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The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock

T0 review · 2 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read Two JWST eclipses show GJ 3929b's dayside matches a bare rock and rule out thick CO2 atmospheres.

desk verdict JWST MIRI eclipses give a likely bare-rock GJ 3929b; the CO2-exclusion claim is the model-dependent part to audit, but the core measurement is new and deserves refereeing. read the letter →

arxiv 2508.12516 v1 pith:2STKVSQT submitted 2025-08-17 astro-ph.EP

Qiao Xue , Michael Zhang , Brandon P. Coy , Madison Brady , Xuan Ji , Jacob L. Bean , Michael Radica , Andreas Seifahrt
show 9 more authors
Julian Sturmer Rafael Luque Ritvik Basant Nina Brown Tanya Das David Kasper Caroline Piaulet-Ghorayeb Eliza M.-R. Kempton Edwin S. Kite
This is my paper · ORCID
classification astro-ph.EP
keywords GJ3929bsecondaryeclipseterrestrialexoplanetbarerockJWSTMIRIbrightnesstemperatureatmosphericescapeatmosphere
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper reports two secondary eclipses of the rocky exoplanet GJ 3929b observed at 15 $\mu$m with JWST and combines these with fresh photometry and radial-velocity data. The joint eclipse depth gives a dayside brightness temperature $T_{\mathrm{p,dayside}} = 782 \pm 79$ K, consistent within uncertainties with the $T_{\mathrm{max}} = 737 \pm 14$ K ceiling for a zero-albedo, no-heat-redistribution black rock. The paper argues this agreement rules out CO2-rich atmospheres thicker than 100 mbar at more than $3\sigma$, so GJ 3929b has likely lost any significant secondary atmosphere. If correct, the planet is a directly observed bare rocky world, a useful calibration case for thermal emission from small exoplanets and for atmosphere-loss theory around M-dwarf planets.

What carries the argument

The load-bearing comparison is between the measured 15 $\mu$m brightness temperature and the bare-rock maximum temperature $T_{\mathrm{max}} = 737 \pm 14$ K, defined as the equilibrium dayside temperature of a body with zero Bond albedo and no heat redistribution. The observable that carries this comparison is the secondary-eclipse depth, the ratio of planet to stellar flux during eclipse, which is converted directly into a dayside temperature. The atmospheric limit is carried by a forward-model grid that maps the single 15 $\mu$m eclipse depth to a CO2 column, with thermal structure, composition, and heat-redistribution choices built in.

What would settle it

A decisive check is a 3–5 $\mu$m secondary eclipse of GJ 3929b: a bare rock should match the Rayleigh–Jeans extrapolation of the 15 $\mu$m temperature, while a CO2 layer above roughly 10 mbar would alter the 4.3 $\mu$m band and shift the inferred brightness temperature. A full 15 $\mu$m phase curve would also settle the matter, because the no-redistribution model predicts a large, sharply peaked day–night contrast while heat redistribution would flatten and phase-shift the curve.

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Extended reading notes

Core claim

The central claim is that GJ 3929b's dayside emits like a bare rock. A joint fit to two MIRI 15 $\mu$m secondary eclipses yields a dayside brightness temperature $T_{\mathrm{p,dayside}} = 782 \pm 79$ K, indistinguishable from the maximal value $T_{\mathrm{max}} = 737 \pm 14$ K expected when the planet absorbs all incident starlight and re-emits it from the dayside with no heat transport to the night side. Interpreting the same eclipse depths with atmospheric forward models, the paper excludes CO2-rich atmospheres thicker than 100 mbar at more than $3\sigma$ and concludes the planet has likely lost any significant secondary atmosphere. The accompanying radial-velocity measurements refine the

Load-bearing premise

The exclusion of CO2 atmospheres thicker than 100 mbar assumes the forward-model grid faithfully covers the range of atmospheres GJ 3929b could actually have—thermal structure, composition, clouds, and heat redistribution—so that one 15 $\mu$m eclipse depth maps uniquely and completely to a CO2 column.

