Ian Czekala

The physical properties of circumplanetary disks offer insights into the formation of giant planets and their satellites. The circumbinary planetary-mass companion SR 12 c (1200 au from SR 12 AB) hosts an isolated dust disk directly detected with ALMA. We present JWST/MIRI imaging of SR 12 c from 5.6 to 21 μ m and analyze its spectral energy distribution from infrared to submillimeter wavelengths. Phenomenologically, the dust disk can be described by a two-component blackbody with large contrasts in characteristic temperatures and radii. Physically motivated models with varied geometries and heating mechanisms suggest a predominantly flat, irradiation-heated disk with a smaller contribution from viscous dissipation. The disk is inferred to be compact, with an inner radius of ∼3 R _Jup . The outer radius is only loosely constrained, with a characteristic value of ∼3 au. The lack of strong 10 μ m excess emission suggests significant grain growth and dust settling. We also find that SR 12 c is broadly consistent with the empirical star–disk scaling relations, providing context for its possible formation pathway. These results demonstrate the power of combining mid-infrared and submillimeter observations with disk modeling to better understand the physical properties of circumplanetary disks.

HATS-6 b is one of several recently discovered giant exoplanets orbiting M dwarf stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ( T _eq ∼ 700 K), Saturn-mass ( M _p ∼ 0.3 M _Jup ), Jupiter-radius ( R _p ∼ 1 R _Jup ) planet that transits its star every ∼3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of 0.6−5.3 μ m. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low-metallicity ( log [ M / H ] = − 1.9 9 − 0.2 + 0.2 ) subsolar-C/O ( log [ C / O ] = − 0.4 6 − 0.2 + 0.2 ) atmosphere, and find strong evidence for H ₂ O, CH ₄ , and NH ₃ at volume mixing ratios (in log [ X ] ) of − 4.8 8 − 0.24 + 0.25 , − 5.3 8 − 0.19 + 0.18 , and − 6.0 3 − 0.19 + 0.18 , respectively. We consistently retrieve a significantly lower T _eq than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a nonzero Bond albedo. Our planetary interior models retrieve bulk metallicities 3 orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well mixed. We find an excess feature around 3 μ m, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C ₂ H ₄ . Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.

Many details of the gas accretion phase during giant planet formation remain untested. We present new 0.2–0.7 μ m UV-through-red optical imaging of the young, wide-orbit planetary-mass companion SR 12 c from the Wide Field Camera 3 (WFC3) instrument on board the Hubble Space Telescope. SR 12 c exhibits strong accretion-related continuum excess blueward of ∼5000 Å and clear signs of the Balmer jump at 3646 Å. We derive a total accretion luminosity of 1.65 ± 0.19 × 10 ⁻⁵ L _⊙ and a mass accretion rate of 8 ± 2 × 10 ⁻¹² M _⊙ yr ⁻¹ . Based on its mass and age, SR 12 c will not grow by an appreciable amount at its current accretion rate; it is at the end stages of assembly. No accretion variability is evident between the two epochs of the WFC3 observations spanning a month-long baseline, but the H α emission line strength decreases by 90% compared to the reported flux from five years earlier. Combined with previous observations of SR 12 c, we assemble one of the most complete spectral energy distributions of a young giant planet to date, spanning the UV through sub-mm wavelengths (0.2–880 μ m). This adds SR 12 c to the small yet growing sample of planets with detailed accretion and disk constraints, which together are beginning to establish the diversity of timescales and physical processes governing the formation of giant planets.

The growing sample of giant exoplanets around M dwarf stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293 A b, a 0.5 M _Jup planet orbiting an early M dwarf with a period of ∼3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope at μ m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ( log [ M / H ] = − 1.0 3 − 0.44 + 0.53 × solar) and supersolar C/O ratio ( 1.2 3 − 0.75 + 2.94 ). The retrievals yield Bayes factors that indicate strong evidence for CH ₄ as well as low-significance detections of CO ₂ , H ₂ O, and NH ₃ . Finally, using thermal evolution models we find that the radius of TOI-5293 A b is inflated above theoretical expectations (∼1.07 R _Jup ), despite it having an temperature of ∼700 K, and hence we were unable to constrain its bulk composition.

