Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS

Abstract: How aerosols form, are transported, and cycle between condensation and evaporation across exoplanet temperature regimes remains poorly understood. Recent models and observations suggest that warm giant planets near $800$--$1000$ K may span a transition between homogeneous and longitudinally heterogeneous aerosol distributions. We present a robust detection of aerosol asymmetry in a giant planet with $T_{\rm eq}\lesssim1000$ K, using the $0.86$--$2.82~μ$m JWST NIRISS/SOSS transmission spectrum of WASP-69 b. The evening limb shows prominent 1.4 $μ$m H$<em>2$O absorption ($Δ\mathrm{BIC}</em>{\rm H_2O}=+22.7$), whereas H$<em>2$O is not detected on the morning limb ($Δ\mathrm{BIC}</em>{\rm H_2O}=-8.7$). Atmospheric retrievals reveal significant aerosol opacity on both limbs, with high-altitude, optically thick clouds muting molecular features on the morning limb and lower cloud opacity allowing H$<em>2$O to emerge on the evening limb. The evening terminator is hotter by $304^{+62}</em>{-91}$ K, consistent with morning-limb condensates partially evaporating during transport toward the evening limb. This mechanism is independently verified with 3D general circulation models. Stellar contamination or aerosols dominated by photochemical haze do not readily explain the asymmetry. From a limb-resolved analysis, we infer a stellar-to-superstellar atmospheric metallicity, with $\rm[M/H]=0.11<sup>{+0.40}_{-0.46}$ from the equilibrium retrieval and [O/H]$=1.38<sup>{+0.44}_{-0.79}$ from the free retrieval. We also detect an escaping metastable-helium tail extending to $3.08<sup>{+0.50}_{-0.45}\,R_p$. WASP-69 b anchors the cooler edge of the emerging population of planets with asymmetric aerosol distributions and suggests that substantial aerosol opacity may persist on both limbs across this transition.

• The paper reveals a robust morning–evening aerosol asymmetry in WASP-69 b, with a 1.4 μm water feature detected on the evening limb but absent on the morning limb, supported by a 304 K evening-side temperature excess.
• Limb retrievals show a high-altitude cloud deck on the morning side and thinner evening aerosols, while stellar contamination and strong photochemical haze are disfavored and equilibrium models yield broadly consistent metallicity estimates.
• A 3D circulation and cloud model qualitatively reproduces the pattern through condensate evaporation during transport, extending detected giant-planet terminator asymmetries into the cooler 800–1000 K regime while motivating panchromatic follow-up observations.

Overview and motivation

This paper presents a limb-resolved analysis of the JWST NIRISS/SOSS transmission spectrum (0.86–2.82 μm) of the warm Saturn WASP-69 b ($T_{\rm eq}\approx960$ K, $M_p\approx0.26\,M_{\rm J}$, $R_p\approx1.06\,R_{\rm J}$), obtained as part of the "Grand Tour" survey (GO 5924). The central result is a robust detection of morning–evening aerosol asymmetry in a giant planet at $T_{\rm eq}\lesssim1000$ K, a regime where no such detection had previously been reported. The observation bears on an open question in aerosol physics: whether warm giants near 800–1000 K occupy a transition between longitudinally homogeneous aerosol coverage, expected at cooler temperatures where hazes persist at observable pressures across longitudes, and the cloudy-morning/clear-evening regime seen on hotter planets such as WASP-39 b, WASP-94A b, and WASP-17 b (Espinoza et al., 2024, Mukherjee et al., 16 May 2025, Fu et al., 21 Jul 2025).

Observations and data reduction

The transit was observed over 5.67 hours with NIRISS/SOSS in SUBSTRIP96 mode (2064 integrations). Two independent pipelines, FIREFLy and Eureka!, were used for reduction, including group-level 1/f noise subtraction (FIREFLy) and curved-trace straightening with column-by-column background estimation (Eureka!). The white-light curve fit yields consistent orbital parameters between pipelines, e.g., $b=0.670\pm0.002$ and $i=86.87^\circ\pm0.01^\circ$, with residual scatter of 106 ppm.

A notable data-quality issue arose: free-fitted limb-darkening coefficients implied unphysical limb brightening ($(u_1+u_2)=-0.25\pm0.04$), inconsistent with both the Stagger 3D stellar models and contemporaneous NIRSpec results. The authors attribute this to a common-mode correlated-noise structure also seen in other SUBSTRIP96 observations, and mitigate it via a Gaussian-smoothed common-mode systematic derived from residuals with fixed 3D-model limb darkening. This treatment is empirical; its adequacy rests on the assumption that the common-mode component is spectrally flat relative to atmospheric features, which the authors assert but do not fully demonstrate.

