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TESS Discovers a Second System of Transiting Exocomets in the Extreme Debris Disk of RZ Psc

Adalyn Gibson, Meredith A. MacGregor, Ward S. Howard, Ann Marie Cody, Mark Swain, Jennifer A. Burt, Laura Venuti, Evgenya Shkolnik, Neal J. Turner, Alan Didion, Jaime Nastal, David Makowski

TL;DR

This study reports the discovery and characterization of a second exocomet system around the young star RZ Psc using TESS data from three sectors, identifying 24 transiting exocomets with absorption depths between 1% and 20%. The exocomet radii (0.4–11 km) imply a broken power-law size distribution with a break near $r_{\text{break}} \approx 2.3$ km and slopes $\gamma_{AD>\mathrm{break}} = 2.32 \pm 0.12$ and $\gamma_{AD<\mathrm{break}} = 0.11 \pm 0.04$, analogous to Solar System Kuiper Belt object distributions. The results show a higher incidence of small bodies compared to Beta Pictoris, consistent with a dynamically active, collision-dominated debris disk. The paper also outlines the EVE SMEX mission concept, which could detect hundreds of exocomet transits across multiple bands, enabling grain-size and composition discrimination and improving constraints on the debris-disk fraction in young clusters.

Abstract

We present the TESS discovery of only the second system of transiting exocomets with a sufficient number of events to measure the size distribution in the RZ Psc system, enabling comparisons with the $β$ Pictoris and Solar System size distributions. Twenty-four transits with absorption depths (AD) of 1--20\% were observed across three TESS sectors of the 20-50 Myr K0V star, detected as part of our TESS survey of extreme debris disks identified by their IR excess. We discover that the ADs (and hence exocomet radii) follow a broken power-law cumulative frequency distribution not previously seen in extrasolar contexts but similar to that observed in Solar System Kuiper Belt Object sizes, with power-law slopes above and below the break of $γ_\mathrm{AD>break}$=2.32$\pm$0.12 and $γ_\mathrm{AD<break}$=0.11$\pm$0.04, respectively. We derive size distributions of 1--7~km from two independent lines of evidence. We use the RZ Psc exocomet rate to predict exocomet yields for the Early eVolution Explorer (EVE) NASA astrophysics Small Explorer (SMEX) mission concept to obtain simultaneous photometry of 10$^4$ young stars in NUV, optical, and NIR bands. Assuming occurrence rates scaled from RZ Psc, EVE would detect 590 exocomets from $\approx$70 young systems in the optical band, with $\approx$120 simultaneous 5$σ$ detections in all three bands. These data would enable grain sizes of 200--700~nm and graphite--olivine compositions of dozens of events to be distinguished at 2.5--3$σ$, as well as a 4$σ$ determination of the accuracy of the Herschel-derived M-debris disk fraction.

TESS Discovers a Second System of Transiting Exocomets in the Extreme Debris Disk of RZ Psc

TL;DR

This study reports the discovery and characterization of a second exocomet system around the young star RZ Psc using TESS data from three sectors, identifying 24 transiting exocomets with absorption depths between 1% and 20%. The exocomet radii (0.4–11 km) imply a broken power-law size distribution with a break near km and slopes and , analogous to Solar System Kuiper Belt object distributions. The results show a higher incidence of small bodies compared to Beta Pictoris, consistent with a dynamically active, collision-dominated debris disk. The paper also outlines the EVE SMEX mission concept, which could detect hundreds of exocomet transits across multiple bands, enabling grain-size and composition discrimination and improving constraints on the debris-disk fraction in young clusters.

Abstract

We present the TESS discovery of only the second system of transiting exocomets with a sufficient number of events to measure the size distribution in the RZ Psc system, enabling comparisons with the Pictoris and Solar System size distributions. Twenty-four transits with absorption depths (AD) of 1--20\% were observed across three TESS sectors of the 20-50 Myr K0V star, detected as part of our TESS survey of extreme debris disks identified by their IR excess. We discover that the ADs (and hence exocomet radii) follow a broken power-law cumulative frequency distribution not previously seen in extrasolar contexts but similar to that observed in Solar System Kuiper Belt Object sizes, with power-law slopes above and below the break of =2.320.12 and =0.110.04, respectively. We derive size distributions of 1--7~km from two independent lines of evidence. We use the RZ Psc exocomet rate to predict exocomet yields for the Early eVolution Explorer (EVE) NASA astrophysics Small Explorer (SMEX) mission concept to obtain simultaneous photometry of 10 young stars in NUV, optical, and NIR bands. Assuming occurrence rates scaled from RZ Psc, EVE would detect 590 exocomets from 70 young systems in the optical band, with 120 simultaneous 5 detections in all three bands. These data would enable grain sizes of 200--700~nm and graphite--olivine compositions of dozens of events to be distinguished at 2.5--3, as well as a 4 determination of the accuracy of the Herschel-derived M-debris disk fraction.

