arXiv subjects

Jeehyun Yang

Publications and source records attributed to Jeehyun Yang.

18 recordsLinked to original sources

arXiv2026

Phase-dependent chemistry of WASP-43 b revealed with a suite of one-, two-, and three-dimensional models

Our goal is to investigate the chemistry of the hot Jupiter WASP-43 b in detail using theoretical models, considering the constraints of the James Webb Space Telescope MIRI phase curve. With a suite of pseudo-two-dimensional and three-dimensional photochemical models, we simulate the composition of WASP-43 b in various configurations, and compare them with atmospheric retrieval models. We confirm that disequilibrium chemistry in our theoretical models reduces the methane concentration on the planet night side for wind jet speeds > 500 m/s. Varying the metallicity in the models induces large changes in the CO$_2$ and SO$_2$ concentrations, with SO$_2$ producing mid-infrared absorption features in synthetic emission spectra of the night side at atmospheric metallicities > 10x solar. Our models provide evidence for pole-to-equator circulation enhancing the CH$_4$, NH$_3$, and HCN abundances, which is nonetheless insufficient for detectable spectral features. Finally, we show that H$_2$O, CO, and CO$_2$ are robustly modeled, but species affected by photochemistry are more sensitive to model-specific assumptions and pathways. We conclude that horizontal quenching is the prime mechanism that explains the non-detection of methane in the MIRI phase-curve of WASP-43 b. This mechanism requires only moderate wind speeds and is operative at various thermal structures and atmospheric metallicities. Furthermore, coupled carbon-sulfur chemistry leads to an additional decrease in methane compared to previous models in the literature that did not contain sulfur chemistry. We do not favor a high metallicity as it would have led to observable SO$_2$ features in the MIRI spectra. Our study shows that phase-dependent photochemistry models are essential tools in the interpretation of hot-Jupiter phase curves, but benchmarking is needed to improve the accuracy of photochemical models in the future.

astro-ph.EP↗

arXiv2026

C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894b, an exceptional 420 K sub-Saturn that is the only known giant planet transiting a late M dwarf. Remarkably, both the light curve and the transit spectrum exhibit almost no stellar contamination. The spectrum is dominated by prominent absorption features from CH$_4$ and the photochemical product CS$_2$. For the first time in a transit spectrum, NH$_3$ is visually evident, while subtler features from H$_2$O, and CO$_2$ can also be seen. We significantly improve upon state-of-the-art photochemical reaction networks, and use our new network to run radiative-convective photochemical models at different metallicities. These models show that the spectrum--in particular the size of the NH$_3$ and CO$_2$ features relative to the CH$_4$ and H$_2$O features--is most consistent with a metallicity of 3--10$\times$ solar. Using a semi-free retrieval framework that perturbs the self-consistent model's abundance and temperature profiles to fit the data, we find that the planet's C/O, N/O, and S/O ratios are broadly consistent with solar values. A grid retrieval on 1D radiative-convective photochemical equilibrium (RCPE) models reveals a similar result: $[M/H]=0.46 \pm 0.08$ and C/O=$0.69 \pm 0.06$. The planet's atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to that of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.

astro-ph.EP↗

arXiv2026

Photochemical Production of CS2 in Temperate-to-Warm Gas Giant Exoplanet Atmospheres

Sulfur chemistry has emerged as an important probe of exoplanet atmospheres in the JWST era, although observational constraints have thus far been largely limited to SO2 and H2S in warm and hot exoplanets. Recent JWST observations have revealed CS2 in several cooler gas-giant exoplanets, yielding a new tracer of sulfur chemistry. However, the detailed chemical pathways responsible for the formation of CS2 remain poorly understood. Here, we use TOI-6894 b, a temperate gas giant with evidence for CS2, as a test case for one-dimensional photochemical kinetic-transport modeling and sensitivity analyses of CS2 chemistry. We show that CS2 is produced through coupled thermochemical and photochemical processes involving CH4 and H2S as the primary carbon and sulfur reservoirs, with S2 photolysis driving disequilibrium sulfur chemistry. Our models provide a physically consistent explanation for the observed CS2 feature in TOI-6894 b. Extending our analysis to gas giant exoplanets spanning a wide range of Teq, we find that CS2 abundance peaks in temperate to warm atmospheres (Teq ~ 500 - 700 K), and declines toward both lower and higher temperatures. This temperature dependence provides a unified framework for interpreting current CS2 observations, accounting for reported detections in temperate to warm planets and the lack of detections in colder and hotter giant exoplanets. Our results establish CS2 as a complementary probe of sulfur inventories and atmospheric metallicity in cool gas giant exoplanets

