{"id":"e58f608d-ab15-4def-8320-30333f2e9398","arxiv_id":"2509.00151","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":15,"one_line_summary":"Joint HARPS, NIRPS and CRIRES+ transit spectra of WASP-121b detect Fe, CO, and V, retrieve near-solar abundance ratios, and measure a -15 ± 3 km/s circulation offset favoring weak-drag circulation.","lead":"Astronomers combined three high-resolution spectrographs to watch the ultra-hot Jupiter WASP-121b pass in front of its star, detecting iron, carbon monoxide, and vanadium in its atmosphere. The same data yield a wind-induced shift of the planet's orbital signal that favors weak or absent atmospheric drag and a composition close to solar.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"ΔKp interpretation lacks injection/recovery calibration: 1D retrieval against 3D GCM spectra, so the circulation offset could be partly a model-mismatch artifact.","rationale":"The paper is carefully executed and honestly hedged, and I do not find a fatal internal inconsistency. The reader's weakest assumption—stellar-mass calibration of Kp_orbital—is real and is explicitly quantified in Table 6 and Figure 10. However, it is not the most load-bearing issue: even at the extreme literature masses 1.33 and 1.42 Msun, ΔKp remains -13±3 to -18±3 km/s, so the non-zero circulation offset and the exclusion of the specific strong-drag models considered are preserved. The stronger threat is methodological: the paper compares a 1D retrieval Kp to 3D GCM predictions without validating that the retrieval recovers the GCM-injected Kp offset. The retrieval's single rotational-broadening parameter can absorb phase-dependent 3D velocity structure, and the CCF peaks themselves are broad (SNR 4.7-5.8). The paper's own caveat in Section 5.1 that the blueshift 'might have a non-physical meaning' reinforces this. I therefore recommend keeping the reader's CONDITIONAL verdict: the interpretation should be accepted only after a GCM injection/recovery test demonstrates that the retrieval pipeline preserves the predicted ΔKp. My agreement with the reader is partial because I agree on the need for conditionality but disagree that the stellar mass is the single weakest link.","tokens_in":30166,"tokens_out":8732,"duration_ms":115414,"concrete_test":"Inject the four GCM transmission spectra used for Figure 10 (Parmentier et al. 2018 drag-free; Tan et al. 2024 drag-free, weak-drag, strong-drag) at the known orbital Kp=218.42 km/s and Vsys=38.35 km/s into the HARPS/NIRPS/CRIRES+ wavelength grids with realistic noise and telluric/continuum residuals, then run the exact Section 4.2 SCARLET retrieval. Check whether the recovered ΔKp matches the vertical-line GCM values within the retrieval uncertainty. Also run a null test with a 1D Keplerian, wind-free spectrum: if the retrieval returns a non-zero ΔKp, the pipeline itself biases the circulation measurement.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that ΔKp = -15±3 km/s is a real circulation signature and rules out strong drag rests on comparing a 1D retrieval Kp (Table 5: 202.99+2.84/-2.92 km/s) with an orbital Kp computed from an assumed Mstar (Section 5.3). The comparison is judged against vertical-line GCM predictions in Figure 10, but the paper never shows how those GCM predictions were mapped onto the retrieval's Kp definition, nor any injection/recovery test. The retrieval is 1D, isothermal, well-mixed, with 3D effects collapsed into a single rotational-broadening FWHM (Section 5.3); the authors themselves note that 3D effects are 'modelled as one free rotational broadening parameter'. If Wardenier/Tan/Parmentier GCM spectra with known input Kp are passed through this exact SCARLET retrieval, would the recovered Kp equal the input Kp minus the GCM's true ΔKp? Without that calibration, the observed offset could be biased by line-shape/continuum/rotational-broadening degeneracies rather than reflecting true atmospheric circulation. This is more load-bearing than the stellar-mass dependence flagged by the reader: across the 1.33-1.42 Msun range in Table 6, ΔKp stays negative and strong drag is still disfavoured, so the non-zero offset is robust to that calibration. The missing GCM-injection test directly threatens the physical interpretation.