A second planetesimal collision in the Fomalhaut system (2512.15861v1)

Abstract: The nearby star Fomalhaut is orbited by a compact source, Fomalhaut b, which has previously been interpreted as either a dust-enshrouded exoplanet or a dust cloud generated by the collision of two planetesimals. Such collisions are rarely observed but their debris can appear in direct imaging. We report Hubble Space Telescope observations that show the appearance in 2023 of a second point source around Fomalhaut, resembling the appearance of Fomalhaut b twenty years earlier. We interpret this additional source as a dust cloud produced by a recent impact between two planetesimals. The positions and motion of two impact-generated dust clouds over twenty years provide constraints on the collisional dynamics in the debris belt.

• The paper presents high-sensitivity HST imaging evidence for a second transient dust source, confirming recurrent high-mass planetesimal collisions.
• The study details cs2’s photometric brightness, rapid radial motion, and high eccentricity, aligning with theoretical models of dust dispersal.
• The findings constrain debris production rates and collisional cascade dynamics, offering new insights into the evolution of mature planetary systems.

Evidence for Recurrent Planetesimal Collisions in the Fomalhaut System

Introduction

This paper presents high-sensitivity HST/STIS coronagraphic imaging of the Fomalhaut system, revealing a second transient, compact dust source—Fomalhaut cs2—appearing in 2023 at the debris belt's inner edge. The recurrent detection of such sources, including the well-studied Fomalhaut cs1 (formerly Fomalhaut b), provides empirical evidence for high-mass planetesimal collisions in an extrasolar debris disk. The observations constrain the physical and dynamical properties of debris production and dispersal over decadal timescales, refining models of collisional evolution in mature planetary systems.

Observational Results

Deep STIS imaging acquired in 2023 and 2024 using four independent PSF subtraction methodologies robustly detected Fomalhaut cs2 (SNR 7–9) as a compact, point-like source at 12.76" separation (PA = 311.27°), on the inner edge of the outer belt. No trace of cs1 was found in 2023, consistent with its expected rapid radial evolution and fading. Injection-recovery experiments and analysis of prior HST data rule out cs2 as a background or pre-existing bound source, given its absence at previous epochs and its photometric brightness of 24.67 ± 0.15 mag—0.3 mag brighter than cs1 in 2012.

Follow-up in 2024 yielded a candidate detection of cs2 with similar brightness, exhibiting a northward offset corresponding to 0.113" yr⁻¹, indicative of an eccentric orbit ($e\sim 0.8$). Orbital modeling (OFTI) supports a high-eccentricity solution for cs2, akin to that inferred for cs1 in 2010–2013. Constraints from failed attempts to detect cs1 in 2023 are consistent with theoretical models of dust cloud expansion and acceleration under radiation pressure, predicting significant radial migration and reduced detectability.

Physical Interpretation of Dust Cloud Sources

The paper delivers strong evidence supporting the collisional dust cloud hypothesis for both cs1 and cs2, rather than self-luminous or dust-enveloped planets. Radial acceleration and photometric fading of cs1, together with the sudden appearance and subsequent motion of cs2, match the expected behavior for unbound, optically thick dust clouds expelled in high-mass planetesimal collisions. The observed properties restrict the grain population to sub-micron to mm-scale, with surface brightness and spatial extent evolving on multi-year timescales due to dispersal and radiation pressure.

Estimated dust mass for cs1 is $\sim10^{20}$ g, requiring destruction of parent bodies of $\sim30$ km radius. The inferred frequency—roughly 22 million cs1-like events over the system's 440 Myr age—implies that such catastrophic collisions are a non-dominant but significant evolutionary process, contributing $\sim0.04$ Earth masses to belt depletion.

The relative proximity (8.1° in belt azimuth; 23.4 au projected separation) of the cs1 and cs2 events suggests non-random collisional dynamics, potentially hinting at spatial concentration mechanisms such as mean-motion resonances rather than random belt-wide collisions. Nonetheless, emission and density mapping at mm wavelengths do not show significant overdensities, and intersection with the misaligned intermediate belt is geometrically disfavored.

