{"id":"70eaa09b-5a83-4c52-b43f-d9b7bacfd61f","arxiv_id":"2506.23795","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"A 10-Jupiter-mass planet flying through a protoplanetary disk creates observable spiral arms that differ between prograde (two arms) and retrograde (one arm) encounters.","lead":"This paper runs computer simulations of a giant free-floating planet (10 times Jupiter's mass) crashing through a young star's planet-forming disk. It predicts that the crash creates spiral patterns visible in infrared and radio images, with the number of arms depending on the direction of the planet's fall.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"No convergence test supports the central one-arm/two-arm morphology; the 144×60×144 grid resolves the disk scale height with only 1–2 cells at the interaction region, so the arm count may be numerical.","rationale":"The central claim is a qualitative morphological discriminator: prograde flybys produce two arms, retrograde produce one. The reader's stated weakest assumption is the fixed parabolic orbit and neglected self-gravity. I find that assumption less threatening: the disk is gravitationally stable (Toomre Q ≳ 10 across most of the disk; Q≈55 at 40 AU for the adopted Σ(r) and T(r)), and the dynamical-friction timescale for a 10 MJup planet moving at ~40 km/s through the midplane density of ~10^-12 g/cm^3 is ~10^5 yr, far longer than the ~10^2 yr encounter. So a fixed orbit is a reasonable first approximation. The absent resolution study is more damaging because the one-arm/two-arm distinction is a small morphological difference extracted from a grid that barely resolves the vertical structure at the interaction radius. The paper reports no convergence check, and the 'merging' of the second arm in the retrograde case is exactly the kind of behavior that numerical diffusion can produce. I therefore propose a concrete 2× resolution rerun as the single test that would settle the concern. If the morphology persists, the paper's claim would be substantially strengthened. If not, the headline prediction would be invalid. This does not change the reader's conditional verdict but sharpens the condition.","tokens_in":16378,"tokens_out":14257,"duration_ms":161869,"concrete_test":"Rerun the two fiducial models (q=5 AU, i=10°, ω=90°, prograde and retrograde) with at least 2× resolution in every dimension, e.g. 288×120×288, keeping the same physical setup, initial conditions, and viscosity prescription. Compare at t=200, 290, and 500 yr: (1) the number of spiral arms in the pole-on density isosurface and in the 3 µm synthetic images; (2) the density contrast of the hump at R=40 AU relative to the background; (3) the time at which the retrograde second hump merges with the first. If the one-arm/two-arm asymmetry and the contrast values persist within ~20%, the resolution concern is resolved; if the arm count changes or the contrast degrades substantially, the central claim is not converged and should be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central, novel prediction is that a prograde parabolic flyby leaves a two-armed spiral while a retrograde one leaves a single arm in pole-on images. This prediction is extracted solely from the PLUTO finite-volume runs on a 144×60×144 grid covering R∈[0.2,107.2] AU and θ∈[15°,165°]. The radial resolution is ΔR≈0.74 AU and the polar resolution is Δθ=2.5°, so near the perihelion region (q=5 AU, ω=90°), the vertical cell height is ≈0.22 AU while the local pressure scale height is H≈0.34 AU; at 10 AU the cell height is ≈0.44 AU and H≈0.8 AU. The disk scale height is therefore resolved by only 1–2 cells in exactly the region where the planet crosses the disk plane and excites the 'humps' that define the arms. The Hill radius of a 10 MJup planet at 5 AU (~0.75 AU) is comparable to one cell. No resolution study, no AMR, and no convergence check is reported. The retrograde case's one-arm pattern is explicitly attributed to the second hump 'merging' with the first; on an under-resolved grid with explicit viscosity and thermal conductivity, such merging can be a numerical diffusion effect rather than a physical one. Since the synthetic images are generated from the same grid, all observational claims inherit this resolution uncertainty. If the true resolution-converged morphology differs, the headline discriminator (one versus two arms, and by extension the prograde/retrograde diagnostic) would be invalid.","agreement_with_reader":"disagree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents three-dimensional gas-dynamical simulations of a protoplanetary disk (0.01 M_sun) around a solar-mass star perturbed by a 10-Jupiter-mass free-floating planet on a parabolic orbit. Using the PLUTO code on a 