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript presents a joint analysis of high-resolution transit spectroscopy of the ultra-hot Jupiter WASP-121b obtained with HARPS, NIRPS, and CRIRES+, together with TESS and EulerCam photometry and a large radial-velocity dataset. The authors detect Fe, CO, and V via cross-correlation, run a free chemical retrieval with the SCARLET model, and update the system's orbital parameters. The headline result is a non-zero offset between the retrieved planetary velocity semi-amplitude and the orbital value, ΔKp = -15 ± 3 km/s (for Mstar = 1.38 ± 0.02 Msun), which they interpret as atmospheric circulation consistent with drag-free or weak-drag 3D GCM predictions. The retrieved abundance ratios are reported to be broadly consistent with a solar-composition chemical-equilibrium atmosphere at ~1e-4 to 1e-3 bar.","tokens_in":30652,"tokens_out":3737,"duration_ms":48299,"significance":"If correct, the paper adds a valuable multi-instrument, multi-wavelength data set for a benchmark ultra-hot Jupiter and uses the largest RV sample to date for WASP-121b. The explicit treatment of stellar-mass sensitivity in Table 6 and Figure 10, the use of a free retrieval with uniform abundance priors rather than equilibrium-chemistry priors, and the comparison with multiple GCM drag regimes are strengths. The ΔKp measurement is a potentially important dynamical constraint. However, the central dynamical interpretation currently lacks an end-to-end validation of the retrieval on simulated 3D GCM spectra, which is necessary to support the claim that the measured offset is a circulation signature rather than a model-mismatch artifact.","major_comments":[{"comment":"The central claim that ΔKp = -15 ± 3 km/s is a genuine circulation offset and rules out strong drag is not supported by an injection/recovery calibration. The retrieval is a 1D, isothermal, well-mixed SCARLET model in which all 3D dynamical effects are absorbed into a single free rotational-broadening FWHM (Table 5, §4.2). The GCM predictions are shown as vertical lines in Fig. 10, but the paper does not demonstrate that passing a synthetic GCM spectrum with a known input Kp through this exact retrieval returns the input Kp plus the GCM's true ΔKp, without bias from line-shape, continuum, or broadening degeneracies. Without this test, the observed offset could be partly a model-mismatch artifact. Please add such an injection/recovery test, or substantively weaken the strong-drag exclusion claim.","section":"§5.3, Fig. 10, Table 5"},{"comment":"The chemical conclusions rest on retrieval outputs that are not validated by injection tests. In particular, H2O is constrained to log10(H2O) = -6.52+0.49/-0.68 despite a non-detection in the CCF analysis (SNR ≲ 2, Fig. 7), and the paper itself notes the constraint arises because the retrieval 'prefers to add water.' Given that the same retrieval also collapses 3D effects into a single broadening parameter, the reported abundance ratios (e.g., log(H2O/CO) = -1.23 ± 1.00) may be subject to unquantified biases. An injection/recovery demonstration for the abundances would substantially strengthen the solar-composition conclusion. If such tests are not feasible, the composition claims should be framed as more tentative.","section":"§5.2, Table 5, Fig. 7"}],"minor_comments":[{"comment":"Typo: 'forth panel' should be 'fourth panel.'","section":"§5.2"},{"comment":"The retrieved temperature is reported as 2828+691/-238 K in Table 5 but as 2861+396/-418 K in the Conclusions. Please harmonize.","section":"§6 vs §5.2/Table 5"},{"comment":"The typesetting introduces spaces in 'W ASP-121b' throughout; this should be corrected to 'WASP-121b.'","section":"Throughout"},{"comment":"The vertical GCM predictions are shown without uncertainties or a precise definition of how each GCM's ΔKp was computed from the simulated spectra. Since the observational ΔKp is a single retrieval value, please specify the species, pressure weighting, and mapping used for the model lines.","section":"Fig. 10"},{"comment":"The prior on Kp is given as U(156, 276) km/s. A brief justification of this range, especially its lower bound relative to the expected ~218 km/s, would help readers assess prior influence on the retrieved value.","section":"§4.2"}],"recommendation":"major_revision","confidential_remarks":"The reader's conditional verdict is fair. The missing GCM-injection/recovery test is the key load-bearing issue; it is feasible and within the scope of a major revision. If the authors provide that calibration and the ΔKp offset survives, I would be supportive of acceptance. The stellar-mass dependence is already handled transparently in Table 6 and does not, by itself, undermine the non-zero offset. No concerns about scope or novelty."