astro-ph.EP↗

arXiv2026

Novel Chemical Pathways for the Formation of Nucleobase Precursors via Benzene {\pi}-Bond Addition to HCN

We propose a simple and efficient pathway for the formation of precursors to core nucleobases in DNA and RNA using a suite of computational chemistry methods. Benzene, which is thermochemically stable in N2- or CO2-dominated atmospheres, could have formed via upper-atmospheric photochemistry or surface lightning and accumulated on the early Earth or Mars. However, nitrogen insertion into the benzene ring to form pyrimidine and purine is widely considered to be challenging. We propose that nitrogen incorporation occurred through HCN 1,4-cycloaddition to benzene's {\pi}-system, followed by a C2H2 fragmentation mechanism, as confirmed by quantum chemistry calculations. This pathway, potentially facilitated by photochemistry at the ocean surface or episodic impact events on local reservoirs, can lead to pyrimidine formation, which can further react with NH3 and HCN to produce purine. Extending this pathway to early Mars, our photochemical model simulates heterocyclic compound formation under cold, dry surface conditions that favor high benzene and HCN concentrations but lack liquid water. We thus propose that organics formed during dry phases may have later dissolved into surface waters during wet phases and become concentrated as ocean sediments. This result supports Mars Sample Return efforts focused on ancient aqueous environments likely to retain prebiotic signatures.

physics.chem-ph↗

arXiv2026

Sub-Neptunes as Soot Factories: Deep Atmosphere Hydrocarbon Formation and Quenching as the Origin of Sub-Neptune Aerosol Trends

Recent population-level studies of sub-Neptune atmospheres have identified a tentative parabolic trend in transmission spectrum amplitude for planets with Teq ~ 500-800 K. While the trend has been commonly attributed to hydrocarbon aerosols, we lack a first-principles explanation of its underlying chemical mechanism. Previous work has focused on the role of methane photolysis and subsequent polymerization, but with limited reaction networks that truncated at C2-species and couldn't reproduce the observed parabolic trend. In this work, enabled by a computer-automated, rate-based chemical network generator, we construct the most comprehensive carbon reaction network for exoplanet atmospheres to date. We explicitly model the formation of polycyclic aromatic hydrocarbons (PAHs), which are well established as soot precursors in combustion chemistry. We calculate the chemical timescales of hydrocarbon species through an eigenvalue timescale method and model their quenched abundances across a range of C/O, metallicities, and Teq. In this framework, the deep atmosphere acts as a "soot factory" analogous to a combustion engine, transporting the ingredients for hydrocarbon aerosol formation to the JWST-observable region of the atmosphere, where it may be further augmented by photochemistry. We find that the predicted abundances of PAHs peak near 600 K, and fall off toward higher and lower Teq, consistent with the observed muted-spectra regime suggested in observational studies by HST and JWST. We also show that PAH abundances are expected to vary with C/O and metallicity, thus providing a natural explanation for observed diversity among planets with similar Teq.

astro-ph.EP↗

arXiv2025

A Comprehensive Sulfur Chemistry Network Including Excited S(1D) and SO(1{\Delta}) for the XODIAC Photochemical Model: Accounting for Missing Sulfur Processes in Venus and Exo-Venus Analogs

