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The Impact of Different Haze Types on the Atmosphere and Observations of Hot Jupiters: 3D Simulations of HD 189733b, HD209458b and WASP-39b

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Pith's one-line read 3D simulations show that 1.5-nanometre haze, carried by the superrotating jet and trapped in nightside vortices, can make the morning terminator of a hot Jupiter more opaque than the evening terminator.

desk verdict Solid 3D haze modeling with a testable morning-limb prediction that is a scenario, not a robust forecast. read the letter →

arxiv 2507.20366 v1 pith:ZHNBLV45 submitted 2025-07-27 astro-ph.EP

Mei Ting Mak , Denis Sergeev , Nathan Mayne , Maria Zamyatina , Maria E. Steinrueck , James Manners , Eric Hebrard , David K. Sing
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Krisztian Kohary
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keywords hotJupitersphotochemicalhaze3Dgeneralcirculationmodelsadvectionandsettlingtransmissionspectraterminatorasymmetrysuperrotatingjetnightsidevortices
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The pith

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

The reading

This paper uses three-dimensional atmospheric simulations to argue that tiny (1.5 nm) photochemical haze particles, once produced on the dayside of a hot Jupiter, are carried by the equatorial superrotating jet to the nightside and trapped by vortices there, so the morning terminator ends up more opaque than the evening terminator. The authors show this pattern holds across three benchmark hot Jupiters and three haze compositions, with the same dynamical rules: the jet sets the day-night gradient, eddies set the latitude pattern, and the decomposed flow sets the fine structure. The observable payoff is a terminator asymmetry in transit depth that is strongest in the UV-optical; for WASP-39b with Titan-like or water-world-like haze, the morning limb is deeper than the evening limb, which the paper proposes as a haze indicator. If true, this gives observers a way to detect and roughly size haze in hot Jupiter atmospheres without relying only on the spectral slope.

What carries the argument

The machinery is the transported haze tracer in the 3D general circulation model: the haze mass mixing ratio changes by advection, gravitational settling, a fixed log-normal production term peaking near 0.005 mbar, and a destruction boundary below 100 mbar. The physical load-bearing feature is that a 1.5 nm spherical particle is advected faster than it settles, so the circulation, not gravity, places the haze. To attribute the placement, the horizontal wind is decomposed into divergent, jet, and eddy components; the eddy mass flux convergence piles haze at the jet edges, while cyclonic nightside vortices near the morning terminator trap it there.

What would settle it

Measure WASP-39b's transmission spectrum separately at the morning and evening limbs in the 0.3-0.5 micron range: if the morning-limb transit depth is not larger than the evening-limb depth under a Titan-like or water-world-like haze spectral shape, the prediction fails. A second check is particle microphysics: if the haze particles are retrieved or computed to be at least 30 nm in radius or fractal agglomerates rather than 1.5 nm spheres, the morning-terminator opacity enhancement is expected to disappear or reverse.

Watch

Extended reading notes

Core claim

The central claim is that a single dynamical picture governs where nanometre-sized photochemical haze sits in all three simulated hot Jupiters, regardless of haze type or whether the haze's radiative feedback is switched on. The equatorial superrotating jet exports haze from the dayside production region to the nightside; eddies, measured by the eddy mass flux, concentrate it at mid-latitudes near the jet edges; and the divergent plus eddy components of the wind shape the remaining longitudinal structure. Because the planets are tidally locked, the nightside vortices sit near the morning terminator and trap the haze, giving the morning limb a stronger haze opacity. In the WASP-39b runs with Titan-like or water-world-like haze this opacity wins over the hotter evening-limb temperature in the UV-optical, producing a larger morning-limb transit depth; with the more strongly absorbing soot-like haze the evening limb stays deeper at all wavelengths. The same simulations show that stronger haze absorption strengthens the jet, evens out the day-night haze contrast, and increases the transit depth while muting spectral features.

Load-bearing premise

The whole morning-terminator prediction rests on the assumption that the haze particles are 1.5 nm spheres whose advection time is shorter than their settling time, with coagulation neglected; the paper states that for particles of about 30 nm or larger settling dominates and the limb asymmetry reverses.