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper does solid observational work and reports the things it says it reports: first NIRPS transit of WASP-121b, a joint HARPS+NIRPS+CRIRES+ free retrieval, an updated orbital solution from the largest RV dataset to date (1261 points), and a non-zero ΔKp = -15 ± 3 km/s. The detections of Fe, CO, and V are confirmations, not discoveries, but the paper is honest about that. The abundance-ratio results (CO/Fe, V/Fe, H2O/CO) are consistent with a solar-composition, equilibrium-chemistry picture at ~1e-4–1e-3 bar, and the authors are appropriately cautious about absolute abundances. Credit where due: the RV fitting is thorough (GP detrending, multiple instruments treated separately), the retrieval setup is clearly described, and the paper explicitly discloses its main limitations, including the H- continuum degeneracy and the inability to constrain electron density. That is a trustworthy paper.\n\nThe soft spots are real but not fatal. Most important: the ΔKp interpretation compares a 1D retrieval Kp against an orbital Kp and judges it against 3D GCM predictions, but there is no injection/recovery test showing that a GCM spectrum with known input Kp would produce the same recovered offset when passed through this exact SCARLET retrieval. The 3D effects are folded into a single rotational-broadening FWHM, and the authors say as much, but they do not calibrate the bias this could introduce. The stellar-mass dependence they quantify in Table 6 is a secondary issue—across the published mass range ΔKp stays negative and strong drag remains disfavored—so the missing GCM-injection calibration is the load-bearing concern for the physical interpretation. A skeptical reviewer could argue the offset is partly a model-mismatch artifact, and the paper does not give them a direct answer. I would push the authors to add such a test or at least soften the claim.\n\nTwo smaller items. The abstract states the H2O abundance is 'constrained' while the body says the signal is effectively muted and the retrieval preference comes from a non-detection; that overstates the result. And the retrieved temperature is 2828+691/-238 K in Table 5 but 2861+396/-418 K in the Conclusions—an internal inconsistency that should be fixed.\n\nThe math checks out, the data handling is careful, and the claims are mostly proportional. This is a benchmark-target characterization that the hot-Jupiter community will want to read, and the ΔKp result will be cited and compared with. It deserves a serious referee, but the referee should require the injection/recovery test or a clear statement of why it is unnecessary before the circulation claim is taken at face value.","headline":"A careful, honest multi-instrument characterization of WASP-121b whose headline ΔKp circulation claim would be stronger with an injection/recovery test, and which has a minor internal inconsistency—still deserves a serious referee.","tokens_in":31533,"tokens_out":1347,"would_cite":false,"duration_ms":19345,"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":"WASP-121b's atmosphere is near-solar in composition and only weakly braked by drag.","keywords":["ultra-hot Jupiter","WASP-121b","transit spectroscopy","atmospheric retrieval","cross-correlation","H- continuum","atmospheric circulation","abundance ratios"],"falsifier":"Measure WASP-121's stellar mass independently to about 1 percent precision and recompute ΔKp; if the offset becomes consistent with zero or with the strong-drag global circulation model prediction, the circulation claim fails. A direct check would be phase-resolved Doppler mapping of Fe, V, and CO lines during transit to track the wind velocity field and compare