Editorial extensions

If this is right

  • GJ 3929b joins the short list of rocky exoplanets with directly characterized dayside emission, and its brightness temperature leaves no room for a reflective or heat-circulating atmosphere.
  • CO2-rich secondary atmospheres with pressures at or above 100 mbar are excluded at more than $3\sigma$; only thinner, low-opacity atmospheres remain compatible with the data.
  • The updated ephemeris from photometry and radial velocities makes future eclipse observations predictable, and the two additional non-transiting signals must be folded into the system's dynamical model.
  • If the planet is truly airless, its 15 $\mu$m eclipse depth provides a reference point for the bare-rock null hypothesis in atmospheric interpretations of other small exoplanets.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A shorter-wavelength eclipse measurement near 3–5 $\mu$m would break degeneracies left by a single 15 $\mu$m point: a bare rock should follow the blackbody Rayleigh–Jeans tail, whereas even a thin CO2 layer would imprint structure near 4.3 $\mu$m. This test is not in the paper.
  • The sharpness of the 100 mbar exclusion rests on the atmospheric forward-model grid, which this version of the text does not display; the first step in evaluating the claim is to inspect that grid's assumed thermal profiles, opacities, clouds, and redistribution.
  • If the 6.1 d companion is confirmed, its gravitational pull will shift the eclipse times slightly, producing a testable prediction that the current near-circular ephemeris will need revision.
  • The bare-rock conclusion implies GJ 3929b is not currently outgassing a detectable atmosphere, which bears on its interior volatile budget; the authors do not draw that implication explicitly.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper reports JWST MIRI 15 um secondary-eclipse photometry of the terrestrial exoplanet GJ 3929b, reduced with an updated SPARTA pipeline, together with a new sector of TESS photometry and MAROON-X radial velocities. It finds eclipse depths of 177+47/-45 ppm and 143+34/-35 ppm, a joint dayside brightness temperature of 782 +/- 79 K, and a maximum bare-rock temperature of 737 +/- 14 K. The paper interprets the consistency of these values as evidence that GJ 3929b is likely a bare rock, and claims that CO2-rich atmospheres thicker than 100 mbar are excluded at >3 sigma. It also refines the planet parameters and reports a new non-transiting planet candidate in a 6.1 d orbit.

Significance. If the central atmospheric-model exclusion is robust, this is a significant result for the JWST Rocky Worlds program: a directly observed terrestrial exoplanet whose thermal emission is compatible with a zero-albedo, zero-redistribution bare rock, with a sharp upper bound on a CO2-dominated secondary atmosphere. The comparison to Tmax is parameter-free and therefore a useful benchmark. The additional MAROON-X evidence for a 6.1 d planet candidate is also notable. However, the sharpest claim, the >3 sigma exclusion of CO2 atmospheres, depends on an atmospheric forward-model grid that is not inspectable in the supplied text, so the published conclusion cannot currently be audited.

