{"id":"7c87eee7-4353-4dec-8911-03c3f7b0f751","arxiv_id":"2603.02395","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"Planets in tight binaries may form by gravitational fragmentation of a massive disc before the secondary star forms; massive, early-formed planets survive as S-types, low-mass planets are ejected as free-floating planets.","lead":"This paper argues that planets in very close binary stars can form by gravitational fragmentation of a massive disc before the companion star is born, rather than after the binary exists. Simulations show that big or early-forming planets survive close to the main star while small, slowly migrating planets are flung out as free-floating planets.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Mass-dependent survival trend depends on untested migration/accretion split in Assumptions (iv)-(v); if low-mass fragments accrete, the central result may not hold.","rationale":"The reader's weakest_assumption identifies exactly the load-bearing issue: the migration/accretion split in Assumptions (iv) and (v) is necessary for the paper's central mass-dependent survival trend. The grid experiments show that in the absence of planetary gas accretion, massive planets migrate inward faster and survive, while low-mass planets are caught by the secondary. But this trend is only meaningful if low-mass fragments really do not accrete substantially. The authors rely on previous, not yet published, work (Nayakshin 2017b, Papers I and II) for this split, and the current paper does not test the sensitivity of the 28-run grid to allowing accretion on planetary-mass fragments. The paper's own caveats—inner-boundary artifacts, single stochastic realizations per grid point, selected proof-of-concept, and the explicit 'whether it can do so is unclear at the present' about producing the field binary population—are appropriately reflected in the CONDITIONAL verdict. The concern does not invalidate the paper; it identifies a concrete, testable assumption that must hold for the central claim. Therefore the reader's verdict remains CONDITIONAL, and no change is needed. The proposed test directly addresses whether the mass-dependent branching ratio survives when the accretion assumption is relaxed.","tokens_in":21784,"tokens_out":8741,"duration_ms":87212,"concrete_test":"Rerun the §5.2 grid (or at minimum the three representative cases Rp30_Mp0.1, Rp30_Mp1.0, and Rp90_Mp1.0) with gas accretion enabled for the planet-mass fragments, using the same Kley prescription as for the secondary but with a small accretion efficiency f_acc, e.g., 0.01, bracketed by 0.001 and 0.1. If the 0.1 M_J case grows to ≳1 M_J before the oligarch encounter and survives as an S-type planet, the central mass-dependent branching ratio is not robust to the assumed accretion split.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central mass-dependent survival trend (massive planets survive as S-types, low-mass planets ejected) is largely an output of the input assumptions (iv)-(v) in §2, not an independent result of the dynamics. In the §5.2 grid, all planetary-mass fragments (0.1-3 M_J) are treated as non-accreting test particles, while only the 12 M_J oligarch is given the Kley (1999) accretion prescription and grows to ~100 M_J. The paper's explanation is type I migration scaling t_mig ∝ M_p^-1: massive planets reach the inner stable region before the secondary arrives; low-mass planets are caught and ejected. But this only holds if low-mass fragments indeed accrete negligibly. The authors cite Nayakshin (2017b) and Paper II for this split, but those are unpublished companion papers and no sensitivity test of the accretion treatment is present here. If a 0.1 or 0.3 M_J fragment accretes gas on a timescale shorter than the secondary's growth/migration time, it could become a ~1 M_J planet, migrate faster, and survive, erasing the predicted deficiency of low-mass S-type planets. Both the observational comparison (deficiency of low-mass planets in tight binaries) and the predicted steep FFP mass function depend directly on this assumption. The paper itself only claims success 'under the assumptions spelled out in §2', so the central claim is conditional on a split that is plausible but not demonstrated here.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper proposes that planets in close binary systems form by gravitational fragmentation of a massive circumstellar disc before the secondary star is born. The authors use 2D FARGO-ADSG simulations: in §4 they present a proof-of-concept run in which the disc fragments ab initio, a planetary fragment migrates inward and survives as an S-type planet, and a more massive 'oligarch' fragment accretes to become the secondary; in §5 they present a 28-run grid in which a secondary seed and a single planet are injected into a non-fragmenting, gravito-turbulent disc. They report that planets with Mp ≳ 1–3 MJ preferentially survive as S-type planets in the resulting tight binary, while lower-mass planets are ejected as FFPs or left as wide P-type planets. They argue this explains the observed deficiency of low-mass planets in tight binaries and