arXiv subjects

Michael Radica

Publications and source records attributed to Michael Radica.

At least 19 recordsLinked to original sources

arXiv2026

Atmospheric Reconnaissance of TRAPPIST-1 f with JWST NIRISS SOSS: No Evidence for the Transit Light Source Effect

In just over three years of operation, JWST has observed all seven planets of the TRAPPIST-1 system. The two innermost planets were found to have little to no atmosphere, barring the presence of high-altitude aerosols. Here we present the first JWST transit spectra of the habitable-zone exoplanet TRAPPIST-1 f, which were obtained with NIRISS SOSS over the course of five transits. At least one stellar flare occurred in each visit, but unlike observations of closer-in TRAPPIST-1 planets, no evidence for contamination of the transit spectra from unocculted stellar surface heterogeneities was found. This non-detection does not guarantee the absence of unocculted heterogeneities in all future transit observations of this planet, and it could be explained by the transit chord of TRAPPIST-1 f having properties similar to the average, out-of-transit, visible stellar hemisphere at the time of observation. The transit spectra exhibit slopes ranging from -365 ppm/um down to 15 ppm/um, which we attribute to stellar variability, that is, flares and/or smaller-scale events. The visits least affected by flares rule out H2/He-dominated atmospheres with surface pressures higher than about 20 mbar at 95% confidence. For high-mean-molecular-mass atmospheres, the exact upper limits on surface pressures depend on the reduction pipeline and on the treatment of the residual slopes in the transit spectra.

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arXiv2026

Strict Limits on Helium Absorption from LHS 1140 b from Four JWST NIRISS Transits

Orbiting in the habitable zone of its host star, the 1.7 $R_\oplus$, 5.6 $M_\oplus$ planet LHS 1140 b is a target of great interest. Recently Cherubim et al. (2026) published a detection of metastable He escaping from the atmosphere of LHS 1140 b, simultaneously providing the first concrete inference of an atmosphere on this planet and indicating that the atmosphere is He-rich and H-poor as would be expected due to Gyrs of fractionated mass loss. In this work, we analyze four archival transits of LHS 1140 b, spanning Dec 2023 to Jul 2026, taken with the NIRISS instrument on JWST, for evidence of He absorption. Each of the four visits disfavours the presence of He absorption compared to a flat continuum with odds ratios ranging from 3.9--11.6:1. He absorption with an amplitude and width equivalent to that observed by Cherubim et al. (2026) is strongly ruled out by the data with odds ratios from 300--8.6$\times$10$^4$:1 compared to a flat continuum --- though it should be noted that none of the JWST transits are contemporaneous with the Cherubim et al. (2026) detection. We also fit the absolute out-of-transit stellar spectra from these four visits, as well as an additional JWST NIRISS transit of planet c, to search for evidence of stellar variability, but find consistent photosphere and herterogeneity parameters in all five datasets. In all, our work provides a set of strict limits on He escape from LHS 1140 b that will be valuable to future studies into the nature and evolution of this intriguing world.

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arXiv2026

KRONOS II: Solar-like Umbra and Penumbra Properties on the Young Sun V1298~Tau

Transiting exoplanets provide a unique laboratory for studying stellar surface heterogeneities via starspot or facular occultations. When observed at multiple wavelengths, this configuration enables spectroscopic characterization of spot thermal contrasts, distributions, and morphology. In this work, we leverage JWST NIRISS/SOSS transit observations of the 20--30~Myr planets V1298~Tau~bcd to study the surface properties of their solar analog host star V1298~Tau. We identify 14 starspot crossing events across two visits. We derive $0.8-2.8\mu$m starspot contrast spectra and demonstrate the contrasts can only be explained when accounting for the umbral and penumbral components of the starspots, robust to which stellar model grid is assumed. The spot temperatures are broadly consistent between visits, suggesting that V1298~Tau ($T_\mathrm{phot}=4866\pm33$\,K) has starspots with $T_{\mathrm{umbra}}$ = 3300--3600\,K umbrae and $T_{\mathrm{penumbra}}$ = 4400--4600\,K penumbrae, and are $\sim$25--30\% umbrae by area. The differences between these spot components and the stellar photosphere are consistent with sunspots. Additionally, the relation between the spot contrast and the ratio of umbral to penumbral area is similar to that of the Sun. Combining these JWST observations with long baseline multi-band photometry from the Las Cumbres Observatory, we also estimated the global unocculted spot distribution, revealing at least 5 additional large unocculted active regions. Altogether, these measurements suggest that while the total spot coverage evolves in time, the relative temperatures of surface heterogeneities on Sun-like stars may be consistent throughout their lifetimes. Furthermore, these results demonstrate that JWST exoplanet transit observations can be useful for starspot substructure characterization.