Editorial extensions

If this is right

  • If hot-Jupiter hazes are nanometre-sized and non-fractal, the morning terminator should be the more opaque limb in the UV-optical for planets whose haze is not strongly absorbing.
  • A morning-limb transit-depth excess across the UV-optical could serve as a haze indicator that is not spectrally degenerate with cloud opacity or temperature-driven disequilibrium chemistry.
  • Stronger haze absorption, such as soot-like haze, predicts a stronger superrotating jet, a more homogenised day-night haze distribution, and a larger transit depth with muted spectral features.
  • For HD 189733b and HD 209458b the evening limb remains deeper than the morning limb despite more morning haze, because the evening terminator is hotter, so a morning-limb excess is not predicted to be universal among hot Jupiters.
  • The same three-way control by jet, eddies, and decomposed wind should apply to other tidally locked hot Jupiters with small-particle haze, though the specific distribution pattern will be planet-specific.

Reading between the lines

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

  • If the WASP-39b morning-limb excess is confirmed in the UV-optical, a natural extension is to search for the same signature in other low-gravity, high-metallicity hot Jupiters where Titan-like haze is plausible; the paper's mechanism implies the asymmetry amplitude should track the eddy mass flux at the jet edge.
  • The fixed production profile and single particle radius mean the most decisive test is microphysical: computing or measuring the haze size distribution would determine whether the advection-dominated regime assumed here is the right one for a given planet.
  • A retrograde jet or a morning-side cloud opacity could mimic the morning-limb signal, so the wavelength dependence matters: haze should produce a smooth UV-optical rise, whereas clouds and chemical abundance contrasts should produce different spectral signatures.
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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 presents 3D GCM simulations of three hot Jupiters (HD 189733b, HD 209458b, WASP-39b) including a parameterized haze production/removal scheme with three different haze optical properties (Titan-like, water-world-like, soot-like), each in radiatively passive and active configurations. The central dynamical claim is that in every simulated case the haze spatial distribution is set by three mechanisms: the superrotating jet controls the day-to-night distribution, eddies control the latitudinal distribution, and the decomposed divergent/eddy wind components control the finer structure. The radiative feedback of stronger-absorbing haze strengthens the jet, reduces day-night haze contrast, and increases the transit depth. The key observational claim is that for WASP-39b with Titan-like or water-world-like haze, the morning terminator shows a larger transit depth than the evening terminator in the UV-optical, which the authors propose as a strong indicator of the presence of small-particle haze.

Significance. If the results hold, this is one of the few 3D studies of haze radiative feedback on hot-Jupiter circulation and spectra, and it offers a falsifiable observable diagnostic (limb asymmetry in the UV-optical) that is not produced by clouds or disequilibrium chemistry in the paper's own tests. The work is carefully executed within its stated assumptions: the simulations are internally consistent, the wind decomposition analysis is informative, the comparison with Steinrueck et al. (2023) is a valuable cross-model check, and the underlying data are publicly archived (Zenodo, Mak 2025). The significance is tempered by the acknowledged sensitivity of the headline limb-asymmetry prediction to particle size and production rate, which the paper does not map quantitatively.

major comments (2)
  1. [Sec. 4.2, Sec. 3.3.1, Abstract]
  2. [Sec. 2.3, Sec. 3.2, Fig. 18]
minor comments (5)
  1. [Sec. 4.3]
  2. [Sec. 3.3.1, Fig. 18]
  3. [Sec. 2.3]
  4. [Fig. 22]
  5. [Table 3]

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the haze-distribution and limb-asymmetry results are emergent GCM outputs, not fitted or redefined inputs.

full rationale

All three headline findings (jet-controlled day-night transport, eddy-controlled latitudinal distribution, and decomposed-wind fine structure) are diagnostics of a 3D GCM integration with a prescribed haze source/sink and Mie-derived optical properties. The production term (Eq. 2) and loss term (Eq. 3) are inputs, but the claimed night-side accumulation, vortex trapping, and morning-terminator opacity are not encoded in those inputs; they emerge from the modeled circulation. The synthetic transmission spectra are compared with observations only after a common additive offset chosen on the haze-free case (Sec. 3.3.1; the same offset is applied to active cases), so the morning-evening limb difference is internal and unaffected by that offset. The paper explicitly decomposes the limb asymmetry into haze-only spectra (full minus clean) and gas-temperature contributions (Sec. 3.3.2, Fig. 22), which is an analysis of the simulation, not a fitting of the conclusion. Self-citations to Steinrueck et al. (2021, 2023), Mak et al. (2023, 2024), and Zamyatina et al. are used for model parameterizations and comparisons; these are prior modeling choices with independent content, not uniqueness theorems or fitted predictions. The acknowledged sensitivity to particle radius (>=30 nm reverses the asymmetry), production rate, and neglect of coagulation/clouds is a robustness caveat, not a circular reduction. Therefore no step in the derivation reduces to its own input by definition or by fitted-parameter renaming.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