Limb-resolved spectra and evidence for asymmetry

Limb-resolved spectra extracted with catwoman show a prominent 1.4 μm H$_2$O band on the evening limb that is absent on the morning limb. Model comparison via $\Delta{\rm BIC}$ quantifies this starkly: H$_2$O is favored on the evening limb ($M_p\approx0.26\,M_{\rm J}$0) but disfavored on the morning limb ($M_p\approx0.26\,M_{\rm J}$1). An independent diagnostic—the chromatic offset of fitted mid-transit times $M_p\approx0.26\,M_{\rm J}$2—shows structure mirroring the evening-limb spectrum near the water band, as expected when the two limbs have wavelength-dependent effective radii. The authors argue, correctly, that ignoring this asymmetry would bias composition and metallicity inferences from the limb-averaged spectrum.

Atmospheric retrievals

Two retrieval frameworks were applied to the limb spectra using updated stellar parameters from MIST-based isochrone fitting ($M_p\approx0.26\,M_{\rm J}$3 K, [Fe/H]$M_p\approx0.26\,M_{\rm J}$4).

Equilibrium chemistry (PICASO/VIRGA): A joint limb-limb retrieval with Guillot-type $M_p\approx0.26\,M_{\rm J}$5 profiles sharing all parameters except $M_p\approx0.26\,M_{\rm J}$6 finds the evening limb hotter by $M_p\approx0.26\,M_{\rm J}$7 K, with [M/H]$M_p\approx0.26\,M_{\rm J}$8 and C/O$M_p\approx0.26\,M_{\rm J}$9. Cloud opacity dominates the morning-limb spectrum while H$R_p\approx1.06\,R_{\rm J}$0O dominates the evening limb. The thermochemical-equilibrium assumption is justified on the grounds that disequilibrium effects shift H$R_p\approx1.06\,R_{\rm J}$1O by only ~0.1 dex at this temperature, though they strongly affect CH$R_p\approx1.06\,R_{\rm J}$2, NH$R_p\approx1.06\,R_{\rm J}$3, and CO$R_p\approx1.06\,R_{\rm J}$4—species not constrained here.

Free chemistry (POSEIDON): Independent limb retrievals detect only H$R_p\approx1.06\,R_{\rm J}$5O, on the evening limb ($R_p\approx1.06\,R_{\rm J}$6), yielding [O/H]$R_p\approx1.06\,R_{\rm J}$7 ($R_p\approx1.06\,R_{\rm J}$8 solar) when interpreted as an oxygen proxy. This agrees with the equilibrium metallicity within 1.4σ, supporting a stellar-to-superstellar envelope enrichment consistent with core accretion. All other species carry only 3σ upper limits; C/O remains effectively unconstrained without CO and CO$R_p\approx1.06\,R_{\rm J}$9 detections, which the authors acknowledge requires panchromatic NIRSpec coverage.

On the aerosol interpretation, the morning limb requires a high-altitude cloud deck ($T_{\rm eq}\lesssim1000$0), while the evening limb shows enhanced but optically thinner scattering ($T_{\rm eq}\lesssim1000$1). Unlike the nearly clear evening of WASP-94A b, WASP-69 b retains appreciable evening-limb aerosol opacity, plausibly because more efficient heat recirculation at lower $T_{\rm eq}\lesssim1000$2 reduces the longitudinal temperature contrast enough for condensates to persist on both limbs at different altitudes.

Ruling out alternatives: Stellar contamination cannot readily explain the asymmetry. Retrievals including the transit light source effect require spot covering fractions differing by more than 3σ between limbs ($T_{\rm eq}\lesssim1000$3 vs. $T_{\rm eq}\lesssim1000$4) and heterogeneous-region temperatures ~1250–1570 K below the photosphere, while models without contamination are preferred by $T_{\rm eq}\lesssim1000$5 and 8.2. High photochemical haze fractions are disfavored indirectly: comparable muting in 3D models would produce a thermal inversion absent from the dayside emission spectrum (Schlawin et al., 2024). The paper concedes that NIRISS/SOSS alone cannot uniquely identify the condensate composition or exclude a small haze contribution.