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arXiv2026

KRONOS I: The $1{-}2.8\mu$m JWST Transmission Spectrum of the 23 Myr V1298 Tau c

While recent JWST observations of mature super-Earths and sub-Neptunes have frequently revealed featureless transmission spectra, their inflated progenitors offer a unique window into understanding their primordial compositions. As part of the Keys to Revealing the Origin and Nature Of sub-neptune Systems (KRONOS) JWST program, we present the NIRISS/SOSS transmission spectrum of V1298 Tau c, a $\sim$23 Myr super-Earth progenitor orbiting a young Solar analog. We detect H$_2$O in V1298 Tau c's atmosphere with a $\log_{10}$ volume mixing ratio of $-1.83^{+0.68}_{-0.77}$, but no additional molecules from these data alone. We find consistent results for the planetary atmospheric properties in both retrievals with and without informed priors on stellar heterogeneities based on the observed stellar spectrum. We infer an atmospheric metallicity [O/H] of $14.8^{+56.0}_{-12.28}\times$ the solar value. This metallicity is similar to literature measurements for other young planets, including its massive outer companion V1298~Tau~b. In contrast, this measured metallicity is systematically lower than the metallicities of mature planets of similar mass and temperature. Altogether, these results provide tentative but growing evidence that the exoplanet mass--metallicity relation evolves with planetary age.

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arXiv2026

The Days Drag On on WASP-121 b: Interpreting its NIRISS Spectroscopic Phase Curve with General Circulation Models

Ultra-hot Jupiters present extreme atmospheric phenomena not found in the Solar System. These planets' daysides experience strong temperature inversions, molecular species (including H2) dissociate, and magnetism disrupts their atmospheric circulation. On their nightsides H2 can recombine and clouds may form. Spectroscopic phase curves let us measure these spatially inhomogeneous conditions, which can then be interpreted with three-dimensional (3-D) models. In this work we compare the JWST/NIRISS spectroscopic phase curve of the ultra-hot Jupiter WASP-121 b to state-of-the-art 3-D models with varying modeling assumptions, including the aforementioned physical phenomena. We demonstrate the importance of accurately accounting for the planet's radius in comparison between data and models, as it changes the implied overall planetary emission. We find that the 3-D models predict planet emission $\sim$12% higher than observed, contributing to a continued tension between measured and predicted hot Jupiter albedos. We identify multiple pieces of evidence that confirm a strong source of drag operating in this planet's atmosphere. In addition, the nightside emission spectrum is devoid of strong absorption features, which may be best explained by nightside clouds. One feature of the dataset that is not matched by the 3-D models is a trend of increasing eastward phase offset with decreasing wavelength, for wavelengths shorter than $\sim$1.4 \textmu m. This result is not consistent with reflection from dayside clouds, nor can it be explained by removing atmospheric opacity sources. Our analysis highlights the complexities in generating 3-D models and interpreting observations of ultra-hot Jupiters in the JWST era.