predicts that FFPs have a steeper mass function than bound planets in binaries.","tokens_in":22227,"tokens_out":6105,"duration_ms":61076,"significance":"If the result holds, the paper offers a novel and observationally motivated route to form planets in binaries with separations ≲20 au, a regime where core accretion faces severe difficulties. The §4 ab initio run, albeit selected post hoc, demonstrates that the proposed sequence is dynamically possible, and the §5.2 grid is a clean, clearly described parameter study. The authors are also commendably explicit about their assumptions and limitations. The falsifiable predictions — a deficiency of low-mass S-type planets and a steep FFP mass function — are a useful contribution. However, the central mass-dependent survival trend rests on assumptions (iv)–(v) of §2, which are not tested or independently justified within this manuscript, and the statistical basis of the trend is thin (one realization per grid cell).","major_comments":[{"comment":"The central mass-dependent survival trend is substantially imposed by the input assumptions. In §5.1, the 0.3, 1, and 3 MJ planets are explicitly not allowed to accrete gas, while the 10 and 12 MJ objects grow to ~0.1 Msun. The paper explains the trend by t_mig ∝ Mp^-1, but if a low-mass fragment accreted gas and became a few-MJ object before the secondary's migration, it would also migrate rapidly and could survive. The authors cite Nayakshin (2017b) and Papers I–II for this split, but those are not available to the reader (Papers I and II are listed as 'subm. to MNRAS'), and no sensitivity test of the accretion treatment is performed here. Since the observational interpretation (§6.3–6.4) depends directly on this split, the authors should either include runs with gas accretion enabled for low-mass fragments or provide a quantitative justification for neglecting it in these discs.","section":"§2, Assumptions (iv)–(v); §5.1, Fig. 6"},{"comment":"The paper's quantitative claims rest on very limited statistics. The §4 run was 'selected' as the one that produced the tightest binary and an S-type planet, which is a post hoc selection from 'a few such simulations' (the authors state the outcomes are 'quite stochastic' in §3). The §5.2 grid has 28 cells but only one realization per (Mp, Rp) pair. Given the acknowledged stochasticity of planet–secondary encounters, the statement that the branching ratio between S-type survival and FFP ejection is 'strongly planet-mass dependent' (§6.1) would be more convincing with at least a few repeat runs for representative cells (e.g., Rp30_Mp0.1 and Rp30_Mp1.0) or with a quantitative measure of run-to-run variance.","section":"§4 and §5.2; Fig. 8"},{"comment":"The abstract claims that 'survival depends strongly on formation time and mass', but formation time is not varied in the §5.2 grid: the planet and the secondary are injected simultaneously at t = 14 kyr. The only evidence for a formation-time effect comes from the single §4 case (P1 injected at 2.1 kyr, S1 at 3.9 kyr) and the Appendix A mass variations, which do not change injection times. To support the abstract claim, the grid should include a subset of runs in which the planet is injected earlier than the secondary, allowing the planet to migrate inward before the oligarch becomes massive.","section":"Abstract and §5.2"},{"comment":"The simulations place the inner boundary at R_in = 0.5 au (§4) or 1 au (§5), and the paper repeatedly notes that the final positions of S-type planets are influenced by open-boundary artifacts near R_in. The simulated binary separation is ~14–15 au, not the tightest observed systems such as DMPP-3 (a_bin ≈ 1.2 au) or KOI-1257 (a_bin ≈ 5.3 au), which the paper invokes in §6.2. The authors state that smaller secondary masses would produce tighter binaries but would require a smaller R_in. This means the regime most relevant to the most extreme observed systems is not actually simulated. I request either a subset of runs with a smaller inner boundary (to follow planets to <1 au) or a clearly stated argument for how the current results extrapolate to a_bin ≲ 5 au.","section":"§4, §5.2, §6.1; inner boundary artifacts"}],"minor_comments":[{"comment":"The terms 'S-type' and 'P-type' are used without explicit definitions in the text. Please define them at first use (e.g., S-type = planet orbiting one binary component; P-type = circumbinary).","section":"Throughout"},{"comment":"Typo: 'Assumng' should be 'Assuming'.","section":"§1"},{"comment":"The paper refers to 'P1', 'S1', and 'P2' before Table 1 is introduced; please ensure the reader can identify these objects at first mention.","section":"§4, Fig. 2 and Table 1"},{"comment":"In Fig. 8, the caption says 'Blue dots represent the P1 with purple bars indicating eccentricity' — this reads as a typo; presumably it should be 'represent the planets'.","section":"§5.2"},{"comment":"The sentence 'per Mdwarf star' should be 'per M-dwarf star'.","section":"§6.4"},{"comment":"The papers 'Calovic et al. 2025, subm.' and 'Nayakshin et al. 2025, subm.' are load-bearing for assumptions (iv)–(v). If they are not yet published, please provide the arXiv IDs