144×60×144 spherical grid, the authors follow the disk response for about 500 years after the encounter, then post-process density distributions with RADMC-3D and the CASA simulator to produce synthetic infrared and submillimeter images. The central claim is that a prograde flyby produces two observable spiral arms in pole-on images, while a retrograde flyby produces a single arm, with additional signatures such as disk warping and a trailing gas tail. Variations in pericenter distance, inclination, argument of pericenter, and planet mass are explored, and the paper concludes that such structures are observable in nearby star-forming regions but persist for only ~500 years.","tokens_in":16729,"tokens_out":9535,"duration_ms":97005,"significance":"If correct, this work establishes a new, morphologically distinctive diagnostic for free-floating planet encounters with protoplanetary disks: the number and handedness of spiral arms would indicate the encounter geometry. The use of state-of-the-art radiative transfer and ALMA image synthesis makes the predictions falsifiable with current facilities. The paper also demonstrates that such structures persist only ~500 years, implying that frequent encounters (e.g., via highly eccentric bound planets) would be needed for detection. The strengths are the broad parameter scan, the use of publicly available validated codes (PLUTO, RADMC-3D, CASA), and the explicit synthetic observations. The main caveat is the absence of a resolution study and the simplified treatment of the planet's orbit and disk self-gravity, which bear directly on the headline result.","major_comments":[{"comment":"The central one-arm versus two-arm discriminator is based on a single grid resolution of 144×60×144 cells over R∈[0.2,107.2] AU and θ∈[15°,165°]. In the region of interest (r≈5–20 AU), the vertical cell height is ~0.22–0.44 AU, while the disk pressure scale height H is ~0.34–0.8 AU; thus the disk thickness is resolved by only 1–2 cells. The Hill radius of a 10 M_Jup planet at 5 AU (~0.74 AU) is comparable to the radial cell width (~0.74 AU). The retrograde one-arm pattern is attributed to the second hump 'merging' with the first (Results, paragraph after Fig. 4), which could be a numerical diffusion artifact at this resolution. No convergence test or resolution study is presented. I request a resolution study, at least one higher-resolution run for the fiducial prograde and retrograde cases, to demonstrate that the arm count and merging behavior are converged.","section":"Model and method / Results"},{"comment":"The planet is assumed to move on a fixed parabolic orbit, and disk self-gravity is neglected. The disk mass (0.01 M_sun) is ~10.5 M_Jup, almost identical to the planet mass (10 M_Jup). The gravitational back-reaction of the disk on the planet could decelerate or deflect the planet during its two disk-plane crossings, changing the epochs and locations of the hump excitation; self-gravity could also modify spiral wave propagation. Please provide an order-of-magnitude estimate of the dynamical friction or deflection timescale against the crossing time (~10^2 yr) to justify the fixed-orbit approximation, or quantify the effect on the predicted morphology.","section":"Model and method"},{"comment":"The abstract claims that 'two spiral arms can be observed in case of the prograde fall, and one with retrograde case.' However, the Results state that 'In the inner part of the disk in both cases bright two-armed spirals are visible' (paragraph after Fig. 4). Thus the clean prograde/retrograde discriminator applies only to the outer part of the disk, or to the overall large-scale pattern, and not to the full pole-on image. Please reconcile the abstract with the detailed morphology and specify exactly which radial range or brightness level separates the two cases.","section":"Abstract / Results"}],"minor_comments":[{"comment":"Table 1 lists the unit of velocity as v0 = 4.74×10^5 cm s^-1 with the comment '2πL0/t0'; however, 2πL0/t0 = 2π × 1 AU / 1 year ≈ 2.98×10^6 cm s^-1, so the numerical value corresponds to L0/t0 without the factor 2π. Please correct the table or the value.","section":"Table 1"},{"comment":"The symbol ρ0 is used both as the normalization density (Table 1, 5.94×10^-7 g cm^-3) and as the midplane density scale in Eq. (6) and the subsequent text ('ρ0 = Σ0/√(2π) H0'). This double use is confusing; please rename one of them.","section":"Initial and boundary conditions / Eq. (6)"},{"comment":"In the Image generation section, the total fine-dust mass is stated as ~2×10^-5 M_sun, but later the text says the structures can be observed 'if the mass of fine dust is at least 10^-4 M_sun'. Please clarify which dust mass was used for the images in Figs. 8 and 9 and