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arXiv2026

On the Information Content of Ariel Transmission Spectra: Reassessing the Tier System

The European Space Agency's Ariel mission will conduct a survey of the atmospheric properties of exoplanets around bright stars. The mission is nominally divided into three Tiers. The Tier 1 survey will consist of low-precision observations of ~1000 planets, with a subset of these included in the higher-precision Tier 2 survey expected to be necessary for atmospheric characterization. Tier 3 will be repeated observations of a small number of benchmark planets. Though previous studies have assessed the ability of Ariel to uncover population-level trends, they have generally presupposed a given Tier. Here we interrogate this assumption and assess the information content of Ariel transmission spectra as a function of Tier for three benchmark planets: a hot-Saturn, warm-Neptune, and temperate sub-Neptune. We simulate a grid of Ariel transit spectra at different Tiers for each target and use retrievals to assess which chemical species are well constrained. We find that for giant planets like a hot-Saturn or warm-Neptune, Tier 1-quality observations are sufficient for $\lesssim$1.5\,dex constraints on H$_2$O and CO$_2$, even in the presence of grey clouds --- meaning important chemical insights are already obtainable in the Tier 1 survey. Moving to Tiers 2 and 3 results in an incremental increase in precision as well as other molecules becoming constrainable in certain scenarios (e.g., H$_2$S, CO). Tier 1 observations are also sufficient to identify CH$_4$ in a temperate sub-Neptune, whereas observations with at least Tier 2 precision are necessary for CO$_2$. The number of transits necessary to reach this observational S/N, however, may be prohibitive for the inclusion of temperate sub-Neptunes in even the Tier 1 survey.

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arXiv2026

Super-Solar Metallicity and Tentative Evidence for Photochemistry on WASP-96b from JWST and Ground-Based VLT Transmission Spectroscopy

With its expanded wavelength coverage and increased precision compared to previous space-based observatories, JWST provides the opportunity to revisit benchmark planets and view them in a new light. Here, we conduct an in-depth study of the atmosphere of the hot-Saturn WASP-96b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and VLT/FORS2 transmission spectra. The combined spectrum shows clearly-visible features from H2O, CO2, and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly super-stellar atmospheric metallicity of 2-6x stellar. When combined with a roughly stellar C/O ratio ($0.41^{+0.10}_{-0.09}$ from self-consistent grids), we find that WASP-96b potentially formed via core-accretion beyond the H2O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference (ln B=2.69) for models with SO2 versus without. WASP-96b falls directly on the proposed "SO2 shoreline" and the retrieved SO2 abundance is well-matched to predictions from photochemical models. Our combined spectrum displays an optical slope, which our models fit with opacity from scattering aerosols -- either small-particle condensate clouds or photochemical hazes -- though we cannot completely rule out the broad wings of Na or the effects of stellar contamination. Future observations are necessary to disentangle these effects. Finally, we explore the possibility for limb asymmetry in WASP-96b's transmission spectrum and provide several tests to identify asymmetries in our data. We encourage the community to prioritize the development of a robust pathway to quantify the presence of limb asymmetry -- particularly for low signal-to-noise cases.

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arXiv2026

New Constraints on the M Dwarf Cosmic Shoreline from a Galaxy Far, Far Away

Whether there is a cosmic shoreline that divides terrestrial planets which have atmospheres from those that don't is one of the biggest open questions in exoplanet science. Most atmosphere searches have focused on terrestrial planets around M dwarf stars, since their smaller radii compared to sun-like stars boost planet atmosphere signals. However, the higher activity levels of M dwarfs might also entirely preclude atmosphere retention for their planets. In this work we present a new hope for defining an M dwarf cosmic shoreline, leveraging not only data from exoplanets in our own galaxy, but a comprehensive survey conducted by a commission of the Galactic Republic a long, long time ago in a galaxy far, far away. In this survey, we find definitive proof that M dwarf planets can retain atmospheres, and define an M dwarf cosmic shoreline whose slope agrees well with empirical predictions for Sun-like stars. We then define atmosphere retention metrics for the planets on the JWST Rocky Worlds DDT Targets Under Consideration list. Our analysis highlights the benefits of looking beyond the Milky Way for answers to some of the field's most pressing questions.