major comments (2)
  1. [Abstract, last sentence] The claim that CO2-rich atmospheres thicker than 100 mbar are excluded at >3 sigma is the paper's sharpest and most important result, but the atmospheric forward-model grid from which this limit is derived is not presented in the supplied text. The grid must specify the assumed thermal structure, composition, cloud/haze opacity, and heat redistribution, and it must map a single 15 um broadband depth to a CO2 column. If the grid omits temperature inversions or high-altitude haze, a thicker CO2 atmosphere can reproduce the observed 15 um depth. Please include the model grid, the calculated depth versus CO2-column relation, and the exact statistical threshold. Without this, the exclusion claim is not testable.
  2. [Full text] The supplied manuscript text is severely corrupted: most equations, tables, and figure labels appear as unreadable glyph arrays. In particular, the SPARTA reduction, the joint RV/TESS ephemeris fit, and the joint eclipse fit cannot be checked. This is not a cosmetic issue because the reported error bars, the eclipse timing, and the system parameters are load-bearing for the bare-rock conclusion. A clean, readable manuscript is an essential precondition for any substantive review.
minor comments (5)
  1. [Abstract] The individual eclipse depths (177+47/-45 ppm and 143+34/-35 ppm) agree within uncertainties, but the joint fit should explicitly state the per-visit systematic noise model and the weighting scheme. The supplied text does not allow the reader to verify whether the less precise visit is downweighted appropriately.
  2. [Abstract] The Tp=782 +/- 79 K versus Tmax=737 +/- 14 K consistency is only ~0.6 sigma and is degenerate with many atmospheric compositions. Please state explicitly that this consistency alone does not discriminate a bare rock, and that the atmospheric-model grid is the actual discriminator for the CO2 exclusion.
  3. [Full text] The text contains an extraneous header 'arXiv:2508.12510v1 [math.ST] 17 Aug 2025' and many character-corruption artifacts. Please provide a clean manuscript without these artifacts.
  4. [Figures/tables] All figures and tables appear as undecipherable numeric arrays. Captions, axis labels, and table column headers are needed to assess the fitted parameters, priors, and residuals.
  5. [Abstract] The newly identified 6.1 d planet candidate is announced without a significance assessment (for example, a false-alarm probability or model comparison) in the readable text. Please include such an assessment or label the candidate more cautiously.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the bare-rock comparison is a measured brightness temperature against an independent, parameter-free Tmax; the CO2-exclusion grid is a forward model, not a fitted quantity.

full rationale

The paper's central claim reduces to comparing a directly measured secondary-eclipse depth, converted to Tp,dayside via the Planck function, to Tmax computed from stellar and orbital parameters under zero Bond albedo and no heat redistribution. Tmax is not fitted to the eclipse depths, so the consistency is not definitional. The >3σ CO2-exclusion rests on an atmospheric forward-model grid that is not shown in the supplied text; while this raises auditability and model-completeness concerns, it is not circularity because no quoted equation identifies the 100 mbar limit with the measured depth by construction. The RV-refined ephemeris is used to time the eclipses, but the eclipse-depth fit is independent of the ephemeris value, and using the same campaign's RVs for timing is standard practice rather than a self-citation chain. No load-bearing self-citation or fitted-input-called-prediction was identified. The result is therefore self-contained against the external Tmax benchmark and receives a circularity score of 0.

Assumptions & free parameters 2 free parameters · 4 assumptions · 1 invented entities

The central measurement is direct photometry, and the bare-rock interpretation is anchored to a parameter-free thermal maximum, so the circularity burden is low. The sharpest claim, the >3 sigma exclusion of >100 mbar CO2, rests on atmospheric forward models that cannot be audited in the provided text. The 6.1 day planet candidate is a new inferred object with an independent falsifiable handle.

