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arxiv: 1906.11774 · v1 · pith:C6IA2PM7new · submitted 2019-06-27 · 🌌 astro-ph.EP

Upper limits on protolunar disc masses using ALMA observations of directly-imaged exoplanets

Sebasti\'an P\'erez , Sebasti\'an Marino , Simon Casassus , Cl\'ement Baruteau , Alice Zurlo , Christian Flores , Gael Chauvin This is my paper

Pith reviewed 2026-05-25 13:59 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords ALMAdirectly-imaged exoplanetscircumplanetary discsprotolunar discsdust growthmoon formationupper limitsyoung giant planets
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The pith

ALMA non-detections of four young directly-imaged exoplanets place upper limits on circumplanetary dust below the mass needed to form regular satellite systems.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports ALMA observations at 1.3 mm of four stars younger than 40 Myr that host directly-imaged giant planet companions. No continuum emission is detected in any system. Radiative transfer models of protolunar discs are used to convert these non-detections into upper limits on dust mass for two disc sizes, 0.4 and 0.04 times the planet's Hill radius. All derived limits lie below 10^{-4} times the planet mass, the benchmark dust reservoir thought necessary to assemble regular satellite systems. The authors conclude that the absence of detectable dust indicates growth of solids into larger bodies on timescales shorter than 10 Myr.

Core claim

The non-detections of 1.3 mm continuum emission around PZ Tel, AB Pic, 51 Eri, and κ And imply dust masses in their circumplanetary discs below 10^{-4} times the planet mass for both extended (0.4 Hill radius) and compact (0.04 Hill radius) disc models, which the authors take as evidence that dust has grown beyond metre sizes to form moonetesimals within less than 10 Myr.

What carries the argument

Radiative transfer models of protolunar discs with two representative sizes (0.4 and 0.04 times the exoplanet's Hill radius) that convert the ALMA non-detections into upper limits on dust mass.

If this is right

  • The upper limits are lower than the dust mass required to explain regular satellite systems.
  • Dust growth to moonetesimals occurs in timescales under 10 Myr.
  • The sample of ALMA non-detections of young companions increases by 50%.
  • Comparison with viscous evolution and debris disc models supports the interpretation of rapid solid growth.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar non-detections in additional systems could indicate that moon formation around giant planets is generally complete by 10 Myr.
  • If future ALMA observations detect dust above these limits in comparable planets, it would point to longer-lived primordial dust or different disc conditions in those cases.
  • These upper limits may suggest that the assembly of regular satellites around extrasolar gas giants follows a timeline comparable to the Solar System's Galilean moons.

Load-bearing premise

The two chosen disc sizes and standard dust properties adequately represent all possible circumplanetary material distributions, and that 10^{-4} planet mass is the correct dust threshold for forming regular satellite systems.

What would settle it

Detection of 1.3 mm emission from any of these four systems at a level corresponding to more than 10^{-4} planet mass of dust, or finding a young directly-imaged planet with sufficient dust to match the threshold without evidence of moon formation.

Figures

Figures reproduced from arXiv: 1906.11774 by Alice Zurlo, Christian Flores, Cl\'ement Baruteau, Gael Chauvin, Sebasti\'an Marino, Sebasti\'an P\'erez, Simon Casassus.

Figure 1
Figure 1. Figure 1: Dust mass upper limits of directly-imaged exoplanets and sub-stellar companions observed at 0.89 and 1.3 mm, as a function of planet mass. The colour of each marker represents the semi-major axis of each planet, which is estimated based on the deprojected separation. The four new ALMA observations are highlighted with black edges. Calculations are shown for two protolunar disc sizes: cutoff radii of 0.4 (l… view at source ↗
Figure 2
Figure 2. Figure 2: Upper limits on the ratio of dust mass to planet mass for directly-imaged exoplanets observed at 0.89 and 1.3 mm as a function of stellar age, based on disc models with two cutoff radii: 0.4 (left) and 0.04 (right) times the planet Hill radius. The dashed and dotted lines represent viscous evolution models with α = 10−4 and 10−3 , respectively, and surrounding planets with masses of 2 (purple) and 20 MJup … view at source ↗
Figure 3
Figure 3. Figure 3: Lower limits on the maximum diameter of solids around each directly-imaged companion based on our dust upper limits and assuming a solid size distribution with an exponent of -3.5 and a total solid mass of 10−4 Mp. Sources are sorted by planet mass, with the less massive compan￾ions on the left, and the most massive on the right. stirring. We used standard values used to model circumstellar de￾bris discs f… view at source ↗
read the original abstract