Sulfur chemistry plays a central role in controlling the atmospheric structure, cloud formation, and composition of Venus and Venus-like exoplanets. However, key reactions involving ground- and excited-state sulfur species remain poorly constrained, and existing photochemical models often rely on incomplete or uncertain kinetic data under high-temperature, CO2-rich conditions. In this work, we compute kinetic parameters for reactions of ground-state S(3P) and excited-state S(1D) with CO2 under Venus-like conditions, forming SO(3Sigma), SO(1Delta), and CO. We characterize the underlying potential energy surfaces, identify intermediate complexes, and derive temperature-dependent rate coefficients using a master-equation framework based on the chemically significant eigenvalue method. We also provide NASA 7-term polynomial coefficients for S and SO in both ground and excited states to enable consistent incorporation into photochemical models. Incorporating these reactions into the one-dimensional photochemical model XODIAC for Venus produces only minor effects above 60 km due to competing pathways. While the model reproduces most observed sulfur species, discrepancies remain for S3 and S4. Introducing a 1 ppm near-surface atomic sulfur source, representing unresolved deep-atmosphere or surface processes, enhances S3 and S4 abundances by 1-2 orders of magnitude and improves agreement with observations. For exo-Venus analogs, the updated chemistry produces modest changes under isothermal conditions. In contrast, in strongly irradiated atmospheres with a high-altitude isotherm and a near-surface sulfur source, it leads to pronounced changes in most sulfur-bearing species, along with significant enhancements in S(1D) and SO(1Delta). These results highlight missing sulfur pathways, including excited states and deep sources, and potential implications for shaping Venus and exo-Venus atmospheres.

astro-ph.EP↗

arXiv2025

Methane on the temperate exo-Saturn TOI-199b

Temperate ($T_{\rm eq}<400$ K) gas giants represent an unexplored frontier in exoplanet atmospheric spectroscopy. Orbiting a G-type star every $\sim100$ days, the Saturn-mass exoplanet TOI-199 b ($T_{\rm eq}=350$ K) is one of the most favorable low-temperature gas giants for atmospheric study. Here, we present its transmission spectrum from a single transit observed with JWST's NIRSpec G395M mode. Despite lower-than-nominal precision due to a pointing misalignment, the spectrum reveals the presence of CH$_4$ (Bayes factor of $\sim$700 in a cloudy atmosphere), corresponding to a metallicity of $\rm{C/H}=13^{+78}_{-12}\times$ solar, although the absence of detectable CO and CO$_2$ at the current precision disfavors metallicities $\gtrsim50\times$ solar. We also tested several haze prescriptions (Titan-like tholin, soot, and water-rich tholin), but the preference for these models is weak (Bayes factors of $\sim 2$ relative to the clear case). The spectrum also shows an increase in transit depth near 3 $\mu$m, which our self-consistent models attribute to either NH$_3$ or, less likely, HCN. Follow-up observations could distinguish between these species, helping determine the planet's vertical mixing regime. The TOI-199 system exhibits strong transit timing variations (TTVs) due to an outer non-transiting giant planet. For planet c, our TTV analysis reduces its mass uncertainty by 50% and prefers a slightly longer orbital period (but still within the conservative habitable zone) and higher eccentricity relative to previous studies. TOI-199 b serves as the first data point for studying clouds and hazes in temperate gas giants, with the detection of spectral features in its transmission spectrum indicating that temperate gas giants are promising targets for detailed atmospheric characterization.

astro-ph.EP↗

arXiv2025

Volcanic Satellites Tidally Venting Na, K, SO2 in Optical & Infrared Light

Recent infrared spectroscopy from the James Webb Space Telescope (JWST) has spurred analyses of common volcanic gases such as carbon dioxide (CO2), sulfur dioxide (SO2), alongside alkali metals sodium (Na I) and potassium (K I) surrounding the hot Saturn WASP-39 b. We report more than an order-of-magnitude of variability in the density of neutral Na, K, and SO2 between ground-based measurements and JWST, at distinct epochs, hinting at exogenic physical processes similar to those sourcing Io's extended atmosphere and torus. Tidally-heated volcanic satellite simulations sputtering gas into a cloud or toroid orbiting the planet, are able to reproduce the probed line-of-sight column density variations. The estimated SO2 flux is consistent with tidal gravitation predictions, with a Na/SO2 ratio far smaller than Io's. Although stable satellite orbits at this system are known to be < 15.3 hours, several high-resolution alkali Doppler shift observations are required to constrain a putative orbit. Due to the Roche limit interior to the planetary photosphere at ~ 8 hours, atmosphere-exosphere interactions are expected to be especially important at this system.