or otherwise make them available to the reader.","section":"References"},{"comment":"The sentence 'Mp = 0.3 MJ planet (or any of the less massive planets that we experimented with)' refers to experiments not shown; the text would benefit from a brief statement about the mass range tested.","section":"§5.1"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is honest and the proposed scenario is interesting, but the central mass-dependent trend is heavily reliant on unpublished companion papers (Papers I and II). I would encourage the editor to consider whether the authors should be asked to include the relevant sensitivity tests (gas accretion on low-mass fragments, multiple realizations, formation-time variation) in the revised manuscript rather than deferring them to future work. The paper's scope is currently somewhat narrow: it demonstrates a mechanism in a selected, stochastic run and a limited grid, and the title's 'new paradigm' claim may be stronger than the evidence supports."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a serious hypothesis paper with a new quantitative result — a 28-run grid mapping S-type survival versus FFP ejection as a function of planet mass and initial radius, plus an ab-initio fragmenting-disc simulation that ends with a tight (~14 au) binary, an S-type planet, and an ejected FFP. The mass-dependent survival trend is clearly present: massive planets migrate inward fast enough to survive, low-mass planets get caught by the growing secondary and ejected. That qualitatively matches the observed deficit of low-mass planets in tight binaries, and the predicted steeper FFP mass function is genuinely testable.\n\nThe paper earns credit for honesty. It flags the inner-boundary artifact, states that the §4 run was selected, notes each grid cell is one stochastic realization, and hedges its claim with \"under the assumptions spelled out in §2.\" The Appendix sensitivity tests on planet and secondary masses are a real plus.\n\nThe soft spots are mostly addressable. The big one is Assumptions (iv)-(v): low-mass fragments don't accrete gas, ≳10 M_J fragments accrete in runaway. The whole survival trend follows from that split, and it is imported from Papers I and II and Nayakshin (2017b) — none published. If a 0.1 M_J fragment accreted gas it could migrate faster and survive, erasing the predicted low-mass deficiency. This is not circular — the simulations do produce the dynamics — but it is a load-bearing input assumption that is not independently tested here. A sensitivity run allowing some accretion on low-mass fragments would substantially stiffen the claim. Second, one realization per grid cell in a strongly stochastic disc limits the quantitative survival fractions — more runs would give error bars. Third, the inner-boundary effect means final semi-major axes close to the star are not reliable, though the survival/ejection distinction probably survives that.\n\nThe observational comparisons (γ Cep, HD 87646, HD 41004) are suggestive rather than quantitative, and the paper doesn't oversell them. The writing is clear.\n\nWho this is for: anyone working on planet formation in binaries, gravitational instability, or FFP populations. It deserves a serious referee; the main requests should be the missing sensitivity tests and multiple realizations, and the manuscript is honest enough that heavy revision is plausible rather than hopeless. I'd cite it as a hypothesis with testable consequences.","headline":"Serious hypothesis paper with a new quantitative survival map for planets in tight binaries; the central mass-dependent trend is real in the simulations but rests on an untested accretion/migration split from the authors' own unpublished papers.","tokens_in":22671,"tokens_out":2872,"would_cite":true,"duration_ms":28752,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Gas giants in tight binaries can form by disc fragmentation before the companion star exists, with survival depending sharply on planet mass.","keywords":["planet formation","close binaries","disc fragmentation","gravitational instability","free-floating planets","S-type planets","circumstellar discs","planetary migration"],"falsifier":"Measure the mass function of free-floating planets from microlensing surveys and compare it with the mass function of bound planets in tight binaries; if free-floating planets are not more bottom-heavy than bound planets, ejection of low-mass planets by the growing secondary is ruled out as the dominant channel.","tokens_in":21619,"feed_emoji":"🪐","tokens_out":3592,"duration_ms":36563,"temperature":0.7,"pith_summary":"Standard planet-formation models assume planets form after their host stars in stable discs, but close binaries truncate discs to only a few au, making classical formation nearly impossible. This paper argues that planet formation and binary formation are simultaneous outcomes of gravitational fragmentation in massive circumstellar discs. As the disc grows, it hatches planetary-mass fragments that migrate inward, while a dominant 'oligarch' fragment accretes in a runaway to become the secondary star, ejecting many low-mass planets. Simulations