The Solar System gas giants are each surrounded by many moons, with at least 50 prograde satellites thought to have formed from circumplanetary material. Just like the Sun is not the only star surrounded by planets, extrasolar gas giants are likely surrounded by satellite systems. Here, we report on ALMA observations of four <40 Myr old stars with directly-imaged companions: PZ Tel, AB Pic, 51 Eri, and $\kappa$ And. Continuum emission at 1.3 mm is undetected for any of the systems. Since these are directly-imaged companions, there is knowledge of their temperatures, masses and locations. These allow for upper limits on the amount of circumplanetary dust to be derived from detailed radiative transfer models. These protolunar disc models consider two disc sizes: 0.4 and 0.04 times the exoplanet's Hill radius. The former is representative of hydrodynamic simulations of circumplanetary discs while the latter a case with significant radial drift of solids. The more compact case is also motivated by the semi-major axis of Callisto, enclosing Jupiter's Galilean satellites. All upper limits fall below the expected amount of dust required to explain regular satellite systems ($10^{-4}$ times the mass of their central planet). Upper limits are compared with viscous evolution and debris disc models. Our analysis suggests that the non detections can be interpreted as evidence of dust growth beyond metre sizes to form moonetesimals in timescales <10 Myr. This sample increases by 50% the number of ALMA non-detections of young companions available in the literature.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper reports ALMA 1.3 mm non-detections toward four young (<40 Myr) directly-imaged exoplanets (PZ Tel b, AB Pic b, 51 Eri b, κ And b). Using radiative-transfer models of protolunar discs with two fixed radial extents (0.4 R_Hill and 0.04 R_Hill) and standard dust properties, the authors derive 3σ upper limits on circumplanetary dust mass that lie below 10^{-4} M_p for all targets. They compare these limits to viscous-evolution and debris-disc models and conclude that the non-detections constitute evidence for grain growth beyond metre sizes (moonetesimal formation) on timescales <10 Myr. The work increases the published sample of such ALMA non-detections by 50%.

Significance. If the upper limits remain robust under reasonable variations in disc geometry and opacity, the result supplies the first direct observational indication that the rapid satellite-formation channel inferred for the Solar System also operates around extrasolar gas giants. The addition of four new systems to the small existing sample is a concrete, incremental contribution to the empirical timeline of circumplanetary dust evolution.

major comments (3)
  1. [Abstract and §5] Abstract and §5: the central interpretive claim—that the non-detections imply dust growth beyond metre sizes within <10 Myr—requires that the radiative-transfer mass limits lie below the 10^{-4} M_p satellite-formation threshold for the adopted disc sizes. No quantitative exploration is presented of how the limits change for disc radii outside the 0.04–0.4 R_Hill range or for grain-size distributions that reduce mm-wave opacity relative to the assumed ISM-like properties; either change would map the same ALMA flux upper bound to a higher total dust mass and could remove the tension with the formation models.
  2. [§3] §3 (radiative-transfer setup): the two disc sizes are motivated by hydrodynamical simulations and by Callisto’s orbit, respectively, but the paper does not demonstrate that these choices bracket the plausible range of circumplanetary material distributions. A brief sensitivity test (e.g., a factor-of-two change in outer radius or a different surface-density power-law index) would directly address whether the reported limits are conservative or optimistic.
  3. [§4] §4 (results): the upper limits are stated to fall below 10^{-4} M_p, yet the precise numerical values, their dependence on the assumed planet parameters, and the conversion from flux to mass for each of the two disc sizes are not tabulated or plotted; without these data it is difficult to assess how close the limits sit to the threshold or how they would shift under modest model variations.
minor comments (2)
  1. [Abstract] The abstract states that the 0.04 R_Hill case is “also motivated by the semi-major axis of Callisto”; a one-sentence clarification of the numerical correspondence would help readers unfamiliar with the Galilean system.
  2. [Throughout] Figure captions and text use “Hill radius” without an explicit definition or reference to the standard expression R_H = a (M_p / 3 M_*)^{1/3}; adding the definition once would improve accessibility.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their detailed and constructive report. We address each major comment below. Where the comments identify areas for improvement, we have revised the manuscript accordingly to strengthen the presentation and robustness of the results.