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arXiv2025

Diversity in the haziness and chemistry of temperate sub-Neptunes

Recent transit observations of K2-18b and TOI-270d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (T$_\mathrm{eq}$=250-400K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO$_2$ abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here, we present the JWST NIRSpec/PRISM 0.7-5.4$\mathbf{\mu}$m transmission spectrum of a third temperate sub-Neptune, the 2.4R$_\oplus$ planet LP 791-18c (T$_\mathrm{eq}$=355K), which is even more favorable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite LP 791-18c's radius, mass, and equilibrium temperature being in between those of K2-18b and TOI-270d, we find a drastically different transmission spectrum. While we also detect methane on LP 791-18c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO$_2$ absorption. Overall, we infer a deep metal-enriched atmosphere (246-415$\times$solar) for LP 791-18c, with a CO$_2$-to-CH$_4$ ratio smaller than 0.07 (at 2$\sigma$), indicating less H$_2$O in the deep envelope of LP 791-18c and implying a relatively dry formation inside the water ice-line. These results show that sub-Neptunes that are near-analogues in density and temperature can show drastically different aerosols and envelope chemistry, and are intrinsically diverse beyond a simple temperature dependence.

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arXiv2025

Separating flare and secondary atmospheric signals with RADYN modeling of near-infrared JWST transmission spectroscopy observations of TRAPPIST-1

Although TRAPPIST-1's temperate planets have the highest transmission signals of any known system, flares contaminate 50-70% of transits at the 1000 ppm level, far above 100 ppm secondary atmospheres. Efforts to mitigate flare contamination and assess impacts on radiation environments are each hampered by a lack of empirical spectral analysis and physics-based modeling. We present spectrotemporal analysis and radiative-hydrodynamic modeling of 5.5 hr of NIRISS and NIRSpec observations of 6 TRAPPIST-1 flares of 2.2-8.7x10^30 erg. Flare lines and continua are characterized using grid searches of RADYN beam-heating models spanning 10$^4\times$ in electron beam parameters. Best-fit models indicate these flares result from moderate-intensity beams with emergent electron fluxes of 10^12 erg s^-1 cm^-2 and energies $\leq$37 keV, although all models over-predict the Paschen jump. These models predict XUV, FUV, and NUV counterparts to the infrared peak fluxes of 8.9-28.9x10^27, 4.3-13.9x10^26, and 3.4-11.4x10^27 erg s^-1, respectively. Scaling the flare rate into the XUV suggests flaring contributes 1.35$_{-0.15}^{+2.0}\times$ quiescence yr$^{-1}$. We bin integrations of similar flare effective temperature to construct fiducial flare spectra from 2000-4500 K in order to develop separate empirical and RADYN-based mitigation pipelines. Both pipelines are applied to all 5.5 hr of R=10 data, resulting in maximum residuals from 1-2.8$\mu$m of 100-140 ppm and typical residuals of 54$\pm$14 and 65$\pm$17 ppm for the empirical and RADYN-based pipelines, respectively. Injection testing supports 3$\sigma$ detection capability for CO2 atmospheres with features of 150-250 ppm, with weak evidence (BF$\approx$3) still obtained at 130 ppm. Our results motivate multi-wavelength observations to improve model fidelity and test high-energy predictions.

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arXiv2025

No TiO detected in the hot Neptune-desert planet LTT-9779 b in reflected light at high spectral resolution

LTT-9779 b is an inhabitant of the hot Neptune desert and one of only a few planets with a measured high albedo. Characterising the atmosphere of this world is the key to understanding what processes dominate in creating the hot Neptune desert. We aim to characterise the reflected light of LTT-9779 b at high spectral resolution to break the degeneracy between clouds and atmospheric metallicity. This is key to interpreting its mass loss history which may illuminate how it kept its place in the desert. We use the high resolution cross-correlation spectroscopy technique on four half-nights of ESPRESSO observations in 4-UT mode (16.4-m effective mirror) to constrain the reflected light spectrum of LTT-9779 b. We do not detect the reflected light spectrum of LTT-9779 b despite these data having the expected sensitivity at the level 100 ppm. Injection tests on the post-eclipse data indicate that TiO should have been detected for a range of different equilibrium chemistry models. Therefore this non-detection suggests TiO depletion in the western hemisphere however, this conclusion is sensitive to temperature which impacts the chemistry in the upper atmosphere and the reliability of the line list. Additionally, we are able to constrain the top of the western cloud deck to $P_{\text{top, western}}<10^{-2.0}$ bar and the top of the eastern cloud deck $P_{\text{top, eastern}}<10^{-0.5}$ bar, which is consistent with the predicted altitude of MgSiO$_3$ and Mg$_2$SiO$_4$ clouds from JWST NIRISS/SOSS. While we do not detect the reflected light spectrum of LTT-9779 b, we have verified that this technique can be used in practice to characterise the high spectral resolution reflected light of exoplanets so long as their spectra contain a sufficient number of deep spectral lines. Therefore this technique may become an important cornerstone of exoplanet characterisation with the ELT and beyond.

