Thomas Mikal-Evans

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes’ transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets’ transmission spectra between 1.1 and 1.7 μ m with the Wide Field Camera 3 (WFC3) and the stars’ UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between 2.0 σ and 3.2 σ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A supersolar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b’s zero-albedo equilibrium temperature of (880 ± 20) K, particularly in atmospheres with supersolar metallicity, possibly additionally muting absorption features.

Understanding the multidimensional structure of strongly irradiated exoplanets is essential for interpreting their atmospheric dynamics, chemistry, and energy transport, yet current analyses remain limited by the difficulty of extracting reliable phase-resolved spectra and by the lack of physically interpretable parameterizations for retrievals. We present a framework combining a data-driven eclipse-normalization method for phase-resolved emission spectra with an analytical three-dimensional temperature parameterization, derived from radiative, advective, and diffusive energy balance and controlled by a small set of characteristic timescales. Applied to JWST/NIRSpec G395H observations of WASP-121b, the method yields spectra consistent with conventional phase-curve fitting, while the parameterization reproduces the large-scale thermal structures predicted by general circulation models. The preferred retrieval reveals a pronounced day–night contrast, a dayside thermal inversion extending to both limbs, an inversion over part of the nightside, and limb temperatures differing by several hundred kelvin. Dynamical transport strengthens with pressure, and the hotspot offset increases from ∼4° to ∼9° across the pressures probed by G395H. The confined dayside hot region and the small, pressure-dependent offsets lie closer to the ∼3 G general circulation model than to its nonmagnetic counterpart, although Rayleigh drag cannot be excluded. The spectra also favour distinct dayside and nightside chemical states, with more nightside CH ₄ than the lower temperatures alone can explain, pointing to disequilibrium chemistry. The retrieved thermal structure further implies an inhomogeneous cloud distribution, with condensation favoured on the nightside and the cooler morning limb. The framework provides a computationally efficient, physically interpretable path from spectroscopic phase curves to multidimensional atmospheric structure.

Discovered deep within the “Neptunian desert,” LTT9779b remains the only known ultrahot Neptune, prompting significant speculation regarding its unique formation and evolutionary history. Its exceptionally high geometric albedo has previously been attributed either to the presence of clouds or to an extremely metal-rich atmosphere. Here, we present a comprehensive panchromatic analysis of its dayside atmosphere using JWST NIRISS and NIRSpec/G395H observations to characterize its atmospheric structure and composition. Leveraging the exceptional signal-to-noise ratio (SNR) in the observed spectra, we report a 3 σ -to-5 σ detection of dayside clouds, with strong evidence for Mg ₂ SiO ₄ (s) (silicate) condensation. This constitutes the first statistically significant detection of clouds on the dayside of a Neptunian-mass exoplanet. We demonstrate that a highly reflective cloud deck, rather than an extremely high-metallicity atmosphere, is the most likely explanation for the planet’s anomalously high optical albedo. Furthermore, our atmospheric retrievals yield robust detections of both CO (∼4.88 σ ) and CO ₂ (∼8.76 σ ), while providing tentative constraints on the H ₂ O abundance and upper limits on SiO, TiO, and VO. Finally, our analysis places a robust constraint on the C/O ratio of 0.984 ± 0.019. This aligns LTT9779b with other known ultrahot Jupiters exhibiting supersolar C/O ratios, suggesting a broader trend driven by the sequestration of oxygen-bearing condensates in ultrahot atmospheres.

Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here we show that two transits of the ultrahot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric light curves caused by the planet’s rotation during transit. We observe increasing CO absorption and slightly decreasing H ₂ O absorption in the transmission spectrum as the planet rotates. These results are indicative of a stronger longitudinal temperature gradient across the evening than across the morning terminator, consistent with higher temperatures in the eastern half than in the western half of the dayside. The observed changes of the transmission spectrum with orbital phase are in line with the temperature increase causing thermal dissociation of H ₂ O, while CO remains abundant. The observation of longitudinal gradients in atmospheric temperature and chemistry from the planet’s rotational transit provides a new probe for constraining atmospheric heterogeneity using JWST beyond differences between morning and evening terminators from limb asymmetries.

We present the combined JWST/NIRSpec/G395H and NIRISS/SOSS transmission spectrum of a warm mini-Neptune, TOI-1130 b (3.66 R _⊕ , 19.8 M _⊕ , and T _eq ∼ 825 K). It is part of a rare and unique multiplanet system, TOI-1130, which hosts an inner mini-Neptune and an outer hot Jupiter locked in a 2:1 mean motion resonance. From the transmission spectrum of TOI-1130 b we detect multiple molecules—H ₂ O (7.5 σ ), CO ₂ (3.3 σ ), and SO ₂ (3.6 σ ), as well as a tentative detection of CH ₄ (∼2 σ ). We find a strong optical slope in the NIRISS/SOSS spectrum, which is consistent with TESS and CHEOPS transit depth measurements. From equilibrium chemistry retrievals we measure the atmospheric metallicity ( log Z / Z ⊙ = 1 . 8 − 0.3 + 0.4 ) and C/O ratio (σ level confidence) and constrain the atmospheric mean molecular weight, μ = 5.5 − 0.8 + 1.3 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 μ m absorption signal and find a mass-loss rate upper limit of 10 ¹¹ g s ⁻¹ . The volatile-rich high mean molecular weight atmosphere of TOI-1130 b along with the “pebble-filtering” effect of the outer hot Jupiter supports the ex situ formation scenario beyond the water ice line and subsequent migration, coherent with its present orbital architecture. A volatile-rich formation scenario could also potentially explain the location of TOI-1130 b at the edge of the “radius cliff.” This result hints that the mini-Neptune population may not have a homogeneous formation history; rather, volatile-rich ex situ formation also contributes to its population.