read point-by-point responses

  1. Referee: [Abstract and §5] Abstract and §5: the central interpretive claim—that the non-detections imply dust growth beyond metre sizes within <10 Myr—requires that the radiative-transfer mass limits lie below the 10^{-4} M_p satellite-formation threshold for the adopted disc sizes. No quantitative exploration is presented of how the limits change for disc radii outside the 0.04–0.4 R_Hill range or for grain-size distributions that reduce mm-wave opacity relative to the assumed ISM-like properties; either change would map the same ALMA flux upper bound to a higher total dust mass and could remove the tension with the formation models.

    Authors: The two adopted disc radii are motivated by specific hydrodynamical results and by the Callisto orbit as representative cases; we agree that a quantitative sensitivity study would better support the robustness of the <10^{-4} M_p conclusion. For grain-size distributions with lower mm-wave opacity, the same flux upper limit would indeed correspond to a higher dust-mass upper limit. We will add a dedicated paragraph in §5 that (i) states the adopted opacities explicitly, (ii) notes that reduced opacity would relax the mass limits, and (iii) discusses why the ISM-like assumption remains the appropriate baseline for testing the rapid-growth hypothesis. A full parameter sweep is beyond the scope of the present work but will be flagged as future work. revision: partial

  2. Referee: [§3] §3 (radiative-transfer setup): the two disc sizes are motivated by hydrodynamical simulations and by Callisto’s orbit, respectively, but the paper does not demonstrate that these choices bracket the plausible range of circumplanetary material distributions. A brief sensitivity test (e.g., a factor-of-two change in outer radius or a different surface-density power-law index) would directly address whether the reported limits are conservative or optimistic.

    Authors: We will perform and report a brief sensitivity test in a new subsection of §3. This will include (a) varying the outer radius by a factor of two around each fiducial value while keeping total mass fixed, and (b) changing the surface-density power-law index from the adopted value. The resulting changes in the derived 3σ dust-mass upper limits will be quantified and compared with the 10^{-4} M_p threshold. revision: yes

  3. Referee: [§4] §4 (results): the upper limits are stated to fall below 10^{-4} M_p, yet the precise numerical values, their dependence on the assumed planet parameters, and the conversion from flux to mass for each of the two disc sizes are not tabulated or plotted; without these data it is difficult to assess how close the limits sit to the threshold or how they would shift under modest model variations.

    Authors: We agree that the numerical values and their dependence on planet mass, radius, and distance should be presented explicitly. We will add a new table in §4 that lists, for each target and each disc size: the 3σ flux upper limit, the corresponding dust-mass upper limit, the assumed planet parameters, and the conversion factor from flux to mass. A supplementary figure showing the mass limits versus planet mass will also be included. revision: yes

Circularity Check

0 steps flagged

No circularity: upper limits computed from external planet parameters and standard RT models

full rationale

The derivation computes ALMA flux upper limits into dust-mass upper bounds via radiative-transfer models whose inputs (planet mass, temperature, location, adopted disc radii 0.4/0.04 R_Hill, and standard dust opacities) are taken from direct-imaging literature and hydrodynamic simulations, not fitted to the present non-detections. The comparison to the 10^{-4} M_p satellite-formation threshold and the subsequent interpretive suggestion of rapid dust growth are downstream inferences, not self-definitional or fitted-input predictions. No load-bearing self-citation, uniqueness theorem, or ansatz smuggling is present in the reported chain; the result remains falsifiable against independent mass thresholds and disc-size assumptions.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim depends on two externally motivated disc size scalings, a benchmark dust-to-planet mass ratio drawn from Solar System analogy, and standard assumptions in radiative transfer for optically thin dust emission.

free parameters (1)
  • disc size scaling factors
    0.4 and 0.04 times Hill radius chosen from hydrodynamic simulations and Callisto's orbit; not fitted to the ALMA data.
axioms (1)
  • domain assumption Expected dust mass required to form regular satellite systems is 10^{-4} times the central planet mass
    Used as the comparison threshold for interpreting the upper limits as evidence of rapid growth.

pith-pipeline@v0.9.0 · 5847 in / 1294 out tokens · 38512 ms · 2026-05-25T13:59:18.833186+00:00 · methodology

discussion (0)

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Reference graph

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