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arXiv2025

Horizontal and vertical exoplanet thermal structure from a JWST spectroscopic eclipse map

Highly-irradiated giant exoplanets known as "ultra-hot Jupiters" are anticipated to exhibit large variations of atmospheric temperature and chemistry as a function of longitude, latitude, and altitude. Previous observations have hinted at these variations, but the existing data have been fundamentally restricted to probing hemisphere-integrated spectra, thereby providing only coarse information on atmospheric gradients. Here we present a spectroscopic eclipse map of an extrasolar planet, resolving the atmosphere in multiple dimensions simultaneously. We analyze a secondary eclipse of the ultra-hot Jupiter WASP-18b observed with the NIRISS instrument on JWST. The mapping reveals weaker longitudinal temperature gradients than were predicted by theoretical models, indicating the importance of hydrogen dissociation and/or nightside clouds in shaping global thermal emission. Additionally, we identify two thermally distinct regions of the planet's atmosphere: a "hotspot" surrounding the substellar point and a "ring" near the dayside limbs. The hotspot region shows a strongly inverted thermal structure due to the presence of optical absorbers and a water abundance marginally lower than the hemispheric average, in accordance with theoretical predictions. The ring region shows colder temperatures and poorly constrained chemical abundances. Similar future analyses will reveal three-dimensional thermal, chemical, and dynamical properties of a broad range of exoplanet atmospheres.

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arXiv2025

A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121\,b

Atmospheric escape of planets on short orbital periods, driven by the host star's irradiation, influences their evolution, composition, and atmospheric dynamics. Our main avenue to probe atmospheric escape is through the near-infrared metastable helium triplet, which has enabled mass loss rate measurements for tens of exoplanets. Among them, only a few studies show evidence for out-of-transit absorption, supporting the presence of a hydrodynamic outflow. However, none of these observations precisely identified the physical extent of the outflow, either due to non-continuous or short-duration observations. This limits our measurements of accurate mass loss rates. Here we present the first continuous, full-orbit helium phase curve monitoring of an exoplanet, the ultra-hot Jupiter WASP-121b, obtained with JWST/NIRISS. It reveals helium absorption for nearly 60% of the orbit at >3sigma significance. Our results show that WASP-121b sustains a strong outflow, separating into two tails trailing and leading the planet. The persistent absorption from these tails, together with their measured radial velocity shifts, suggests that they remain in a collisional fluid regime at large distances from the planet and display very different dynamics. The leading trail has a higher density and moves toward the star, whereas the trailing trail is being pushed away from the star, with the latter being blue-shifted due to stellar irradiation pressure. While qualitatively agreeing with theoretical expectations, the observed structure of helium is not self-consistently reproducible by current models, limiting constraints on the mass loss rate. Furthermore, we show that while ground-based observations of the helium triplet are essential to measure the outflow dynamics precisely, they ideally should be combined with continuous JWST phase curves to constrain the absolute level of helium absorption.