reconcile the factor 5 difference.","section":"Image generation / Results (Fig. 8)"},{"comment":"The tail of gas trailing the planet is claimed as a possible observable, but the authors note that 'due to the limited size of the computational domain, this tail is cut off'. As a result, the prediction for the tail's extent and brightness is incomplete; consider stating this caveat in the abstract or conclusions.","section":"Discussion / Conclusion"},{"comment":"The claim that the minimum planet mass for identifiable spirals is 3 M_Jup appears to be based on only two computed masses (1 and 10 M_Jup) plus possibly an intermediate run; please state the actual grid of masses used or rephrase as 'between 1 and 3 M_Jup' if no intermediate value was run.","section":"Results"}],"recommendation":"major_revision","confidential_remarks":"The paper is likely within scope for a specialist journal in planet formation. The main technical risk is the resolution issue; if the resolution study proves the arm morphology is converged, the paper could be acceptable after revisions. The authors should also address the self-gravity/back-reaction caveat more quantitatively, and correct the normalization table error and the dust-mass discrepancy. The abstract overstates the clean one- versus two-arm dichotomy given the inner two-armed structure in both cases."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The thing to know: this paper makes a concrete, testable prediction, and the authors state it plainly. For a 10-MJup parabolic flyby, pole-on images should show two spiral arms for a prograde encounter and one for a retrograde one. That is a genuinely useful diagnostic if true. The modeling chain is standard—PLUTO hydrodynamics, RADMC-3D radiative transfer, CASA image synthesis—and the paper is honest about uncertainties: the structures last only about 500 years, and the claimed tail of gas is cut off by the computational domain and inferred rather than simulated.\n\nThe serious soft spot is resolution. The grid is 144×60×144 over a radial range of 0.2 to 107.2 AU and a polar range of 15° to 165°. That gives ΔR ≈ 0.74 AU and, in the key interaction region near 5 AU, a vertical cell height of about 0.22 AU against a local pressure scale height of about 0.34 AU. So the vertical disk structure is resolved by only one or two cells. The planet's Hill radius at that radius is comparable to one cell. No convergence test, no AMR, no higher-resolution check is reported. The retrograde single-arm pattern is attributed to the second hump merging with the first; on such a coarse grid with explicit viscosity and thermal conductivity, that merging could just be numerical diffusion. I am not saying the result is wrong, only that the evidence is incomplete. The synthetic images inherit the same uncertainty.\n\nThe physical assumptions also bite. The planet is prescribed on a fixed parabolic orbit, and the disk's self-gravity is neglected even though the disk mass equals the planet mass. Back-reaction and deflection could matter. The authors list these as caveats, but they do not quantify the potential effect.\n\nThe citation pattern is fine. The authors build on their own prior infall simulations and on the stellar-flyby literature, and I do not see any obvious missing references.\n\nWho gets value from this? People working on flyby signatures might cite the proposed prograde/retrograde distinction, but I would not build on it until it passes a resolution test. The paper deserves a serious referee, but that referee should ask for at least two resolution levels and a discussion of the back-reaction. My recommendation: send it to peer review, but expect major revision. The idea is worth exploring; the current numerical evidence is not conclusive.","headline":"Clearly stated and potentially useful prediction (two vs one spiral arms for prograde vs retrograde flyby) but the grid is too coarse to support it; a convergence study is needed before trusting the arm-count claim.","tokens_in":766,"tokens_out":1586,"would_cite":false,"duration_ms":56661,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A 10-Jupiter-mass free-floating planet passing through a protoplanetary disk would leave a two-arm spiral if it moves with the disk's rotation and a one-arm spiral if it moves against it, along with a warped plane and a trailing gas…","keywords":["protoplanetary disks","free-floating planets","planet-disk interaction","numerical simulation","spiral arms","radiative transfer","synthetic ALMA observations","parabolic flyby"],"falsifier":"Target nearby young clusters with ALMA at 740 µm to search for disks with a single dominant spiral arm, an asymmetric warp, and a trailing gas tail. If surveys of several hundred disks in Taurus and