HATS-75 b is one of the recently discovered Giant Exoplanets Around M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying seven GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6–5.3 μ m). The planet’s spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (e.g., stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity/TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity ( log [ M / H ] = − 1.7 4 − 0.76 + 0.92 ) and supersolar carbon-to-oxygen ( C / O = 1.0 4 − 0.09 + 0.40 ), which paired with a best-fit interior model with bulk metallicity of Z _p = 0.20 ± 0.04 implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH ₄ , CO, and CO ₂ . We obtain only an upper limit for H ₂ O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M dwarfs with JWST.

Recent discoveries of transiting giant exoplanets ( R _p ≳ 8 R _⊕ ) around M dwarfs present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ( P = 1.63 days) Jupiter-like planet ( M _p = 1.08 M _J and R _p = 0.94 R _J ) orbiting an M4 dwarf ( M _⋆ = 0.392 M _⊙ , R _⋆ = 0.394 R _⊙ ). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph spanning 0.6–5.3 μ m. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths λ ≲ 3.0 μ m, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of H _2O . The wavelength coverage enabled a robust detection of CH ₄ and H ₂ S, which have detectable molecular features between 3.0 and 5.0 μ m. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with subsolar metallicity (3 σ upper limit of log [ M / H ] ≲ − 1.24 ) and supersolar C/O ratio (3 σ lower limit of log [ C / O ] ≳ 0.09 ), although this may partly be driven by the nondetection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10%–20%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

Protoplanetary disks are the birthplaces of planetary systems, and deviations from Keplerian rotation imprinted in disk gas kinematics serve as key tracers of physical processes and the presence of protoplanets within disks. Using the CO ( J = 3–2) data from the exoALMA Large Program, encompassing 15 disks, we constructed two-dimensional (2D) maps of centroid velocity, line width, and peak intensity, and extracted non-Keplerian deviations by subtracting smooth Keplerian models. This Letter provides the first systematic and uniform overview of 2D gas substructures across the entire exoALMA sample. We find that all targets exhibit large-scale deviations from smooth Keplerian disks, displaying a variety of morphologies, including spiral-like structures, arc- or ring-like features, and patterns indicative of variations in the emitting surface height. Nonaxisymmetric spiral-arm features are detected or suggested in five disks (CQ Tau, MWC 758, HD 135344B, HD 34282, and SY Cha), and are preferentially found in Herbig Ae/Fe systems. In contrast, some other sources (J1852, PDS 66, and V4046 Sgr), despite exhibiting noticeable deviations, appear to be dynamically quieter. This 2D atlas suggests that kinematic substructures are ubiquitous in large (≳100 au) protoplanetary disks with ages of a few million years, based on the observations obtained with sufficient sensitivity at a moderate-to-high spatial resolution of ∼20 au and high-velocity resolution of ∼0.1 km s ⁻¹ .

Vertical gas flows—such as winds and meridional circulations—are natural outcomes of protoplanetary disk processes and play a critical role in the earliest stages of planet formation. We analyze the vertical gas motions in 14 disks, as part of the exoALMA Large Program, focusing on the ¹² CO J = 3–2 and ¹³ CO J = 3–2 emission lines. Using discminer to model the Keplerian velocity field, we extract line-of-sight velocity residuals and measure the radial and vertical components of the gas motion. Vertical motions are detected in most disks. Two types of patterns emerge: (1) oscillatory up/down flows, likely linked to instabilities; and (2) transitions from downward to upward motions that we interpret as the bases of disk winds. In most cases, the velocity amplitudes are of a few tens of m s ⁻¹ . However, two disks, MWC758 and CQ Tau, show two spiral velocity features in their residual maps, red- and blueshifted, which we interpret as vertical velocities reaching up to ∼350 m s ⁻¹ (∼0.7 c _s ), consistent with the gas motion in eccentric disks. Fast upward motions (up to 500 m s ⁻¹ ; ∼1.8 c _s ) are also detected in the outer disk of MWC758. Synthetic observations from (magneto)hydrodynamic simulations validate the reliability of our method. Although strong molecular winds appear to be relatively rare in ¹² CO and ¹³ CO, our study shows that, when traced by deep high-spectral-resolution line data, protoplanetary disks exhibit ubiquitous vertical flows. However, their overall velocity structure is highly complex, preventing the identification of a coherent dominant physical mechanism driving the vertical motions across all disks, thus requiring further theoretical investigation.