it with the GCM-predicted day-to-night flow.","tokens_in":30082,"feed_emoji":"🪐","tokens_out":7077,"duration_ms":77925,"temperature":0.7,"pith_summary":"This paper combines high-resolution transit spectra of the ultra-hot Jupiter WASP-121b from three spectrographs across nine transits, together with TESS photometry and the largest radial-velocity dataset yet assembled for the system. The central claim is that the atmosphere probed near 10^-4 to 10^-3 bar is chemically close to a solar-composition atmosphere in equilibrium, and that its measured circulation offset — a negative 15 ± 3 km/s shift between the orbital velocity recovered from atmospheric lines and the velocity from stellar reflex motion — matches drag-free or weak-drag 3D circulation models, ruling out strong atmospheric drag. If true, WASP-121b's probed layers retain a largely primordial composition, and the Doppler shift of transit lines becomes a usable probe of how strongly ultra-hot Jupiter atmospheres brake their winds. The paper also updates the planet's orbital parameters and shows that water is partially dissociated, with its absorption muted by the H− continuum.","feed_headline":"WASP-121b's air is near-solar and barely braked","feed_subtitle":"A 15-km/s circulation offset and solar-like chemistry emerge from nine transits with three spectrographs.","key_machinery":"The load-bearing object is the velocity offset ΔKp = Kp(retrieved) − Kp(orbital), computed by comparing a cross-correlation and free-retrieval analysis of the planet's spectral lines with a Keplerian fit to 1261 radial velocities and five TESS sectors. The retrieval models absorption from H2O, CO, OH, Fe, V, TiO, H−, and electrons, fitting abundances together with temperature, Kp, systemic velocity, and rotational broadening, while reporting abundance ratios as the robust chemical outputs. The drag interpretation rests on a ladder of 3D circulation models — drag-free, weak-drag, and strong-drag regimes, parameterized by a drag timescale representing how long an air parcel takes to lose a sub","core_discovery":"On the paper's own terms, the core discovery is that WASP-121b's atmosphere, seen in transmission, is simultaneously near-solar in its volatile/refractory ratios and dynamically unbraked. Cross-correlation detects Fe, CO, and V with signal-to-noise ratios of 5.8, 5.0, and 4.7; a free retrieval on the combined datasets yields log(H2O/CO) = -1.23 (+1.00/-0.97), log(CO/Fe) = 1.04 (+0.80/-1.32), log(V/Fe) = -3.56 (+0.66/-1.11), and log(H−/Fe) = -3.98 (+0.81/-1.14), all consistent with 1× solar equilibrium chemistry at 10^-4 to 10^-3 bar; the slightly sub-solar H2O/CO is attributed to thermal dissociation into OH and O. The retrieval gives Kp = 202.99 (+2.84/-2.92) km/s, while a global fit to rad","pith_inferences":[],"forward_implications":["If the non-zero offset is real, strong atmospheric drag is excluded for WASP-121b's probed layers, and Doppler shifts of transit lines can measure how efficiently ultra-hot Jupiter atmospheres brake their winds.","The retrieved abundance ratios imply that the 10^-4 to 10^-3 bar atmosphere is close to solar equilibrium, with no strong alteration from rainout, photochemistry, or deep vertical mixing.","Water is present but partially dissociated and masked by the H− continuum, so near-infrared non-detections of H2O in ultra-hot Jupiters should not be read as absence.","The improved orbital solution, with stellar reflex motion Kb = 167.97 +5.78/-6.98 m/s, tightens all future atmospheric studies that depend on the ephemeris and the velocity scale.","The method — joint optical and near-infrared high-resolution transits with simultaneous photometry and radial velocities — can be repeated on other ultra-hot Jupiters to measure drag and composition together.","Inference — An independent stellar mass at roughly 1 percent precision would sharpen ΔKp enough to separate weak-drag from drag-free predictions; currently that separation is blurred by the adopted stellar mass.","Inference — The consistently blueshifted Fe, V, and CO signals suggest that phase-resolved transit spectroscopy could map wind speed as a function of pressure, giving a vertical profile of the drag timescale.","Inference — The H−/Fe degeneracy between extra hydride