astro-ph.EP↗

arXiv2025

Coupled 1D Chemical Kinetic-Transport and 2D Hydrodynamic Modeling Supports a modest 1-1.5x Supersolar Oxygen Abundance in Jupiter's Atmosphere

Understanding the deep atmospheric composition of Jupiter provides critical constraints on its formation and the chemical evolution of the solar nebula. In this study, we combine one-dimensional thermochemical kinetic-transport modeling with two-dimensional hydrodynamic simulations to constrain Jupiter's deep oxygen abundance using carbon monoxide (CO) as a proxy tracer. Leveraging a comprehensive chemical network generated by Reaction Mechanism Generator (RMG), we assess the impact of updated reaction rates, including the often-neglected but thermochemically significant Hidaka reaction (CH3OH + H -> CH3 + H2O). Our 1D-2D coupled approach supports a modest supersolar oxygen enrichment of 1.0-1.5x the solar value. We also present a method for deriving Jupiter's eddy diffusion coefficient Kzz = 3e6 to 5e7 cm2/s) from 2D hydrodynamic simulations using the quasi steady-state approach. This method is applicable to exoplanet atmospheres, where Kzz remains highly uncertain despite its strong influence on atmospheric chemistry. Finally, our results imply a significantly elevated planetary carbon-to-oxygen (C/O) ratio of ~2.9, highlighting the importance of clarifying the mechanisms behind the preferential accretion of carbon-rich material during Jupiter's formation. By integrating thermochemical and hydrodynamic processes, our study offers a more complete framework for constraining chemical and dynamical processes in (exo)planetary atmospheres.

astro-ph.EP↗

arXiv2025

A water-rich interior in the temperate sub-Neptune K2-18 b revealed by JWST

Temperate sub-Neptunes are compelling targets for detecting liquid-water oceans beyond the Solar System. If water-rich and lacking massive hydrogen-helium envelopes, these planets could sustain liquid layers beneath their atmospheres despite sizes larger than Earth. Previous observations of the temperate sub-Neptune K2-18 b revealed an H2-dominated atmosphere rich in CH4, with moderate evidence for CO2 and tentative signs of dimethyl sulfide (DMS). Here we present four new JWST/NIRSpec transit observations of K2-18 b. The resulting high-precision transmission spectrum robustly detects both CH4 and CO2, precisely measuring their abundances and firmly establishing the planet's water-rich nature: either a thick envelope with >10% H2O by volume or a thin atmosphere above a liquid-water ocean. The spectrum reveals no detectable H2O, NH3, or CO. The absence of atmospheric water vapor suggests an efficient cold trap, while the nondetections of NH3 and CO support the scenario of a small H2-rich atmosphere overlying a liquid reservoir. However, alternative models that include these gases can also reproduce the spectrum within uncertainties, highlighting the need for deeper observations. The spectrum only contains marginal signals of DMS, methyl mercaptan (CH3SH), and nitrous oxide (N2O), with none exceeding 3 sigma in model preference and all falling below ~2 sigma without imposing a strong super-Rayleigh haze. Meanwhile, our self-consistent photochemical models show that DMS and CH3SH may form abiotically in massive H2-rich atmospheres of high metallicity, making it important to consider additional indicators for their potential use as biosignatures. K2-18 b, a cool, water-rich world, stands out as one of the most promising temperate sub-Neptunes for exploring the emergence of liquid-water environments in non-Earth-like planets, motivating further characterization of its atmosphere and interior.