Paper Structure

This paper contains 15 sections, 10 equations, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and Light Curve Generation
  3. Results and Analysis: The Exocomets of Rz Psc
  4. Exocomet Identification
  5. Exocomet Modeling
  6. Exocomet Size Distribution
  7. Discussion
  8. Size Distribution Comparison
  9. The Early eVolution Explorer - EVE
  10. Other Future Directions for Exocomet Discovery and Characterization
  11. Conclusions
  12. Acknowledgements
  13. Stellar Variability Model
  14. Exocomet Radii Calculation
  15. Exocomet Yield Prediction Details with the Early eVolution Explorer

Figures (8)

  • Figure 1: The top three panels show all three TESS sectors from start to finish, with the stellar variability and outliers subtracted. Each exocomet is denoted by the dashed line (orange). The bottom three panels are zoomed in examples in sectors 57 and 84.
  • Figure 2: Examples of exocomet models across all three sectors, in the left panel sector 17 is shown with the highest AD ($AD\simeq0.20$, $R = 5.27 \pm 0.069$ km) exocomet modeled, the middle panel shows an ($AD\simeq0.14$, $R = 5.086 \pm 0.13$ km) exocomet in sector 57, and the right panel shows a smaller exocomet ($AD\simeq0.019$, $R = 1.16 \pm 0.14$ km) modeled in sector 84.
  • Figure 3: Non-cumulative histogram of the number of exocomets as a function of $AD$, the bin width is 0.02. We report a power law index $\alpha = 0.42 \pm 0.18$, which was calculated from Equation \ref{['eqn:2']}.
  • Figure 4: Left panel: (blue) Cumulative size distribution of the 24 exocomets identified in the RZ Psc system with radii in units of km, calculated from the Equations \ref{['eqn:AD']}, \ref{['eqn:q']}, and \ref{['eqn:Mdot']} in Appendix \ref{['exocomet_radii_calculation']}. We report a power law index $\gamma_\mathrm{R<break} = -0.12 \pm 0.02$ for the power law fit before the turnover point at $\sim$2.5 km, and a power law index $\gamma_\mathrm{R>break} = 3.45 \pm 0.17$ for the power law fit after the turnover point, which was calculated from Equation \ref{['eqn:powerlaw2']}. (green) Cumulative size distribution of the 24 exocomets identified in the RZ Psc system with radii in units of km, calculated from the Equation \ref{['eqn:AD_to_rad']} in Appendix \ref{['exocomet_radii_calculation']}. We report a power law index $\gamma_\mathrm{R<break} = -0.39 \pm 0.03$ for the power law fit before the turnover point at around $\sim$2.26 km, and a power law index $\gamma_\mathrm{R>break} = 2.48 \pm 0.05$ for the power law fit after the turnover point. (yellow) We compare to the size distribution of $\beta$ Pictoris from lecavelier:2022. (grey, dashed) We also compare it to solar system populations from lecavelier:2022. (grey, dotted) We plot a vertical line represented the estimated radius of the most recent interstellar object, 3I/ATLAS, of $5.6 \pm0.7$ km from Chandler:2025. Right panel: A comet frequency distribution in AD space to highlight our detection ability for smaller ADs. We report a power law fit: $\gamma_\mathrm{AD>break}$=2.32$\pm$0.12 and $\gamma_\mathrm{AD<break}$=0.11$\pm$0.04, where the ADs in the region of incomplete recovery were not fit. We expect that far fewer than 100% of exocomets will be detectable in the region of incomplete recovery. This calculation was performed with a cadence of 200 s, matching the cadence of sectors 57 and 84.
  • Figure 5: Top: Simulated EVE observation of an exocomet transit of a K7V at 100 pc expected during a 20 d stare at $\sim$20 Myr Theia 43 or 45 associations. We scale the optical light curve template used for the RZ Psc events into the NUV and NIR bands using a 50-50% weighted average of empirical extinction curves of $\sim$200 nm water ice kratschmer:1985 and olivine dust huffman:1975. Bottom: Distribution of optical ADs versus SpT for 590 simulated detections. The SNR of the NUV detections (left) enable color differences due to composition (middle) and grain size (right) to be distinguished given the color predictions for the narrow-tail multiband comet models of kalman:2024. Each panel highlights the region in which EVE should detect exocomets given a 20% M-star disk fraction, but not for a 2.1% fraction.
  • ...and 3 more figures