arXiv subjects

Lindsey S. Wiser

Publications and source records attributed to Lindsey S. Wiser.

14 recordsLinked to original sources

arXiv2026

The Identification of CS2 and Evidence for Carbon-Sulfur Chemical Coupling in a Warm Giant Exoplanet Atmosphere

Transmission spectroscopy with the James Webb Space Telescope (JWST) is revealing growing chemical complexity in giant exoplanet atmospheres. Of particular interest is sulfur, which had essentially no observational constraints before JWST. Recent work has shown that a planet's atmospheric sulfur content traces its refractory budget and is therefore a sensitive indicator of formation pathways. But despite the growing library of JWST data, the sulfur inventory of giant exoplanets remains poorly constrained: sulfur-bearing species are governed by disequilibrium chemistry and by kinetic networks that are still being revised. Here we present a transmission spectrum of the warm giant planet WASP-80 b obtained with JWST/NIRCam and MIRI over 2.4 $μ$m--10$μ$m in three transits. We uniquely identify CS$_2$ in our transmission spectrum using the combination of the two absorption features in NIRCam and MIRI at a significance of $\ln (B)=17.89$ ($σ= 6.3$). Our grid-based retrievals yield $\mathrm{[M/H] = \:} 0.54^{+0.17}_{-0.12}$ and $\mathrm{C/O =\:}0.43^{+0.12}_{-0.08}$ which result in $\log(\mathrm{X_{CS_2}})$ abundances of $\sim-4.5$. The latest carbon-sulfur kinetics networks produce substantially greater amounts of CS$_2$ than past works, enabling good fits ($χ^2/\mathrm{N_{data}}\sim1.2$) to the data without invoking extreme abundance patterns. These results identify CS$_2$ as an observable tracer of sulfur disequilibrium chemistry and provide observational support for theoretically predicted carbon-sulfur chemical coupling in giant exoplanet atmospheres.

astro-ph.EP↗

arXiv2026

The tale of the 3 planets: 3D cloud feedback enhances the spectral diversity of warm Jupiters

JWST has shown a large diversity in warm Jupiter spectra, despite only small variations in the planetary parameters. However, the main driver of this diversity remains unclear. We aim to identify the mechanisms responsible for the spectral difference of three warm Jupiter-size exoplanets observed by JWST: whereas WASP-80b appears mostly cloud-free, both WASP-107b and WASP-69b have spectra dominated by clouds. We model each planet using the same framework, ADAM (formerly SPARC/MITgcm), which solves for the interactions among cloud transport, radiative transfer, and atmospheric circulation in 3D. We investigate the role of three condensate species, Na$_2$S, KCl, and MgSiO$_3$, and four particle sizes (0.1, 1, 5, and 10 $μ$m). Clouds settle deeper in the atmosphere of the higher-gravity planet WASP-80b than in WASP-107b, reproducing their spectral difference naturally. For WASP-107b, three clouds can reproduce the NIRCam observations: 5 $μ$m Na$_2$S, 1 $μ$m KCl, and 5 $μ$m MgSiO$_3$ models. However, these cannot match the scattering slope observed at shorter wavelengths in NIRISS and the possible silicate feature in the MIRI bandpass, suggesting a multi-modal distribution of clouds. Our model predicts that small silicate particles should be homogeneously distributed and thus cannot account for the difference between the two limb spectra in the MIRI bandpass. Finally, applying the same model to WASP-69b does not yield a partially cloudy dayside solution that fits the emission spectra, as proposed in a previous study. Coupling among 3D circulation, clouds, and radiative transfer can enhance the spectral diversity of warm Jupiter exoplanets, particularly through changes in cloudiness with gravity. The combination of multi-phase, wide-wavelength coverage and models that couple clouds, circulation, and radiative transfer is key to advancing our understanding of these new objects.

astro-ph.EP↗

arXiv2026

Cloudy solutions for the clear skies of WASP-80b: 3D cloud feedback on the atmosphere and spectra of a warm Jupiter