free parameters (2)
  • Secondary eclipse depth (joint fit) = Visit 1: 177 (+47/-45) ppm; visit 2: 143 (+34/-35) ppm; joint fit gives Tp,dayside = 782 +/- 79 K
    Fitted to MIRI photometry. This is the primary observable being measured, not an ad hoc nuisance parameter introduced to force a conclusion.
  • RV/TESS orbital elements for GJ 3929b and companions (period, semi-amplitude, epoch, eccentricity) = GJ 3929b period about 1.5 days (refined); additional signals at 15.0 d and 6.1 d
    Refined from new MAROON-X RVs and TESS data; used to predict the secondary eclipse time. Standard orbital fit with independent falsifiable handles (future RVs, transits).
assumptions (4)
  • domain assumption Zero Bond albedo and zero heat redistribution define the maximum possible dayside temperature benchmark Tmax = 737 +/- 14 K for GJ 3929b.
    Stated in the abstract as the bare black rock comparison point. The benchmark is parameter-free, which makes the bare-rock conclusion robust, but it assumes the planet emits as a blackbody at 15 um.
  • domain assumption The adopted stellar parameters (effective temperature, radius, luminosity) from prior literature determine Tmax to the quoted +/- 14 K.
    Tmax is computed from stellar inputs; an offset in stellar parameters shifts the benchmark and changes the significance of the bare-rock consistency.
  • domain assumption The atmospheric forward-model grid used to translate a single 15 um eclipse depth into the 100 mbar CO2 limit is complete and physically accurate.
    This is the load-bearing premise for the sharpest claim. The abstract references the exclusion without presenting the grid, and the provided text is unreadable, so the grid cannot be audited.
  • domain assumption SPARTA pipeline systematics are controlled at the tens-of-ppm level needed to recover a roughly 150 ppm eclipse depth.
    The eclipse depths are small relative to the stellar flux; unmodeled systematics of 30 to 50 ppm would shift both eclipse depths and the inferred dayside temperature.
invented entities (1)
  • Planet candidate 'GJ 3929b d' in a 6.1 day orbit independent evidence
    purpose: Explains an additional radial velocity signal in the system and sets up future dynamical studies
    A Keplerian signal in new MAROON-X RVs is a falsifiable handle: future radial velocities and possible transit searches can confirm or refute it. This is a standard inferred object from orbital fitting, not a new physics entity.

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Cite this review

Pith. "Pith review of The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock." pith.science (2026). https://pith.science/paper/2STKVSQT

@misc{pith2026250812516,
  author       = {Pith},
  title        = {Pith review of: The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2STKVSQT}},
  note         = {Machine review of arXiv:2508.12516}
}
read the original abstract

We report first results from the JWST Rocky Worlds Director's Discretionary Time program. Two secondary eclipses of the terrestrial exoplanet GJ 3929b were recently observed using MIRI photometric imaging at 15 um. We present a reduction of these data using the updated SPARTA pipeline. We also refine the planet mass, radius, and predicted time of secondary eclipse using a new sector of TESS data and new, high-precision radial velocities from the MAROON-X spectrograph. For the two JWST observations, we recover secondary eclipse depths of 177+47-45ppm and 143+34-35ppm at times consistent with a nearly circular orbit, as expected from the radial velocity data. A joint fit of the two visits yields a dayside brightness temperature Tp,dayside = 782+/-79K for GJ 3929b, which is consistent with the maximum brightness temperature Tmax = 737+/-14K for a bare, black rock (i.e., assuming zero Bond albedo and no heat redistribution). These results rule out CO2-rich atmospheres thicker than 100mbar at >3sigma, suggesting that GJ 3929b has lost any significant secondary atmosphere. The radial velocity data also indicate two additional non-transiting planets in the system: a previously-identified planet in a 15.0d orbit, and a newly-identified planet candidate in a 6.1d orbit.

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Forward citations

Cited by 4 Pith papers

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  1. Sulfur photochemistry observationally traces mantle redox states of rocky planets

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  2. GJ 3929 b as the First Complete Rocky Worlds DDT Data Set

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    GJ 3929 b's full four-eclipse JWST/MIRI data set yields an eclipse depth of 118±22 ppm, consistent with a bare rocky surface and ruling out thick CO2 atmospheres at >3σ.

  3. SCoRE: the Surface Composition of Rocky Exoplanets

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    In over 150,000 equilibrium crust-atmosphere models, the thermal stability of 23 minerals is tied to atmospheric type, independent of the six tested refractory-element abundance sets.

  4. Exploration of Exoplanet Atmospheres with the James Webb Space Telescope

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    A review of JWST's first cycles of exoplanet atmosphere observations, spanning direct imaging, transit spectroscopy, and the prospect of a ~10,000-hour population survey.

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1 extracted references · 1 canonical work pages · cited by 4 Pith papers

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