Collision Rate and Belt Mass Budget

The analysis leverages both empirical and modeled size-frequency distributions for planetesimals. Assuming a primordial Dohnanyi-like power law ($a=3.5$), the parent body population required to sustain the observed collision rate is consistent with the belt's estimated mass ($\sim1.8\,M_{\oplus}$ in bodies up to 0.3 km). For cs1-like events, parent objects of radius $\sim30$ km and total mass $\sim18\,M_{\oplus}$ are plausible, provided $\sim4\%$ of their mass is efficiently liberated as $<3\,\mu$m-sized grains during collisions. This requires a regolith-dominated planetesimal structure resulting from numerous prior non-catastrophic shattering events, consistent with collisional cascade models.

The dust injection rate from cs1-like events is significantly lower than required for steady-state belt maintenance, implying that most mass loss is sustained by smaller-scale, more frequent collisions among sub-0.3 km bodies. The magnitude of the ejected dust in these events is orders of magnitude greater than the mass released in observed Solar System asteroid disruptions (e.g., P/2010 A2, Scheila), underscoring the scale of these extrasolar processes.

Dynamical Evolution of Ejected Dust

Multi-epoch astrometry of cs1 reveals a transition from a high-eccentricity Keplerian orbit (2004–2012) to rapid radial acceleration (2013), quantitatively consistent with the onset of optically thin dispersal and runaway acceleration due to stellar radiation pressure ($a_{\rm rad} = 1.946 \times 10^{-7}$ km s⁻²). Theoretical models suggest expansion velocities of $\sim5$ m s⁻¹ during the optically-thick phase, with grain blowout commencing over $\sim8$ years, analogous (but with lower escape speeds) to ejection velocities in lunar-forming giant impacts. The lack of observable photometric brightening during expansion is attributed to instrument sensitivity and photometric uncertainties.

Candidate detection of cs1 at greater stellocentric separation in 2023 is consistent with faded and radially propagated dust, though confirmatory observations are precluded by noise and search depth limitations.

Implications for Planetary System Evolution

The repeated observation of high-mass planetesimal collisions in Fomalhaut's debris disk directly constrains the collisional regime in mature planetary systems. This supports the view that catastrophic disruptions of $\sim10-30$ km planetesimals are active but not dominant in mass loss, with steady-state belt erosion driven by a collisional cascade among smaller objects. These events create substantial, transient circumstellar dust clouds, analogous to, but far larger than, Solar System asteroid breakup events. Observational cadence and photometric depth achievable via HST provide critical temporal and spatial baselines for constraining dust production, dispersal, and dynamical evolution.

Theoretical implications extend to modeling dust replenishment, mass transport efficiency, primordial planetesimal size distribution, and the role of regolith-dominated bodies in belt longevity. Non-random spatial event distribution hints at possible substructure or resonance trapping, motivating higher-resolution mapping and expanded multi-wavelength campaigns.

Prospects for Future Research

Continued multi-epoch high-contrast imaging, especially with advanced coronagraphic instrumentation (JWST, ELTs), holds promise for detailed monitoring of debris belt dynamics. Increased sensitivity and resolution may reveal further instances of catastrophic collisions, enable tracking of dust cloud expansion and fading, and refine our understanding of resonance dynamics and belt substructure. Parallel spectroscopic and multi-wavelength campaigns can elucidate dust compositions and grain-size distributions, informing models of long-term planetary system evolution.

Empirical constraints from systems such as Fomalhaut can be generalized to address key questions in extrasolar planetesimal physics: What sets the dominant collision regime? What is the efficiency of mass loss and dust injection? Are catastrophic collisions stochastic or spatially concentrated due to resonant or dynamical substructure? Answers to these questions are central for comparative studies with Solar System analogs and for tracing the processes influencing planet formation and belt stability.

Conclusion

This work establishes robust observational evidence for recurrent catastrophic planetesimal collisions in the Fomalhaut system, manifested as transient, expanding dust clouds on timescales of decades. Detailed photometric, astrometric, and dynamical analysis shows that these events are consistent with theoretical models of collisional cascades and debris dispersal in evolved extrasolar systems. The results constrain both the physical and statistical properties of debris production, as well as underlying population parameters critical for long-term belt evolution modeling. Future observational campaigns leveraging improved sensitivity and cadence will further clarify the role and frequency of such events in shaping planetary architectures.