astro-ph.EP↗

arXiv2025

Evidence for a volcanic atmosphere on the sub-Earth L98-59b

Assessing the prevalence of atmospheres on rocky planets around M-dwarf stars is a top priority of exoplanet science. High-energy activity from M-dwarfs can destroy the atmospheres of these planets, which could explain the lack of atmosphere detections to date. Volcanic outgassing has been proposed as a mechanism to replenish the atmospheres of tidally-heated rocky planets. L 98-59 b, a sub-Earth transiting a nearby M dwarf, was recently identified as the most promising exoplanet to detect a volcanic atmosphere. We present the transmission spectrum of L 98-59 b from four transits observed with JWST NIRSpec G395H. Although the airless model provides an adequate fit to the data based on its $\chi^2$, an SO$_2$ atmosphere is preferred by 3.6$\sigma$ over a flat line in terms of the Bayesian evidence. Such an atmosphere would likely be in a steady state where volcanism balances escape. If so, L 98-59 b must experience at least eight times as much volcanism and tidal heating per unit mass as Io. If volcanism is driven by runaway melting of the mantle, we predict the existence of a subsurface magma ocean in L 98-59 b extending up to $R_p\sim 60-90\%$. An SO$_2$-rich volcanic atmosphere on L 98-59 b would be indicative of an oxidized mantle with an oxygen fugacity of $f\rm{O}_2>IW+2.7$, and it would imply that L 98-59 b must have retained some of its volatile endowment despite its proximity to its star. Our findings suggest that volcanism may revive secondary atmospheres on tidally heated rocky planets around M-dwarfs.

astro-ph.EP↗

arXiv2024

Chemical mapping of temperate sub-Neptune atmospheres: Constraining the deep-interior H2O/H2 using the atmospheric CO2/CH4

Understanding the planetary envelope composition of sub-Neptune-type exoplanets is challenging due to the inherent degeneracy in their interior composition scenarios. Particularly, the planetary envelope's H2O/H2 ratio, which can also be expressed as the O/H ratio, provides crucial insights into its original location relative to the ice line during planetary formation. Using self-consistent radiative transfer modeling and a rate-based automatic chemical network generator combined with 1D photochemical kinetic-transport atmospheric modeling, we investigate various atmospheric scenarios of temperate sub-Neptunes, ranging from H2-dominated to H2O-dominated atmospheres with equilibrium temperatures (Teq) of 250-400 K. This study includes examples such as K2-18 b (Teq = 255 K), LP 791-18 c (Teq = 324 K), and TOI-270 d (Teq = 354 K). Our models indicate that the atmospheric CO2/CH4 ratio can be used to infer the deep-interior H2O/H2 ratio. Applying this method to recent JWST observations, our findings suggest K2-18 b likely has an interior that is 50% highly enriched in water, exceeding the water content in a 100xZ scenario and suggesting a planetary formation mechanism involving substantial accretion of ices. In contrast, our model suggests that approximately 25% of TOI-270 d's interior is composed of H2O, which aligns with the conventional metallicity framework with a metallicity higher than 100xZ. Furthermore, our models identify carbonyl sulfide (OCS) and sulfur dioxide (SO2) as strong indicators for temperate sub-Neptunes with at least 10% of their interior composed of water. These results provide a method to delineate the internal composition and formation mechanisms of temperate sub-Neptunes (Teq < ~500 K) via atmospheric characterization through transmission spectroscopy.

astro-ph.EP↗

arXiv2024

Sulphur dioxide in the mid-infrared transmission spectrum of WASP-39b

The recent inference of sulphur dioxide (SO$_2$) in the atmosphere of the hot ($\sim$1100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations suggests that photochemistry is a key process in high temperature exoplanet atmospheres. This is due to the low ($<$1 ppb) abundance of SO$_2$ under thermochemical equilibrium, compared to that produced from the photochemistry of H$_2$O and H$_2$S (1-10 ppm). However, the SO$_2$ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 $μ$m, and therefore the detection of other SO$_2$ absorption bands at different wavelengths is needed to better constrain the SO$_2$ abundance. Here we report the detection of SO$_2$ spectral features at 7.7 and 8.5 $μ$m in the 5-12 $μ$m transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). Our observations suggest an abundance of SO$_2$ of 0.5-25 ppm (1$σ$ range), consistent with previous findings. In addition to SO$_2$, we find broad water vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 $μ$m. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy element content (metallicity) for WASP-39b of $\sim$7.1-8.0 $\times$ solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