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arXiv2025

Possible Evidence for the Presence of Volatiles on the Warm Super-Earth TOI-270 b

The search for atmospheres on rocky exoplanets is a crucial step in understanding the processes driving atmosphere formation, retention, and loss. Past studies have revealed the existence of planets interior to the radius valley with densities lower than would be expected for pure-rock compositions, indicative of the presence of large volatile inventories which could facilitate atmosphere retention. Here we present an analysis of the JWST NIRSpec/G395H transmission spectrum of the warm ($T_\mathrm{eq,{A_B}=0}$ = 569 K) super-Earth TOI-270 b ($R_\mathrm{p}$ = 1.306 $R_\oplus$), captured alongside the transit of TOI-270 d. The JWST white light-curve transit depth updates TOI-270 b's density to $\rho_\mathrm{p}$ = 3.7 $\pm$ 0.5 g/cm$^3$, inconsistent at 4.4$\sigma$ with an Earth-like composition. Instead, the planet is best explained by a non-zero, percent-level water mass fraction, possibly residing on the surface or stored within the interior. The JWST transmission spectrum shows possible spectroscopic evidence for the presence of this water as part of an atmosphere on TOI-270 b, favoring a H$_2$O-rich steam atmosphere model over a flat spectrum ($\ln\mathcal{B}$ = $0.3-3.2$, inconclusive to moderate), with the exact significance depending on whether an offset parameter between the NIRSpec detectors is included. We leverage the transit of the twice-larger TOI-270 d crossing the stellar disk almost simultaneously to rule out the alternative hypothesis that the transit-light-source effect could have caused the water feature in TOI-270 b's observed transmission spectrum. Planetary evolution modeling furthermore shows that TOI-270 b could sustain a significant atmosphere on Gyr timescales, despite its high stellar irradiation, if it formed with a large initial volatile inventory.

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arXiv2025

Precise Constraints on the Energy Budget of WASP-121 b from its JWST NIRISS/SOSS Phase Curve

Ultra-hot Jupiters exhibit day-to-night temperature contrasts upwards of 1000 K due to competing effects of strong winds, short radiative timescales, magnetic drag, and H2 dissociation/recombination. Spectroscopic phase curves provide critical insights into these processes by mapping temperature distributions and constraining the planet's energy budget across different pressure levels. Here, we present the first NIRISS/SOSS phase curve of an ultra-hot Jupiter, WASP-121 b. The instrument's bandpass [0.6 - 2.85 micron] captures an estimated 50-83% of the planet's bolometric flux, depending on orbital phase, allowing for unprecedented constraints on the planet's global energy budget; previous measurements with HST/WFC3 and JWST/NIRSpec/G395H captured roughly 20% of the planetary flux. Accounting for the unobserved regions of the spectrum, we estimate effective day and nightside temperatures of T_day = 2717 +/- 17 K and T_night = 1562 +/- 19 K corresponding to a Bond albedo of A_B = 0.277 +/- 0.016 and a heat recirculation efficiency of epsilon = 0.246 +/- 0.014. Matching the phase-dependent effective temperature with energy balance models yields a similar Bond albedo of 0.3 and a mixed layer pressure of 1 bar consistent with photospheric pressures, but unexpectedly slow winds of 0.2 km/s, indicative of inefficient heat redistribution. The shorter optical wavelengths of the NIRISS/SOSS Order 2 yield a geometric albedo of A_g = 0.093 +/- 0.029 (3 sigma upper limit of 0.175), reinforcing the unexplained trend of hot Jupiters exhibiting larger Bond albedos than geometric albedos. We also detect near-zero phase curve offsets for wavelengths above 1.5 micron, consistent with inefficient heat transport, while shorter wavelengths potentially sensitive to reflected light show eastward offsets.

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arXiv2025

Enriched volatiles and refractories but deficient titanium on the dayside atmosphere of WASP-121b revealed by JWST/NIRISS