Close-in warm Jupiters orbiting M dwarf stars are expected to exhibit diverse atmospheric chemistry, with clouds playing a key role in shaping their albedo, heat distribution, and spectral properties. We study WASP-80b, a warm Jupiter orbiting an M dwarf star, using the latest JWST panchromatic emission and transmission spectra to comprehensively characterise its atmosphere, including cloud coverage, chemical composition, and particle sizes, and compare the observations with predictions from the general circulation models (GCMs). We used a GCM, ADAM (ADvanced Atmospheric MITgcm, formerly known as SPARC/MITgcm), combined with the latest JWST data to study the atmosphere of WASP-80b. A cloud module with radiatively active, tracer-based clouds was integrated with the GCM to study the effects on the atmosphere and the spectrum. We find that the emission and transmission spectra of WASP-80b are only compatible with cloudless atmospheres or with clouds composed of sufficiently large particles, namely Na$_2$S ($\geq 10 μ$m), KCl ($\geq 1 μ$m), and MgSiO$_3$ ($\geq 5 μ$m). For these large-particle cloud cases, efficient gravitational settling confines the clouds to deeper atmospheric layers, resulting in weak spectral signatures. Smaller particles are ruled out due to their strong radiative feedback on the atmospheric structure. Overall, our results suggest that WASP-80b's atmosphere is either effectively cloud-free or contains clouds composed of large, settled particles whose opacity has little impact on the observable atmosphere. This underscores the importance of particle size and vertical cloud distribution in interpreting exoplanet spectra. Future observations at shorter wavelengths may help distinguish between large-particle cloud scenarios and a truly cloudless atmosphere.

astro-ph.EP↗

arXiv2026

NASA's Pandora SmallSat Mission: Simulated Modeling and Retrieval of Near-Infrared Exoplanet Transmission Spectra

Pandora is a SmallSat mission dedicated to understanding exoplanets and their host stars by disentangling the impact of stellar heterogeneity on exoplanet transmission spectra. Selected as a NASA Astrophysics Pioneers mission in 2021, Pandora will provide simultaneous long-term visible photometric monitoring (0.4--0.7 $μ$m) and low-resolution near-infrared (NIR) spectroscopy (0.9--1.6 $μ$m) of transiting systems for the purposes of monitoring host star variability and characterizing exoplanetary atmospheres. Pandora's year-long prime mission from 2026 to 2027 coincides with the middle of a decade defined by targeted efforts for atmospheric characterization of exoplanets, offering a key opportunity to leverage this new resource to maximize science with JWST and other observatories. Here we investigate Pandora's anticipated performance for the general exoplanet population accessible to transit spectroscopy, from hot Jupiters to temperate sub-Neptunes. By modeling the atmospheres of five test cases broadly consistent with the bulk properties of HD~209458~b, HD~189733~b, WASP-80~b, HAT-P-18~b, and K2-18~b, we find that Pandora may provide abundance constraints as precise as $\sim$1.0\,dex for main atmospheric absorbers such as H$_2$O and CH$_4$. Then, we explore the synergies between Pandora and JWST. Our results suggest that targets with JWST data in the near-infrared can benefit from the addition of Pandora observations and result in more reliable abundance estimates than with JWST data alone. Moreover, Pandora can serve the community by providing precursory observations of targets of interest for JWST atmospheric characterization. We conclude by outlining strategies for the use of Pandora as a standalone observatory and in synergy with JWST.

astro-ph.EP↗

arXiv2025

Challenges in the detection of gases in exoplanet atmospheres

Claims of detections of gases in exoplanet atmospheres often rely on comparisons between models including and excluding specific chemical species. However, the space of molecular combinations available for model construction is vast and highly degenerate. Only a limited subset of these combinations is typically explored for any given detection. As a result, apparent detections of trace gases risk being artifacts of incomplete modeling rather than robust identification of atmospheric constituents, especially in the low signal-to-noise regime. Using the sub-Neptune K2-18 b as a case study, we show that recent biosignature claims vanish when the model space is expanded, with numerous alternatives providing equally good or better fits. We demonstrate that the significance of a claimed detection relies on the choice of models being compared, and that model preference does not in itself imply the presence of a specific gas. We recommend treating model comparisons instead as relative adequacy tests, which should be supported by theoretical predictions and complementary metrics of statistical significance in order to attribute a signal to a particular gas.

astro-ph.EP↗

arXiv2025

A Comparison of 1D and 3D Exoplanet Atmosphere Model Grids: ScCHIMERA and the SPARC/MITgcm