The central results are simulation outputs, not derived from first principles. They depend on a small set of adopted parameters (production rate, particle radius, production profile, loss timescale) and on modeling choices (no clouds, no chemistry, capped production). The paper acknowledges these dependencies explicitly.

free parameters (6)
  • Column haze mass production rate F0 = 1e-12 kg m-2 s-1
    Fixed production rate adopted, not fitted, but central. Authors note lower values like 1e-15 to 1e-16 kg m-2 s-1 would weaken opacity and preserve spectral features, so results depend on this choice (Sec. 2.1, Sec. 3.3.1).
  • Haze particle radius = 1.5 nm
    Fixed particle radius for all haze types. Small size makes advection dominate over settling, directly producing the morning-terminator accumulation. Authors state particles >=30 nm would reverse the limb asymmetry (Sec. 2.2, Sec. 4.2).
  • Median production pressure m = 0.005 mbar
    Adjusted downward from Steinrueck et al.'s 0.002 mbar for model stability. Shifts peak production above the dayside upper boundary for two planets, with production capped there (Sec. 2.1).
  • Production distribution width sigma = 0.576
    Adopted from Steinrueck et al. (2021, 2023) to match their production profile width (Sec. 2.1).
  • Haze loss timescale tau_loss = 1e3 s
    Haze removal timescale below p_deep, adopted from Steinrueck et al. (2021, 2023) (Sec. 2.1).
  • Deep haze removal boundary p_deep = 100 mbar
    Pressure below which haze is removed, adopted from Steinrueck et al. (2021, 2023) (Sec. 2.1).
assumptions (6)
  • domain assumption Haze particles are spherical and Mie scattering applies.
    Optical properties are computed via Mie theory. Authors note fractal agglomerates or DDA can change absorption, especially for soot (Sec. 2.2, Sec. 4.2).
  • domain assumption Haze production follows a fixed log-normal pressure profile proportional to cos(zenith angle), independent of chemistry and radiative feedback.
    Equation 2; no photochemical network, no self-consistent haze formation. Authors state this may overestimate haze MMR (Sec. 2.1, Sec. 4.1).
  • domain assumption No clouds, UV photolysis, thermal chemistry, or aerosol-cloud microphysics are included.
    Authors isolate haze effects but note clouds could produce spectrally degenerate flat spectra and limb asymmetry (Sec. 2.3, Sec. 4.4).
  • domain assumption The model reaches a pseudo-steady state in the upper atmosphere after 1200 Earth days; the deeper atmosphere is still evolving.
    Pseudo-steady state is defined by <=1% changes; analysis focuses on p<=10 mbar; deeper layers may not be converged (Sec. 2.3).
  • ad hoc to paper Upper boundary truncation and capping of haze production above the dayside top has little effect on final haze concentration.
    For HD189733b and WASP-39b the peak production lies above the model top. Authors assert capping has little effect, supported only by comparison with Steinrueck et al. 2023 (Sec. 2.3, Sec. 4.3).
  • domain assumption Linear additive offset of synthetic spectra to match observed water features does not affect relative limb differences.
    Spectra are adjusted vertically to match observations; the offset cancels in limb differences but affects full-spectrum comparison (Sec. 3.3.1).