With dayside temperatures elevated enough for all atmospheric constituents to be present in gas form, ultra-hot Jupiters offer a unique opportunity to probe the composition of giant planets. We aim to infer the composition and thermal structure of the dayside atmosphere of the ultra-hot Jupiter WASP-121b from two NIRISS$/$SOSS secondary eclipses observed as part of a full phase curve. We extract the eclipse spectrum of WASP-121b with two independent data reduction pipelines and analyse it using different atmospheric retrieval prescriptions to explore the effects of thermal dissociation, reflected light, and titanium condensation on the inferred atmospheric properties. We find that the observed dayside spectrum of WASP-121b is best fit by atmosphere models possessing a stratospheric inversion with temperatures reaching over 3000K, with spectral contributions from H2O, CO, VO, H-, and either TiO or reflected light. We measure the atmosphere of WASP-121b to be metal enriched (~10x stellar) but comparatively titanium poor (~1x stellar), potentially due to partial cold-trapping. The inferred C/O depends on model assumptions such as whether reflected light is included, ranging from being consistent with stellar if a geometric albedo of zero is assumed to being super-stellar for a freely fitted Ag = 0.16 +/- 0.02. The volatile-to-refractory ratio is measured to be consistent with the stellar value. We infer that WASP-121b has an atmosphere enriched in both volatile and refractory metals, but not in ultra-refractory titanium, suggesting the presence of a nightside cold-trap. Considering H2O dissociation is critical in free retrieval analyses, leading to order-of-magnitude differences in retrieved abundances for WASP-121b if neglected. Simple chemical equilibrium retrievals assuming that all species are governed by a single metallicity parameter drastically overpredict the TiO abundance.

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arXiv2025

The JWST Rocky Worlds DDT Program reveals GJ 3929b to likely be a bare rock

We report first results from the JWST Rocky Worlds Director's Discretionary Time program. Two secondary eclipses of the terrestrial exoplanet GJ 3929b were recently observed using MIRI photometric imaging at 15 um. We present a reduction of these data using the updated SPARTA pipeline. We also refine the planet mass, radius, and predicted time of secondary eclipse using a new sector of TESS data and new, high-precision radial velocities from the MAROON-X spectrograph. For the two JWST observations, we recover secondary eclipse depths of 177+47-45ppm and 143+34-35ppm at times consistent with a nearly circular orbit, as expected from the radial velocity data. A joint fit of the two visits yields a dayside brightness temperature Tp,dayside = 782+/-79K for GJ 3929b, which is consistent with the maximum brightness temperature Tmax = 737+/-14K for a bare, black rock (i.e., assuming zero Bond albedo and no heat redistribution). These results rule out CO2-rich atmospheres thicker than 100mbar at >3sigma, suggesting that GJ 3929b has lost any significant secondary atmosphere. The radial velocity data also indicate two additional non-transiting planets in the system: a previously-identified planet in a 15.0d orbit, and a newly-identified planet candidate in a 6.1d orbit.

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arXiv2025

Strict limits on potential secondary atmospheres on the temperate rocky exo-Earth TRAPPIST-1 d

The nearby TRAPPIST-1 system, with its seven small rocky planets orbiting a late-type M8 star, offers an unprecedented opportunity to search for secondary atmospheres on temperate terrestrial worlds. In particular, the 0.8 Earth-radii planet TRAPPIST-1 d lies at the edge of the habitable zone (equilibrium temperature ~262 K). Here we present the first 0.6-5.2 micron NIRSpec/PRISM transmission spectrum of TRAPPIST-1 d from two transits with JWST. We find that stellar contamination from unocculted bright heterogeneities introduces 500-1,000 ppm visit-dependent slopes, consistent with constraints from the out-of-transit stellar spectrum. Once corrected, the transmission spectrum is flat within $\pm$100-150 ppm, showing no evidence for a haze-like slope or molecular absorption despite NIRSpec/PRISM's sensitivity to CH4, H2O, CO, SO2, and CO2. Our observations exclude clear, hydrogen-dominated atmospheres with high confidence (greater than 3$\sigma$). We leverage our constraints on even trace amounts of CH4, H2O, and CO2 to further reject high mean molecular weight compositions analogous to Titan, a cloud-free Venus, early Mars, and both Archean Earth and a cloud-free modern Earth scenario (greater than 95% confidence). If TRAPPIST-1 d retains an atmosphere, it is likely extremely thin or contains high-altitude aerosols, with water cloud formation at the terminator predicted by 3D global climate models. Alternatively, if TRAPPIST-1 d is airless, our evolutionary models indicate that TRAPPIST-1 b, c, and d must have formed with less than approximately 4 Earth oceans of water, though this would not preclude atmospheres on the cooler habitable-zone planets TRAPPIST-1 e, f, and g.

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