astro-ph.EP↗

arXiv2024

LHS 1140 b is a potentially habitable water world

LHS 1140 b is a small planet orbiting in the habitable zone of its M4.5V dwarf host. Recent mass and radius constraints have indicated that it has either a thick H$_2$-rich atmosphere or substantial water by mass. Here we present a transmission spectrum of LHS 1140 b between 1.7 and 5.2 $\mu$m, obtained using the NIRSpec instrument on JWST. By combining spectral retrievals and self-consistent atmospheric models, we show that the transmission spectrum is inconsistent with H$_2$-rich atmospheres with varied size and metallicity, leaving a water world as the remaining scenario to explain the planet's low density. Specifically, a H$_2$-rich atmosphere would result in prominent spectral features of CH$_4$ or CO$_2$ on this planet, but they are not seen in the transmission spectrum. Instead, the data favors a high-mean-molecular-weight atmosphere (possibly N$_2$-dominated with H$_2$O and CO$_2$) with a modest confidence. Forming the planet by accreting C- and N-bearing ices could naturally give rise to a CO$_2$- or N$_2$-dominated atmosphere, and if the planet evolves to or has the climate-stabilizing mechanism to maintain a moderate-size CO$_2$/N$_2$-dominated atmosphere, the planet could have liquid-water oceans. Our models suggest CO$_2$ absorption features with an expected signal of 20 ppm at 4.2 $\mu$m. As the existence of an atmosphere on TRAPPIST-1 planets is uncertain, LHS 1140 b may well present the best current opportunity to detect and characterize a habitable world.

astro-ph.EP↗

arXiv2024

Automated chemical reaction network generation and its application to exoplanet atmospheres

With the advent of JWST and spectroscopic characterization of exoplanet atmospheres with unprecedented detail, there is a demand for a more complete picture of chemical and photochemical reactions and their impact on atmospheric composition. Traditionally, building reaction networks for (exo)planetary atmospheres involves manually tracking relevant species and reactions, a time-consuming and error-prone process. This approach's applicability is also often limited to specific conditions, making it less versatile for different planetary types. (i.e., photochemical networks for Jupiters may not be directly applicable to water-rich exoplanets). We introduce an automated approach using a computer-aided chemical reaction network generator, combined with a one-dimensional photochemical kinetic-transport model, offering significant advantages. This approach automatically selects reaction rates through a rate-based iterative algorithm and refinement steps, enhancing model reliability. Also, this approach allows for the efficient simulation of diverse chemical environments, from hydrogen to water, carbon dioxide, and nitrogen-dominated atmospheres. Using WASP-39b and WASP-80b as examples, we demonstrate our approach's effectiveness. Our WASP-39b model aligns with prior studies and JWST observations, capturing photochemically produced sulfur dioxide. The WASP-80b model reveals an atmosphere influenced by deep interior thermochemistry and vertical mixing, consistent with JWST NIRCam observations. Furthermore, our model identifies a novel initial step for the N2-NH3-HCN pathway that enhances the conversion efficiency in high-temperature/pressure environments. This automated chemical network generation offers a novel, efficient, and precise framework for studying exoplanetary atmospheres, marking a significant advancement over traditional modeling techniques.

astro-ph.EP↗

arXiv2024

JWST Reveals CH$_4$, CO$_2$, and H$_2$O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet