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Cite this review

Pith. "Pith review of The Impact of Different Haze Types on the Atmosphere and Observations of Hot Jupiters: 3D Simulations of HD 189733b, HD209458b and WASP-39b." pith.science (2026). https://pith.science/paper/ZHNBLV45

@misc{pith2026250720366,
  author       = {Pith},
  title        = {Pith review of: The Impact of Different Haze Types on the Atmosphere and Observations of Hot Jupiters: 3D Simulations of HD 189733b, HD209458b and WASP-39b},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZHNBLV45}},
  note         = {Machine review of arXiv:2507.20366}
}
read the original abstract

We present the results from the simulations of the atmospheres of hot-Jupiters HD189733b, HD209458b and WASP-39b, assuming the presence of three different types of haze. Using a 3D General Circulation Model, the Unified Model, we capture the advection, settling and radiative impact of Titan-like, water-world-like and soot-like haze, with a particle radius of 1.5 nm. We show that the radiative impact of haze leads to drastic changes in the thermal structure and circulation in the atmosphere. We then show that in all our simulations, 1) the superrotating jet largely determines the day-to-night haze distribution, 2) eddies drive the latitudinal haze distribution, and 3) the divergent and eddy component of the wind control the finer structure of the haze distribution. We further show that the stronger the absorption strength of the haze, the stronger the superrotating jet, lesser the difference of the day-to-night haze distribution, and larger the transit depth in the synthetic transmission spectrum. We also demonstrate that the presence of such small hazes could result in a stronger haze opacity over the morning terminator in all three planets. This could lead to an observable terminator asymmetry in WASP-39b, with the morning terminator presenting a larger transit depth than the evening terminator. This work suggests that, although it might not be a typical detection feature for hot-Jupiters, an observed increase in transit depth over the morning terminator across the UV and optical wavelength regime could serve as a strong indicator of the presence of haze.

Figures

Figures reproduced from arXiv: 2507.20366 by the authors.