Inferring the properties of transiting exoplanet atmospheres relies on comparing models to spectroscopic observations. Atmosphere models, however, make a range of assumptions, from one-dimensional (1D, varying with altitude) radiative-convective equilibrium (RCE) to three-dimensional (3D) general circulation models (GCMs). The goal of this investigation is to determine the causes of differences in dayside thermal emission spectra resulting from 3D-GCMs (using SPARC/MITgcm) and 1D-RCE models (using ScCHIMERA). We conduct a one-to-one comparison of 1D-RCE models and 3D-GCMs with the same outgoing bolometric thermal flux over a grid of equilibrium temperatures, gravities, metallicities, and rotation periods. Each 1D-RCE model assumes heat redistribution in the planet's atmosphere consistent with that in the corresponding 3D-GCM's photosphere. Comparing corresponding models, the dayside average pressure-temperature (PT) structures can be broken into four vertical regions, each influencing wavelength-dependent differences in their spectra. Furthermore, the dayside average 3D-GCM PTs for planets with Teq=1400 K exhibit a temperature inversion, whereas corresponding 1D-RCE models do not. We find that spectral differences between 1D-RCE models and 3D-GCMs with the same parameters decrease for hotter planets because the spectral shapes more closely resemble blackbodies. To a lesser extent, spectral differences increase for planets with longer rotation periods because of smaller day-night temperature contrasts in the photosphere. Finally, we compare spectral differences to realistic observational uncertainties from JWST with the NIRISS SOSS, NIRSpec G395H, and MIRI LRS instrument modes. We find that 1D-RCE models and 3D-GCMs with the same parameters can produce dayside spectral differences larger than JWST's uncertainty, potentially biasing data-model inferences.

astro-ph.EP↗

arXiv2025

A Precise Metallicity and Carbon-to-Oxygen Ratio for a Warm Giant Exoplanet from its Panchromatic JWST Emission Spectrum

WASP-80 b, a warm sub-Jovian (equilibrium temperature ~820 K, 0.5 Jupiter masses), presents an opportunity to characterize a rare gas giant exoplanet around a low-mass star. In addition, its moderate temperature enables its atmosphere to host a range of carbon and oxygen species (H$_2$O, CH$_4$, CO, CO$_2$, NH$_3$). In this paper, we present a panchromatic emission spectrum of WASP-80 b, the first gas giant around a late K/early M-dwarf star and the coolest planet for which the James Webb Space Telescope has obtained a complete emission spectrum spanning 2.4-12 $μ$m, including NIRCam F322W2 (2.4-4 $μ$m) and F444W (4-5 $μ$m), and MIRI LRS (5-12 $μ$m). We report confident detections of H$_2$O, CH$_4$, CO, and CO$_2$, and a tentative detection of NH$_3$. We estimate WASP-80 b's atmospheric metallicity and carbon-to-oxygen ratio and compare them with estimates for other gas giants. Despite the relative rarity of giant planets around low-mass stars, we find that WASP-80 b's composition is consistent with other hot gas giants, suggesting that the formation pathway of WASP-80 b may not be dissimilar from hot gas giants around higher-mass stars.

astro-ph.EP↗

arXiv2025

Escaping Helium and a Highly Muted Spectrum Suggest a Metal-Enriched Atmosphere on Sub-Neptune GJ3090b from JWST Transit Spectroscopy

Sub-Neptunes, the most common planet type, remain poorly understood. Their atmospheres are expected to be diverse, but their compositions are challenging to determine, even with JWST. Here, we present the first JWST spectroscopic study of the warm sub-Neptune GJ3090b (2.13R$_\oplus$, Teq~700 K) which orbits an M2V star, making it a favourable target for atmosphere characterization. We observed four transits of GJ3090b; two each using JWST NIRISS/SOSS and NIRSpec/G395H, yielding wavelength coverage from 0.6-5.2 $μ$m. We detect the signature of the 10833 Å metastable Helium triplet at a statistical significance of 5.5$σ$ with an amplitude of 434$\pm$79 ppm, marking the first such detection in a sub-Neptune with JWST. This amplitude is significantly smaller than predicted by solar-metallicity forward models, suggesting a metal-enriched atmosphere which decreases the mass-loss rate and attenuates the Helium feature amplitude. Moreover, we find that stellar contamination, in the form of the transit light source effect, dominates the NIRISS transmission spectra, with unocculted spot and faculae properties varying across the two visits separated in time by approximately six months. Free retrieval analyses on the NIRSpec/G395H spectrum find tentative evidence for highly muted features and a lack of CH4. These findings are best explained by a high metallicity atmosphere (>100x solar at 3$σ$ confidence, for clouds at $\sim μ$bar pressures) using chemically-consistent retrievals and self-consistent model grids. Further observations of GJ3090b are needed for tighter constraints on the atmospheric abundances, and to gain a deeper understanding of the processes that led to its potential metal enrichment.