Ophiuchus find no such morphology despite a substantial population of massive free-floating planets, the predicted ~500-year visibility window or the one-arm retrograde signature would be called into question.","tokens_in":16171,"feed_emoji":"🪐","tokens_out":7153,"duration_ms":66495,"temperature":0.7,"pith_summary":"This paper argues that a close encounter between a ~10-Jupiter-mass free-floating planet and a protoplanetary disk would leave observable scars: spiral arms when the disk is viewed pole-on, a warped disk plane when viewed edge-on, and a gas tail following the planet. The authors simulate the gas-dynamic response of a 0.01-solar-mass disk to a planet on a parabolic orbit, then post-process the density fields with radiative transfer and an ALMA simulator to produce synthetic infrared and submillimeter images. They find that a prograde flyby (planet moving with the disk's rotation) produces two spiral arms, while a retrograde flyby produces one, with the difference tracing how the disk's rotation merges the two disturbances. The structures remain identifiable for roughly 500 years, which is short compared with disk lifetimes, but the scenario repeats if the planet is on a highly eccentric bound orbit that crosses the disk each periastron passage.","feed_headline":"A 10-Jupiter flyby leaves one or two spiral arms in a disk","feed_subtitle":"A prograde flyby makes two arms, a retrograde one makes one—a new diagnostic of rogue-planet encounters in young disks","key_machinery":"The argument rests on a three-stage numerical pipeline. First, the PLUTO finite-volume code evolves the Euler equations with the star's potential plus a smoothed planetary potential (Klahr-Kley form) on a 144x60x144 spherical grid, with the planet on a fixed parabolic orbit—either prograde or retrograde relative to the disk's rotation. Second, RADMC-3D computes Monte Carlo radiative transfer through the resulting density field, assuming dust opacities for magnesium-iron silicates. Third, the CASA simulator turns the fluxes into synthetic ALMA images at 740 µm with realistic noise. The key physical mechanism is the gravitational wake: the planet raises two spiral humps at the two disk-plane crossings; the disk's rotation then determines whether the second hump survives as a separate arm (prograde) or merges into the first (retrograde). The diagnostic power comes from the one-arm/two-arm count, the warp asymmetry, and the trailing tail.","core_discovery":"The central claim is that a 10-Jupiter-mass planet passing through a protoplanetary disk creates a specific, observationally identifiable morphology: two spiral density humps on the disk surface for a prograde encounter, one for a retrograde encounter, plus a warped disk plane and a trailing gas tail, all detectable in infrared (3 µm) and submillimeter (740 µm) images at distances of ~140 pc. The distinction between one and two arms arises because the planet intersects the disk plane twice; for retrograde motion the second hump merges with the first under the disk's rotation, whereas for prograde motion the two remain separate. Detectability requires a planet of at least 3 Jupiter masses and a fine-dust mass of roughly $10^{-4}$ solar masses; close flybys (pericenter ~5 AU) favor single-arm spirals, while more distant encounters (~20 AU) make double-arm spirals resolvable. The authors also show that the flyby does not change the disk's accretion rate onto the star.","pith_inferences":["If an eccentric bound planet re-encounters the disk every ~680 years, the one-arm and two-arm morphologies could appear repeatedly in the same object; a time series of images could then pin down the unseen planet's orbital period and pericenter.","The mass threshold of ~3 Jupiter masses implies that surveys for such scars should prioritize disks around stars in dense clusters, where massive free-floating planets are most common; a null detection in a large cluster sample would constrain the product of encounter rate and 500-year visibility time.","The computational domain truncates the gas tail, so its full length and detectability remain open; a follow-up simulation with a larger radial grid could test whether the tail is visible in CO or HI line emission, not just continuum."],"forward_implications":["A pole-on image of a young disk with two bright spiral arms could be evidence of a recent prograde planetary flyby rather than an embedded planet or gravitational instability.","A single-arm spiral with a warped outer plane and a gas tail would point to a retrograde encounter, offering a way to infer the encounter geometry from a single snapshot.","The structures fade within ~500 years, so detections would imply either a very recent flyby or a planet on a highly eccentric bound