Figure 1
Figure 1. Comparison of complex refractive indices (i–ii), extinction effi￾ciency (iii) and single scattering albedo (iv) from Titan-like (Khare et al. 1984; He et al. 2022), water-world-like (He et al. 2024) and soot-like haze (Lavvas & Koskinen 2017). The extinction efficiency (iii) and single scatter￾ing albedo (iv) are calculated from the Mie theory using SOCRATES (see details in Sec. 2.3). 2.3 3D Simulations We use the U… view at source ↗
Figure 2
Figure 2. Zonal mean of haze mass mixing ratio as a function of latitude and pressure for all cases of HD 189733b, HD 209458b and WASP-39b. The haze mass mixing ratio is expressed in log10 scale. “[p]” and “[a]” represent the passive and active haze case, respectively. “Ww” stands for water-world. 10 9 10 8 10 7 10 6 10 5 Haze mass mixing ratio 10 1 10 0 Pressure [mbar] (i) HD 189733b [p] Titan-like [a] Ww-like [a] Soot-like … view at source ↗
Figure 3
Figure 3. Dayside- (solid) and nightside-mean (dotted) haze mass mixing ratio profiles for all cases of HD 189733b, HD 209458b and WASP-39b. “[p]” and “[a]” represent the passive and active haze case, respectively. “Ww” stands for water-world. MNRAS 000, 1–28 (2025) [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figures from the paper (19 more)
Figure 4
Figure 4. Figure 4: Zonal mean zonal wind velocity, as a function of latitude and pressure, with contours showing the haze MMR in logarithmic scale for all cases of HD 189733b, HD 209458b and WASP-39b. Only 𝑙𝑜𝑔10(haze MMR)>-10 is shown here. “[p]” and “[a]” represent the passive and activ…
Figure 5
Figure 5. Figure 5: Zonal mean eddy mass flux, as a function of latitude and pressure, with contours showing the haze MMR in logarithmic scale for HD 189733b, HD 209458b and WASP-39b, following the format of [PITH_FULL_IMAGE:figures/full_fig_p009_5.png]
Figure 6
Figure 6. Figure 6: Mass mixing ratio of haze of HD 189733b for all cases at three isobaric surfaces, namely pressures of 0.15, 1 and 10 mbar. The haze mass mixing ratio is expressed in log10 scale. Note that the colour scales are different between different panels. “[p]” and “[a]” repres…
Figure 7
Figure 7. Figure 7: Vertical air velocity (contours) and horizontal wind (arrows) from simulations of HD 189733b, following the format of [PITH_FULL_IMAGE:figures/full_fig_p011_7.png]
Figure 8
Figure 8. Figure 8: Divergent (left column), jet (middle column) and eddy (right column) component of the horizontal wind at a pressure of 0.15 mbar for all cases of HD 189733b. The active haze cases are presented as the difference (indicated by Δ) of the wind velocity of the active haze …
Figure 9
Figure 9. Figure 9: Mass mixing ratio of haze of HD 209458b for all cases at four isobaric surfaces, namely pressures of 0.001, 0.01, 0.1 and 1 mbar. The haze mass mixing ratio is expressed in log10 scale. Note that the colour scales are different between different panels. “[p]” and “[a]”…
Figure 10
Figure 10. Figure 10: Vertical air velocity (contours) and with horizontal wind (arrows) from simulations of HD 209458b, following the same format of [PITH_FULL_IMAGE:figures/full_fig_p014_10.png]
Figure 11
Figure 11. Figure 11: Components of the horizontal wind at a pressure of 0.001 mbar for all cases of HD 209458b, following the format of [PITH_FULL_IMAGE:figures/full_fig_p015_11.png]
Figure 12
Figure 12. Figure 12: Mass mixing ratio of haze of WASP-39b for all cases at three isobaric surfaces, namely pressures of 0.02, 0.1 and 1 mbar. The haze mass mixing ratio is expressed in log10 scale. Note that the colour scales are different between different panels. “[p]” and “[a]” repres…
Figure 13
Figure 13. Figure 13: Vertical air velocity (contours) and with horizontal wind (arrows) from simulations of WASP-39b, following the format of [PITH_FULL_IMAGE:figures/full_fig_p017_13.png]
Figure 14
Figure 14. Figure 14: Components of the horizontal wind at a pressure of 0.001 mbar for all cases of WASP-39b, following the format of [PITH_FULL_IMAGE:figures/full_fig_p018_14.png]
Figure 15
Figure 15. Figure 15: Dayside-mean shortwave (SW) heating rate (dotted) and longwave (LW) cooling rate (solid) for all cases of HD 189733b, HD 209458b and WASP-39b. “[p]” and “[a]” represents the passive and active haze case, respectively. “Ww” stands for water-world. MNRAS 000, 1–28 (2025…
Figure 16
Figure 16. Figure 16: Dayside-mean net heating rate for all cases of HD 189733b, HD 209458b and WASP-39b. “[p]” and “[a]” represents the passive and active haze case, respectively. “Ww” stands for water-world. 800 1000 1200 1400 1600 Temperature [K] 10 5 10 4 10 3 10 2 10 1 10 0 Pressure […
Figure 17
Figure 17. Figure 17: Dayside- (solid) and nightside-mean (dotted) thermal structures for all cases of HD 189733b, HD 209458b and WASP-39b. “[p]” and “[a]” represents the passive and active haze case, respectively. “Ww” stands for water-world. HD 189733b, Titan-like or water-world-like haz…
Figure 18
Figure 18. Figure 18: Transmission spectra of HD 189733b (i-ii), HD 209458b (iii–iv) and WASP-39b (v–vi) showing the full (1st column) and limb (2nd column) spectrum. “[a]” represents the active haze case. A vertical dotted line at 0.4 𝜇m marks the wavelength where the discontinuity in the…
Figure 19
Figure 19. Figure 19: Morning (solid – between longitude of 80◦ and 100◦ ) and evening (dash-dotted – between longitude of 260◦ and 280◦ ) terminator-mean thermal structures for (i) HD 189733b, (ii) HD 209458b and (iii) WASP-39b. “[p]” and “[a]” represents the passive and active haze case,…
Figure 20
Figure 20. Figure 20: Morning and evening terminator-mean haze mass mixing ratio profiles, following the format of [PITH_FULL_IMAGE:figures/full_fig_p021_20.png]
Figure 21
Figure 21. Figure 21: Limb transmission spectra for all cases of WASP-39b plotted against observations from Espinoza et al. (2024). Note that the scale is different in the 1 st column. “[a]” represents the active haze case. A vertical dotted line at 0.4 𝜇m marks the wavelength where the di…
Figure 22
Figure 22. Figure 22: Limb transmission spectra of WASP-39b in all active haze cases. i, iv and vii: a haze-free atmosphere (clean spectrum); ii, v and viii: a hazy only atmosphere (full spectrum minus clean spectrum); iii, vi and ix: the terminator difference of the haze free case is calc…

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