astro-ph.EP↗

arXiv2025

A JWST Panchromatic Thermal Emission Spectrum of the Warm Neptune Archetype GJ 436b

GJ 436b is the archetype warm Neptune exoplanet. The planet's thermal emission spectrum was previously observed via intensive secondary eclipse campaigns with Spitzer. The atmosphere has long been interpreted to be extremely metal-rich, out of chemical equilibrium, and potentially tidally heated. We present the first panchromatic emission spectrum of GJ 436b observed with JWST's NIRCAM (F322W2 and F444W) and MIRI (LRS) instruments between 2.4 and 11.9 $μ$m. Surprisingly, the JWST spectrum appears significantly fainter around 3.6 $μ$m than that implied by Spitzer photometry. The molecular absorption features in the spectrum are relatively weak, and we only find tentative evidence of CO$_2$ absorption at 2$σ$ significance. Under the assumption of a day-side blackbody, we find $T_{\rm day}$=662.8$\pm$5.0 K, which is similar to the zero Bond albedo equilibrium temperature. We use it to obtain a 3$σ$ upper limit on the Bond albedo of $A_B{\le}$0.66. To understand the spectrum we employ 1D radiative-convective models but find that atmospheric constraints depend strongly on model assumptions. If thermochemical equilibrium is assumed, we find a cloudy metal-enriched atmosphere (metallicity $\ge$ 300$\times$solar). We employ 1D photochemical modeling to show that the observed spectrum is also consistent with a cloud-free, relatively lower-metallicity atmosphere (metallicity $\ge$ 80$\times$solar) with a cold internal temperature ($T_{\rm int}$$\sim$60 K). These are much lower metallicities and internal temperatures than inferences from Spitzer photometry. The low $T_{\rm day}$ and non-detection of transmission features at high spectral resolution does suggest a role for cloud opacity, but this is not definitive.

astro-ph.EP↗

arXiv2024

Lessons from Hubble and Spitzer: 1D Self-Consistent Model Grids for 19 Hot Jupiter Emission Spectra

We present a population-level analysis of the dayside thermal emission spectra of 19 planets observed with Hubble WFC3 and Spitzer IRAC 3.6 and 4.5 microns, spanning equilibrium temperatures 1200-2700 K and 0.7-10.5 Jupiter masses. We use grids of planet-specific 1D, cloud-free, radiative-convective-thermochemical equilibrium models (1D-RCTE) combined with a Bayesian inference framework to estimate atmospheric metallicity, the carbon-to-oxygen ratio, and day-to-night heat redistribution. In general, we find that the secondary eclipse data cannot reject the physics encapsulated within the 1D-RCTE assumption parameterized with these three variables. We find a large degree of scatter in atmospheric metallicities, with no apparent trend, and carbon-to-oxygen ratios that are mainly consistent with solar or subsolar values but do not exhibit population agreement. Together, these indicate either (1) formation pathways vary over the hot and ultra-hot Jupiter population and/or (2) more accurate composition measurements are needed to identify trends. We also find a broad scatter in derived dayside temperatures that do not demonstrate a trend with equilibrium temperature. Like with composition estimates, this suggests either significant variability in climate drivers over the population and/or more precise dayside temperature measurements are needed to identify a trend. We anticipate that 1D-RCTE models will continue to provide valuable insights into the nature of exoplanet atmospheres in the era of JWST.

astro-ph.EP↗

arXiv2024

Sulfur Dioxide and Other Molecular Species in the Atmosphere of the Sub-Neptune GJ 3470 b