The exoALMA Large Program has revealed a wealth of substructures in the dust and molecular line emission of several protoplanetary discs, suggesting that planet formation may unfold within highly dynamic environments. Using synthetic observations of planet–disk interactions and disk instabilities, we demonstrate how the origin of these substructures can be investigated through a tomographic study of molecular lines, extending the scope of the analysis beyond line-centroid kinematics alone. Our results indicate that with only a few hours of ALMA integration at moderate angular resolution ( 0 . ″ 15 − 0 . ″ 30 ), it is possible to identify the key signatures driven by planets more massive than 0.1% of the stellar mass. These signatures manifest not only as deviations from Keplerian motion but also as localized line broadening, enabling accurate constraints on the orbital radius and azimuthal location of the planets. We further show that a diagnostic based on line skewness in spectrally resolved observations can help distinguish between planetary and instability-driven signatures, owing to the distinct degrees of velocity coherence associated with each mechanism. Finally, we apply this tomographic analysis to exoALMA CO line data for the disks of HD 135344B and MWC 758. In HD 135344B, we identify strongly localized velocity and line-width perturbations, suggesting the possibility of three massive planets embedded in the disk: one at R = 95 au, exterior to the continuum substructures, and two within dust gaps at R = 41 au and R = 73 au. For MWC 758, the dominance of vertical-velocity spirals over localized signatures is consistent with predictions from models of moderate disk eccentricities or warps, potentially induced by a substellar companion in the inner regions of the system.

GY 91, commonly categorized as a Class I young stellar object, is notable for disk dust substructures that have been hypothesized to trace early planet formation. Using the Atacama Large Millimeter/submillimeter Array 12 m and Atacama Compact Array, we present new Band 7 dust continuum and molecular line observations of GY 91 at an angular resolution of ∼ 0 . ″ 3 (40 au). We report detections of CS J = 6–5, N ₂ H ⁺ J = 3–2, C ¹⁸ O J = 3–2, H ₂ CS J K a , K c = 8 1 , 7 − 7 1 , 6 , H ₂ CO J K a , K c = 4 0 , 4 − 3 0 , 3 , and H ₂ CO J K a , K c = 4 2 , 3 − 3 2 , 2 , as well as a tentative detection of ¹³ C ¹⁸ O J = 3–2. We observe azimuthal asymmetry in CS and H ₂ CS emission, as well as radially structured H ₂ CO 4 _0,4 –3 _0,3 emission outside the dust continuum. C ¹⁸ O and H ₂ CO 4 _0,4 –3 _0,3 show significant cloud contamination, while CS and N ₂ H ⁺ are good tracers of Keplerian rotation originating from the disk. Envelope emission does not appear to contribute significantly either to the continuum or molecular line observations. GY 91’s chemical properties appear in large part to resemble those of Class II disks, although observations of additional molecular probes should be obtained for a fuller comparison. With CS, we estimated a dynamical stellar mass of 0.58 M _⊙ , which is higher than previous estimates from stellar evolutionary models (0.25 M _⊙ ). Using both radiative transfer modeling of the dust continuum and comparison of the C ¹⁸ O and N ₂ H ⁺ fluxes to literature thermochemical models, we estimate a disk mass of ∼0.01 M _⊙ .