LTT 9779 b is an ultra-hot Neptune ( R _p ≈ 4.7 R _⊕ , M _p ≈ 29 M _⊕ ) orbiting its Sun-like host star in just 19 hr, placing it deep within the “hot-Neptune desert,” where Neptunian planets are seldom found. We present new James Webb Space Telescope NIRSpec G395H phase curve observations that probe its atmospheric composition in unprecedented detail. At near-IR wavelengths, which penetrate the high-altitude clouds inferred from previous NIRISS/SOSS spectra, thermal emission reveals a carbon-rich atmosphere with opacity dominated by carbon monoxide (CO) and carbon dioxide (CO ₂ ). Evidence for both species is observed at all orbital phases ( σ < 11.6), with retrieved mixing ratios of ∼10 ⁻¹ for CO and ∼10 ⁻⁴ for CO ₂ , indicating a globally well-mixed reservoir of carbon-bearing gases. We also find moderate evidence of water vapor (H ₂ O; σ < 3.9) on the eastern dayside and dayside (orbital phases 120° and 180°, respectively), and tentatively detect sulfur dioxide (SO ₂ ; σ < 2.5) on the western nightside (orbital phase 300°), providing insight into its chemistry and possible photochemical production under intense stellar irradiation. Our findings show that LTT 9779 b retains a carbon-rich atmosphere at a distance from its host star where hot-Neptune-class worlds are expected to evaporate. This makes LTT 9779 b a valuable laboratory for studying atmospheric escape and chemical processes under extreme conditions, offering new insight into the survival of planets in the hot-Neptune desert.

Correctly interpreting JWST spectra of close-in exoplanets requires a measurement of the X-ray and ultraviolet light that the planets receive from their host stars. Here, we provide spectral energy distributions (SEDs) covering the range ≈5–1 × 10 ⁷ Å for 20 transiting exoplanet host stars observed in JWST Cycle 1. The SEDs are constructed out of new and archival Hubble Space Telescope, Chandra X-ray Observatory, and/or XMM-Newton data combined with spectra from models or stars with similar properties (proxies) filling in unobserved gaps. We have also constructed SEDs of likely Habitable Worlds Observatory targets κ ¹ Ceti, τ Ceti, ϵ Indi, and 70 Oph B for use as proxies. We find that the JWST target planets almost all experience much stronger ultraviolet fluxes than the Earth, especially in the extreme ultraviolet, even for planets with similar overall instellation. Strong ongoing or past atmospheric escape is possible for a majority of these planets. We also assess the now considerable sample of panchromatic stellar SEDs and its applicability for current JWST observations and beyond.

Hot Jupiters (HJs) are 2–3× less common around M dwarfs than around AFGK stars, suggesting that HJs may form and/or migrate via distinct pathways around different types of stars. One source of insight into HJ formation mechanisms is to trace their dynamical histories through measurements of host stellar obliquities via the Rossiter–McLaughlin (RM) effect. Here we present measurements of the RM effect for the HJs TOI-3714 b and TOI-5293 A b using the Gemini-North/MAROON-X spectrograph. Our measurements represent just the second and third hot Jupiters around M dwarfs (HJMD) with a detection of the RM effect. We find that both systems are well-aligned with sky-projected obliquities of λ = 2 1 − 11 + 14 ° and − 1 2 − 14 + 19 ° and deprojected obliquities of ψ = 2 6 − 10 + 11 ° and 2 4 − 10 + 11 ° for TOI-3714 and TOI-5293 A, respectively. Both stars are in wide binary systems. We refine the stellar parameters by decontaminating their unresolved K _s -band photometry and constrain the binary orbits using Gaia DR3 astrometry. We find that the minimum mutual inclination of the planet and binary companion in the TOI-5293 system is sufficiently large to drive Kozai–Lidov (KL) migration, while the result for TOI-3714 is inconclusive. We present a population-level analysis of HJs around AFGK versus early M dwarfs and argue that KL migration is more efficient around the latter, which is expected to produce misaligned stellar obliquities in HJMD systems in the absence of efficient tidal damping. The emerging population of well-aligned HJMD hosts supports the expectation that M dwarfs, with their deep convective envelopes, efficiently dampen misaligned obliquities.