We report observations of the atmospheric transmission spectrum of the sub-Neptune exoplanet GJ 3470 b taken using the Near-Infrared Camera (NIRCam) on JWST. Combined with two archival HST/WFC3 transit observations and fifteen archival Spitzer transit observations, we detect water, methane, sulfur dioxide, and carbon dioxide in the atmosphere of GJ 3470 b, each with a significance of >3-sigma. GJ 3470 b is the lowest mass -- and coldest -- exoplanet known to show a substantial sulfur dioxide feature in its spectrum, at $M_{p}$=11.2${\,{\rm M}_{\oplus}}$ and $T_{eq}$=600$\,$K. This indicates disequilibrium photochemistry drives sulfur dioxide production in exoplanet atmospheres over a wider range of masses and temperatures than has been reported or expected. The water, carbon dioxide, and sulfur dioxide abundances we measure indicate an atmospheric metallicity of approximately $100\times$ Solar. We see further evidence for disequilibrium chemistry in our inferred methane abundance, which is significantly lower than expected from equilibrium models consistent with our measured water and carbon dioxide abundances.

astro-ph.EP↗

arXiv2024

A High Internal Heat Flux and Large Core in a Warm Neptune Exoplanet

Interactions between exoplanetary atmospheres and internal properties have long been hypothesized to be drivers of the inflation mechanisms of gaseous planets and apparent atmospheric chemical disequilibrium conditions. However, transmission spectra of exoplanets has been limited in its ability to observational confirm these theories due to the limited wavelength coverage of HST and inferences of single molecules, mostly H$_2$O. In this work, we present the panchromatic transmission spectrum of the approximately 750 K, low-density, Neptune-sized exoplanet WASP-107b using a combination of HST WFC3, JWST NIRCam and MIRI. From this spectrum, we detect spectroscopic features due to H$_2$O (21$σ$), CH$_4$ (5$σ$), CO (7$σ$), CO$_2$ (29$σ$), SO$_2$ (9$σ$), and NH$_3$ (6$σ$). The presence of these molecules enable constraints on the atmospheric metal enrichment (M/H is 10--18$\times$ Solar), vertical mixing strength (log$_{10}$K$_{zz}$=8.4--9.0 cm$^2$s$^{-1}$), and internal temperature ($>$345 K). The high internal temperature is suggestive of tidally-driven inflation acting upon a Neptune-like internal structure, which can naturally explain the planet's large radius and low density. These findings suggest that eccentricity driven tidal heating is a critical process governing atmospheric chemistry and interior structure inferences for a majority of the cool ($<$1,000K) super-Earth-to-Saturn mass exoplanet population.

astro-ph.EP↗

arXiv2023

Methane Throughout the Atmosphere of the Warm Exoplanet WASP-80b

The abundances of major carbon and oxygen bearing gases in the atmospheres of giant exoplanets provide insights into atmospheric chemistry and planet formation processes. Thermochemistry suggests that methane should be the dominant carbon-bearing species below $\sim$1000 K over a range of plausible atmospheric compositions; this is the case for the Solar System planets and has been confirmed in the atmospheres of brown dwarfs and self-luminous directly imaged exoplanets. However, methane has not yet been definitively detected with space-based spectroscopy in the atmosphere of a transiting exoplanet, but a few detections have been made with ground-based, high-resolution transit spectroscopy including a tentative detection for WASP-80b. Here we report transmission and emission spectra spanning 2.4-4.0 micrometers of the 825 K warm Jupiter WASP-80b taken with JWST's NIRCam instrument, both of which show strong evidence for methane at greater than 6-sigma significance. The derived methane abundances from both viewing geometries are consistent with each other and with solar to sub-solar C/O and ~5$\times$ solar metallicity, which is consistent with theoretical predictions.

astro-ph.EP↗

arXiv2023

A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

Close-in giant exoplanets with temperatures greater than 2,000 K (''ultra-hot Jupiters'') have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble and Spitzer Space Telescopes. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS instrument on JWST. The data span 0.85 to 2.85 $μ$m in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at $>$6$σ$ confidence) and evidence for optical opacity, possibly due to H$^-$, TiO, and VO (combined significance of 3.8$σ$). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy element abundance (''metallicity'', M/H = 1.03$_{-0.51}^{+1.11}$ $\times$ solar), and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the sub-stellar point that decreases steeply and symmetrically with longitude toward the terminators.

astro-ph.EP↗