orbit that re-intersects the disk every ~680 years, making repeated sightings possible.","ALMA Band 8 observations at ~140 pc can resolve the predicted spirals; single-arm spirals are best sought in close-flyby events, double-arm spirals in more distant passings."],"supporting_citations":[{"why":"Supplies the PLUTO finite-volume gas-dynamics code used for the disk evolution.","marker":"Mignone et al., 2012"},{"why":"Provides the smoothed planetary gravitational potential used to model the planet's effect on the gas.","marker":"Klahr, Kley, 2006"},{"why":"Gives the initial vertical density structure of the disk used to set up the simulations.","marker":"Nelson et al. (2013)"},{"why":"Supplies the disk mass normalization (0.01 solar masses) and typical disk properties.","marker":"Williams, Cieza, 2011"},{"why":"Provides the RADMC-3D Monte Carlo radiative-transfer code that converts density fields into synthetic fluxes.","marker":"Dullemond et al., 2012"},{"why":"Provides the magnesium-iron silicate dust opacity used in the radiative-transfer calculations.","marker":"Dorschner et al., 1995"},{"why":"Supplies the CASA simulator used to generate realistic ALMA images with thermal noise.","marker":"Petry, CASA Development Team, 2012"},{"why":"Establishes the comparison case of stellar flybys in protoplanetary disks, whose spiral and warp features the planetary flyby is contrasted with.","marker":"Cuello et al., 2020"}],"fun_headline_variants":["Flyby direction sets spiral arm count in protoplanetary disks","Prograde flyby yields two spiral arms, retrograde one","One or two spiral arms reveal a rogue planet's path through a disk","Spiral arm count in disk tells if flyby was prograde or retrograde"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The planet is treated as a point mass on a fixed parabolic orbit, and the disk gas and the disk's self-gravity never push back on the planet, even though the disk's mass (0.01 solar masses) is about the same as the planet's (10 Jupiter masses).","fun_headline_variants_meta":{"raw":{"variants":["Flyby direction sets spiral arm count in protoplanetary disks","Prograde flyby yields two spiral arms, retrograde one","One or two spiral arms reveal a rogue planet's path through a disk","Spiral arm count in disk tells if flyby was prograde or retrograde"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00082,"raw_usage":{"total_tokens":3590,"prompt_tokens":946,"completion_tokens":2644,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":562,"completion_tokens_details":{"reasoning_tokens":2567}},"tokens_in":562,"tokens_out":2644,"duration_ms":17833,"temperature":1.0,"reasoning_tokens":2567,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T21:32:20.708064+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Target nearby young clusters with ALMA at 740 µm to search for disks with a single dominant spiral arm, an asymmetric warp, and a trailing gas tail. If surveys of several hundred disks in Taurus and Ophiuchus find no such morphology despite a substantial population of massive free-floating planets, the predicted ~500-year visibility window or the one-arm retrograde signature would be called into question.","supporting_citations":[{"cited_title":"M., Muscianisi G","cited_arxiv_id":null,"evidence_quote":"Supplies the PLUTO finite-volume gas-dynamics code used for the disk evolution."},{"cited_title":"3D-radiation hydro simulations of disk-planet interactions","cited_arxiv_id":null,"evidence_quote":"Provides the smoothed planetary gravitational potential used to model the planet's effect on the gas."},{"cited_title":"Linear and non-linear evolution of the vertical shear instability in accretion discs //","cited_arxiv_id":null,"evidence_quote":"Gives the initial vertical density structure of the disk used to set up the simulations."},{"cited_title":"Protoplanetary Disks and Their Evolution //","cited_arxiv_id":null,"evidence_quote":"Supplies the disk mass normalization (0.01 solar masses) and typical disk properties."},{"cited_title":"P., Juhasz A., Pohl A., Sereshti F., Shetty R., Peters T., Commercon B., Flock M","cited_arxiv_id":null,"evidence_quote":"Provides the RADMC-3D Monte Carlo radiative-transfer code that converts density fields into synthetic fluxes."},{"cited_title":"Analysing ALMA Data with CASA // Astronomical Data Analysis Software and Systems XXI","cited_arxiv_id":null,"evidence_quote":"Supplies the CASA simulator used to generate realistic ALMA images with thermal noise."},{"cited_title":"Flybys in protoplanetary discs - II","cited_arxiv_id":null,"evidence_quote":"Establishes the comparison case of stellar flybys in protoplanetary disks, whose spiral and warp features the planetary flyby is contrasted with."}],"review_version":1}