Helium escape

The spectrum shows strong metastable He I 10830 Å absorption with post-transit absorption spanning ~0.5 hr. Harmonica modeling of the envelope as trailing/leading semi-ellipses constrains a trailing tail of $T_{\rm eq}\lesssim1000$6 with no clear leading tail, and envelope opacity $T_{\rm eq}\lesssim1000$7 (with acknowledged α–h degeneracy). These tail sizes are smaller than the $T_{\rm eq}\lesssim1000$8 extent reported from Keck/NIRSPEC (Tyler et al., 2023), which the authors attribute to the low resolving power ($T_{\rm eq}\lesssim1000$9) or temporal variability of the outflow. p-winds fits yield a maximum excess absorption of $b=0.670\pm0.002$0% and equivalent width $b=0.670\pm0.002$1 mÅ, with joint posteriors on mass-loss rate and outflow temperature overlapping ground-based CARMENES constraints—a useful consistency check despite the inability to break the mass-loss/temperature degeneracy at low resolution.

GCM support

A Met Office Unified Model simulation (2000 Earth days, 10× solar metallicity, Venot kinetics chemistry, cloud-free dynamics) reproduces the retrieved thermal contrast: the evening limb is hotter, permitting only MnS condensation, whereas the cooler morning limb permits both MnS and Na$b=0.670\pm0.002$2S, with Na$b=0.670\pm0.002$3S dominating the observable region. Post-processing with VIRGA (using retrieved $b=0.670\pm0.002$4 and GCM-derived $b=0.670\pm0.002$5–$b=0.670\pm0.002$6 cm² s⁻¹, consistent with the retrieval) produces cloudy spectra in which H$b=0.670\pm0.002$7O features are muted on both limbs but remain visible on the evening limb, matching the observations qualitatively. The authors are explicit that this is an exploratory calculation: clouds are decoupled from dynamics, radiative transfer, and microphysics, assumed locally static, and their radiative feedback on the thermal structure is neglected. The agreement therefore supports the evaporation-during-transport mechanism at first order but does not constitute a self-consistent cloud prediction.

Position within the population-level asymmetry trend

Using a modified two-sided continuum definition (interpolating between 0.90–1.30 and 1.52–1.80 μm bands, adopted because WASP-69 b's aerosol slope biases the original single-sided continuum), the paper computes an evening-minus-morning water-index difference of $b=0.670\pm0.002$8 scale heights (a 2.4σ limb difference)—weaker than WASP-39 b and WASP-94A b but larger than WASP-107 b and HAT-P-18 b. Refitting the sigmoid "asymmetry horizon" model of Fu et al. to ten planets favors it over constant asymmetry by $b=0.670\pm0.002$9, but the fit quality is poor ($i=86.87^\circ\pm0.01^\circ$0), indicating that a single diagonal boundary in the $i=86.87^\circ\pm0.01^\circ$1–$i=86.87^\circ\pm0.01^\circ$2 plane does not cleanly separate the population. The authors conclude the horizon is likely a broad, multidimensional transition region shaped by circulation, vertical mixing, particle settling, and nucleation, rather than a binary boundary—an important caveat for any attempt to use the horizon predictively.

Limitations and open questions

Several limitations qualify the conclusions. First, the correlated-noise mitigation for the SUBSTRIP96 common-mode systematics is heuristic, and the fidelity of the limb-resolved extraction depends on it. Second, the aerosol composition (MnS, Na₂S, KCl, or residual haze) is not identifiable within the NIRISS bandpass; mid-infrared MIRI transmission data and UV-optical HST coverage are needed. Third, the GCM cloud treatment is decoupled and neglects radiative feedback, so the quantitative cloud distributions should be regarded as indicative. Fourth, the metallicity constraint rests primarily on H$i=86.87^\circ\pm0.01^\circ$3O alone, with C/O unconstrained pending CO/CO₂ detections. Open questions left by the paper include: what sets the vertical extent difference between limb aerosols at fixed composition; whether the asymmetry horizon sharpens or broadens as more cool planets are observed; and how time-variable stellar activity modulates the apparent limb contrast in repeated transits.

Conclusion

The paper establishes WASP-69 b as the coolest giant planet with a robustly detected cloudy-morning, less-cloudy-evening aerosol asymmetry, extending the known asymmetric population ($i=86.87^\circ\pm0.01^\circ$4) into the 800–1000 K regime. The combination of a $i=86.87^\circ\pm0.01^\circ$5 K limb temperature contrast, differential H$i=86.87^\circ\pm0.01^\circ$6O detection significance ($i=86.87^\circ\pm0.01^\circ$7 of +22.7 versus −8.7), rejection of stellar-contamination and high-haze explanations, and qualitative GCM support converges on condensate clouds forming on the cooler morning limb and partially evaporating toward the evening terminator, while substantial aerosol opacity persists on both limbs. The result demonstrates that limb-resolved transmission spectroscopy can recover aerosol transport signatures invisible in limb-averaged spectra, and positions WASP-69 b as a benchmark target whose panchromatic HST/JWST archive can test three-dimensional chemistry–circulation–aerosol coupling in the coolest regime yet probed.