researchers proposed two planets of several to tens of Earth masses (NASA’s ASASSN-21qj explainer). Neither case is the Fomalhaut event.
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What observations were planned next?
At the time of NASA’s December 2025 report, the team had been granted Hubble time to monitor cs2 over the following three years and planned observations with Webb’s NIRCam. NASA said Webb’s near-infrared color information could help estimate grain size and composition, including whether the cloud contains water ice. Those were plans reported at that time; they are not, by themselves, evidence that the observations have since taken place or that results are available.
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