Vikas Soni

Warm giant exoplanets exhibit strong three-dimensional temperature contrasts that can significantly alter atmospheric chemistry through quenching and photochemistry, yet transmission spectra are commonly interpreted using one-dimensional, limb-averaged models. Such simplifications may bias inferred atmospheric properties, particularly metallicity and C/O ratio. In this work, we investigate the relative influence of atmospheric composition and three-dimensional thermal structure on atmospheric chemistry and transmission spectra using WASP-69 b as a test case. WASP-69 b is a ∼900 K warm Saturn, residing in a thermal regime especially sensitive to disequilibrium chemistry. We use three-dimensional general-circulation-model-derived pressure–temperature profiles as inputs for a one-dimensional photochemical–kinetics model to resolve longitudinal and latitudinal chemical asymmetries across the atmosphere. We find that CH ₄ exhibits strong latitudinal variations linked to deep quench temperatures, while SO ₂ shows longitudinal asymmetries driven by upper-atmospheric photochemistry and irradiation geometry. In contrast, CO ₂ remains comparatively insensitive to spatial thermal variations and emerges as a robust tracer of atmospheric metallicity. Synthetic transmission spectra reveal that three-dimensional chemical asymmetries can produce spectral variations comparable to those induced by metallicity itself, demonstrating that limb-averaged interpretations can mask substantial spatial structure in warm giant exoplanet atmospheres.

In the era of JWST and upcoming space- and ground-based observatories (PLATO, ARIEL, ELT, and TMT), we will see an unprecedented surge in data, including phase curves and morning–evening asymmetry measurements. To understand the physical processes that govern the 3D structure of the observed exoplanet atmosphere, a state-of-the-art 3D climate model and cloud microphysics are required. In this work, we couple ExoRad (a 3D climate model) with the DRIFT (cloud microphysics model), in which clouds (self-consistently generated) are used as a new non-grey opacity source in ExoRad, incorporating both heating and cooling effects. As cloud properties are intrinsically linked to the thermodynamic state and vice versa, we recalculate the cloud properties using the updated thermodynamic structure of the atmosphere and iterate between the DRIFT and the ExoRad, which quickly converges in a few iterations.We apply this method to WASP-107b, an inflated warm gas giant with an equilibrium temperature of 770 K and an abnormally high interior temperature. WASP-107 b has been observed to host silicon clouds, morning–evening asymmetry, and disequilibrium chemical species (CH4 and SO2), linked to atmospheric dynamics. We run the model for several metallicity levels, ranging from solar to 40 times solar metallicity, based on the observed constraints.We find that clouds have a significant impact on the overall thermal structure in all model runs. The presence of clouds makes the planet's atmosphere hotter in the infrared photosphere, producing a weak thermal inversion. The coupling among zonal wind jet, thermal structure and cloud microphysics processes is required to reproduce the observed 150–200 K temperature difference between the morning and evening terminators. We also observe the thermodynamic trends in the metallicity space, which includes the zonal wind jet, day-night and morning-evening temperature differences, and vertical wind structure.

Vertical mixing is a crucial disequilibrium process in exoplanet atmospheres, significantly impacting chemical abundance and observed spectra. While current state-of-the-art observations have detected its signatures, the effect of vertical mixing on atmospheric spectra varies widely based on planetary parameters. In this study, we explore the influence of disequilibrium chemistry across a parameter space that includes eddy diffusion, surface gravity, internal and equilibrium temperature, and metallicity. We also assess the effectiveness of retrieval models in constraining the eddy diffusion coefficient. By running numerous 1D chemical kinetics models, we investigate the impact of vertical mixing on the transmission spectrum. We also built a custom fast-forward disequilibrium model, which includes vertical mixing using the quenching approximation and calculates the model abundance orders of magnitude faster than the chemical kinetics model. We coupled this forward model with an open-source atmospheric retrieval code, used it on the JWST simulated output data of our chemical kinetics model, and retrieved eddy diffusion coefficient, internal temperature, and atmospheric metallicity. We find that there is a narrow region in the parameter space in which vertical mixing has a large effect on the atmospheric transmission spectrum. In this region of the parameter space, the retrieval model can put high constraints on the transport strength and provide optimal exoplanets to study vertical mixing. In addition, the NH₃ abundance can be used to constrain the internal temperature for equilibrium temperature T _equi > 1400 K.

Variations in atmospheric elemental nitrogen can considerably affect the abundance of major nitrogen-bearing species such as NH₃ and HCN. Also, due to vertical mixing and photochemistry, their abundance deviates from thermochemical equilibrium. The goal of this study is to understand the effect of atmospheric metallicity on the composition of NH₃, N₂, and HCN over a large parameter space in the presence of vertical mixing, which when combined with the work on CHO-bearing species in Soni & Acharyya can provide a comprehensive understanding of the effect of atmospheric metallicity. We used quenching approximations and a full chemical kinetics model for the calculations, and a comparison between these two methods was made. To generate thermal profiles, the petitRADTRANS code was used. Chemical timescales of NH₃ and N₂ are found to be complex functions of metallicity, while HCN is inversely proportional. Using quenched abundances of NH₃ and CO, the quenched abundance of HCN can be constrained since it remains in equilibrium with NH₃, CO, and H₂O. Quenched NH₃ increases with increasing K _zz until a particular point, after which it becomes independent of vertical mixing. There is a sweet spot in the K _zz parameter space to maximize the quenched HCN for a given T _int and T _equi; the parameter space moves toward a lower equilibrium temperature, and the abundance of HCN increases with metallicity. Finally, we used a data set of quenched abundances to provide a list of potential candidates in which the observation of HCN is possible.

The atmospheric metallicity greatly influences the composition of exoplanet atmospheres. The effect of metallicity on the thermochemical equilibrium is well studied, though its effect on the disequilibrium abundance is loosely constrained. In this study, we have used the quenching approximation to study the effect of metallicity on the quenched abundance for a range of parameters (temperature: 500–2500 K, pressure: 10⁻⁴–10³ bar, metallicity: 0.1–1000× solar metallicity). We determine the chemical timescale by finding rate-limiting steps in a reduced chemical network with a network-analysis tool and the thermochemical equilibrium abundance. The equilibrium abundance results are similar to the literature. The CO, H₂O, and CO₂ abundances increase with metallicity in the parameter range considered. The CH₄ abundance increases with metallicity for CO/CH₄ < 1 and is unaffected for CO/CH₄ > 1. The chemical timescale of CO shows minimal change with metallicity, while the CH₄ chemical timescale is inversely proportional to atmospheric metallicity. The quench level of CO shifts into the high-pressure region, and the quench level of CH₄ shows complex behavior with metallicity. We benchmarked the quenching approximation with a one-dimensional photochemistry-transport model for two test exoplanets (GJ 1214 b and HD 189733 b) and found it to be in good agreement. We also found that the quenching approximation is a powerful tool to constrain atmospheric parameters. We demonstrated this by constraining the metallicity and transport strength for the test exoplanets HR 8799 b, HD 189733 b, GJ 436 b, and WASP-39 b.