opacity and iron depletion via condensation might be broken by adding dayside emission spectra to the same retrieval; the present data leave that ambiguity open."],"supporting_citations":[{"why":"Provides the grid of drag-free, weak-drag, and strong-drag models whose predicted ΔKp values are compared with the measurement.","marker":"Tan et al. 2024"},{"why":"Provides the drag-free, weak-drag, and strong-drag model grid whose predicted ΔKp values anchor the interpretation.","marker":"Tan et al. 2024"},{"why":"Provides the drag-free GCM benchmark and the H2O dissociation and H− continuum chemistry used to interpret the abundance ratios.","marker":"Parmentier et al. 2018"},{"why":"Equation 1 is used to convert the fitted stellar reflex motion and adopted stellar mass into the planet's orbital Kp.","marker":"Torres et al. 2010"},{"why":"Source of the adopted stellar mass 1.38 ± 0.02 solar masses that calibrates the measured ΔKp.","marker":"Borsa et al. 2021"},{"why":"Supplies the inclination, ephemeris, and prior system parameters used in the global orbital fit.","marker":"Bourrier et al. 2020"},{"why":"Equilibrium-chemistry model grid used to compute solar-metallicity and solar C/O abundance ratios for comparison with the retrieved ratios.","marker":"Kitzmann et al. 2024"},{"why":"Establishes that relative abundances are more robustly constrained than absolute abundances, motivating the ratio-based chemical conclusions.","marker":"Gibson et al. 2022"},{"why":"Provides the data-processing and transmission-model recipe on which the retrieval analysis rests.","marker":"Pelletier et al. 2021"}],"fun_headline_variants":["WASP-121b's air is near-solar, barely braked","Ultra-hot Jupiter shows solar chemistry, weak drag","Near-solar air on WASP-121b, circulation barely braked","WASP-121b: solar-like sky, 15 km/s wind offset","Three spectrographs reveal WASP-121b's solar air"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The measured offset is calibrated by converting the star's wobble into the planet's orbital speed using an adopted stellar mass of 1.38 ± 0.02 solar masses; if that external mass is wrong by more than a few percent, the non-zero offset and the weak-drag versus drag-free interpretation shift appreciably.","fun_headline_variants_meta":{"raw":{"variants":["WASP-121b's air is near-solar, barely braked","Ultra-hot Jupiter shows solar chemistry, weak drag","Near-solar air on WASP-121b, circulation barely braked","WASP-121b: solar-like sky, 15 km/s wind offset","Three spectrographs reveal WASP-121b's solar air"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000513,"raw_usage":{"total_tokens":2454,"prompt_tokens":996,"completion_tokens":1458,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":740,"completion_tokens_details":{"reasoning_tokens":1372}},"tokens_in":740,"tokens_out":1458,"duration_ms":9739,"temperature":1.0,"reasoning_tokens":1372,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T13:54:27.984218+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure WASP-121's stellar mass independently to about 1 percent precision and recompute ΔKp; if the offset becomes consistent with zero or with the strong-drag global circulation model prediction, the circulation claim fails. A direct check would be phase-resolved Doppler mapping of Fe, V, and CO lines during transit to track the wind velocity field and compare it with the GCM-predicted day-to-night flow.","supporting_citations":[{"cited_title":"D., Batalha, N","cited_arxiv_id":null,"evidence_quote":"Provides the grid of drag-free, weak-drag, and strong-drag models whose predicted ΔKp values are compared with the measurement."},{"cited_title":"D., Batalha, N","cited_arxiv_id":null,"evidence_quote":"Provides the drag-free, weak-drag, and strong-drag model grid whose predicted ΔKp values anchor the interpretation."},{"cited_title":"R., Bean, J","cited_arxiv_id":null,"evidence_quote":"Provides the drag-free GCM benchmark and the H2O dissociation and H− continuum chemistry used to interpret the abundance ratios."},{"cited_title":"2021, The Astronomical Journal, 162, 73","cited_arxiv_id":null,"evidence_quote":"Provides the data-processing and transmission-model recipe on which the retrieval analysis rests."}],"review_version":1}