show that planets above roughly 1–3 Jupiter masses tend to survive as bound 'S-type' planets near the primary, while planets below about 0.1 Jupiter masses are usually ejected as free-floating planets. This offers a natural explanation for the observed scarcity of small planets in tight binaries and predicts that free-floating planets have a steeper mass function than bound planets in binaries.","feed_headline":"Disc fragmentation can forge planets before the binary forms","feed_subtitle":"Heavy planets migrate fast enough to survive; light ones get ejected — explaining the scarcity of small planets in tight binaries.","key_machinery":"The central mechanism is the 'oligarch fragment'—a fragment with mass ≳10 Jupiter masses that accretes gas in a runaway regime and becomes the secondary star. The argument runs on a race: in gravitationally unstable discs, type I migration speed scales roughly as planet mass, so massive planets reach the inner disc quickly, while low-mass planets lag behind and are caught by the oligarch. The Holman–Wiegert stability criterion marks the region where planetary orbits are unstable to ejection.","core_discovery":"On the paper's own terms, disc fragmentation can result in the formation of S-type planets in tight binary systems, provided the planets form before the secondary star is born. The branching ratio between survival as an S-type planet and ejection as a free-floating planet is strongly planet-mass dependent: the most massive planets tend to survive because they migrate inward fastest, reaching safe orbits close to the primary before the growing secondary can destabilize them, whereas low-mass planets migrate slowly and are typically ejected.","pith_inferences":["If this model holds, some hot Jupiters in binaries are effectively 'siblings' of the secondary star, having formed in the same fragmenting disc rather than by core accretion after binary formation.","A testable extension would be to run the same simulations with a gas-accretion prescription for low-mass fragments that depends on local disc conditions, instead of neglecting accretion below a few Jupiter masses; this would directly stress the central assumption.","The model implies that binary formation itself acts as a mass filter on the planet population, which could be probed statistically by comparing the super-Earth-to-super-Jupiter ratio among free-floating planets and among bound planets in tight binaries.","The proposed mechanism may also shape the brown-dwarf-to-planet ratio in binaries, since the oligarch channel preferentially forms massive secondaries while lighter fragments are dispersed."],"forward_implications":["Explains why dozens of gas giants exist in tight binaries even though the discs are truncated to only a few au.","Explains the observed deficiency of low-mass planets (≲0.1 Jupiter masses) in tight binaries compared with gas giants.","Predicts that free-floating planets produced by this channel have a steeper (more bottom-heavy) mass function than bound planets in binaries.","Provides a formation path for systems such as HD 87646, HD 72892, and HD 41004 without requiring planet formation inside the present, very small circumprimary disc.","Suggests that hot Jupiter formation in close binaries is suppressed but not impossible, while wider binaries may promote it through a related fragmentation mechanism."],"fun_headline_variants":["Born before the binary: planets from disc fragmentation","Fragmented discs forge planets in close binaries","Fast migration saves massive planets in binary systems","Disc fragmentation births planets, then ejects the light","New paradigm: planets and binaries co-form via fragmentation"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The model assumes that fragments less than a few Jupiter masses migrate inward much faster than they accrete gas, while heavier fragments accrete in a runaway regime; if low-mass fragments actually accreted gas substantially, the mass-dependent survival trend would not follow.","fun_headline_variants_meta":{"raw":{"variants":["Born before the binary: planets from disc fragmentation","Fragmented discs forge planets in close binaries","Fast migration saves massive planets in binary systems","Disc fragmentation births planets, then ejects the light","New paradigm: planets and binaries co-form via fragmentation"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000196,"raw_usage":{"total_tokens":1196,"prompt_tokens":741,"completion_tokens":455,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":485,"completion_tokens_details":{"reasoning_tokens":393}},"tokens_in":485,"tokens_out":455,"duration_ms":4569,"temperature":1.0,"reasoning_tokens":393,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T19:21:46.328507+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the mass function of free-floating planets from microlensing surveys and compare it with the mass function of bound planets in tight binaries; if free-floating planets are not more bottom-heavy than bound planets, ejection of low-mass planets by the growing secondary is ruled out as the dominant channel.","supporting_citations":[],"review_version":1}