Even though sub-Neptunes likely represent the most common outcome of planet formation, their natures remain poorly understood. In particular, planets near 1.5-2.5$\,R_\oplus$ often have bulk densities that can be explained equally well with widely different compositions and interior structures, resulting in grossly divergent implications for their formation. Here, we present the full 0.6-5.2$\,μ\mathrm{m}$ JWST NIRISS/SOSS+NIRSpec/G395H transmission spectrum of the 2.2$\,R_\oplus$ TOI-270d ($4.78\,M_\oplus$, $T_\mathrm{eq}$=350-380 K), delivering unprecedented sensitivity for atmospheric characterization in the sub-Neptune regime. We detect five vibrational bands of CH$_4$ at 1.15, 1.4, 1.7, 2.3, and 3.3$\,μ$m (9.4$σ$), the signature of CO$_2$ at 4.3$\,μ$m (4.8$σ$), water vapor (2.5$σ$), and potential signatures of SO$_2$ at 4.0$\,μ\mathrm{m}$ and CS$_2$ at 4.6$\,μ\mathrm{m}$. Intriguingly, we find an overall highly metal-rich atmosphere, with a mean molecular weight of $5.47_{-1.14}^{+1.25}$. We infer an atmospheric metal mass fraction of $58_{-12}^{+8}\%$ and a C/O of $0.47_{-0.19}^{+0.16}$, indicating that approximately half the mass of the outer envelope is in high-molecular-weight volatiles (H$_2$O, CH$_4$, CO, CO$_2$) rather than H$_2$/He. We introduce a sub-Neptune classification scheme and identify TOI-270d as a "miscible-envelope sub-Neptune" in which H$_2$/He is well-mixed with the high-molecular-weight volatiles in a miscible supercritical metal-rich envelope. For a fully miscible envelope, we conclude that TOI-270d's interior is $90_{-4}^{+3}\,$wt$\,\%$ rock/iron, indicating that it formed as a rocky planet that accreted a few wt % of H$_2$/He, with the overall envelope metal content explained by magma-ocean/envelope reactions without the need for significant ice accretion. TOI-270d may well be an archetype of the overall population of sub-Neptunes.

astro-ph.EP↗

arXiv2023

Photochemically-produced SO$_2$ in the atmosphere of WASP-39b

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Recent observations from the JWST Transiting Exoplanet Early Release Science Program found a spectral absorption feature at 4.05 $μ$m arising from SO$_2$ in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M$_J$) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of $\sim$1100 K. The most plausible way of generating SO$_2$ in such an atmosphere is through photochemical processes. Here we show that the SO$_2$ distribution computed by a suite of photochemical models robustly explains the 4.05 $μ$m spectral feature identified by JWST transmission observations with NIRSpec PRISM (2.7$σ$) and G395H (4.5$σ$). SO$_2$ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H$_2$S) is destroyed. The sensitivity of the SO$_2$ feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of $\sim$10$\times$ solar. We further point out that SO$_2$ also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

astro-ph.EP↗

arXiv2023

High-fidelity reaction kinetic modeling of hot-Jupiter atmospheres incorporating thermal and UV photochemistry enhanced by metastable CO(a3Pi)

A detailed modeling of simultaneous UV-photochemical and thermochemical processes in exoplanet atmosphere-like conditions is essential for the analysis and interpretation of a vast amount of current and future spectral data from exoplanets. However, a detailed reaction kinetic model that incorporates both UV photochemistry and thermal chemistry is challenging due to the massive size of the chemical system as well as to the lack of understanding of photochemistry compared to thermal-only chemistry. Here, we utilize an automatic chemical reaction mechanism generator to build a high-fidelity thermochemical reaction kinetic model later then incorporated with UV-photochemistry enhanced by metastable triplet-state carbon monoxide (a3Pi). Our model results show that two different photochemical reactions driven by Lyman-a photons (i.e. H2 + CO(a3Pi) -> H + HCO and CO(X1Sig+) + CO(a3Pi) -> C(3P) + CO2) can enhance thermal chemistry resulting in significant increases in the formation of CH4, H2O, and CO2 in H2-dominated systems with trace amounts of CO, which qualitatively matches with the observations from previous experimental studies. Our model also suggests that at temperatures above 2000 K, thermal chemistry becomes the dominant process. Finally, the chemistry simulated up to 2500 K does not produce any larger species such as C3 species, benzene or larger (i.e. PAHs). This might indicate that the photochemistry of C2 species such as C2H2 might play a key role in the formation of organic aerosols observed in the previous experimental study.

astro-ph.EP↗